Fact-checked by Grok 2 weeks ago

ATLAS experiment

The ATLAS experiment, whose full name is A Toroidal LHC ApparatuS, is a general-purpose particle physics detector at the Large Hadron Collider (LHC) at CERN in Geneva, Switzerland, designed to probe fundamental questions about the building blocks of matter, the forces governing them, and phenomena such as dark matter and extra dimensions.^[1] Located 100 meters underground near the Franco-Swiss border, it records and analyzes collisions of protons or heavy ions accelerated to nearly the speed of light in the LHC's 27-kilometer ring, aiming to test the Standard Model of particle physics and search for new physics beyond it.^[2] With its massive scale—46 meters long, 25 meters in height and width, and weighing 7,000 tonnes—ATLAS employs over 100 million electronic channels to capture data from billions of collisions per second, using layered subdetectors including silicon and gas-based trackers for particle trajectories, electromagnetic and hadronic calorimeters for energy measurements, and a muon spectrometer within a toroidal magnetic field for identifying long-lived particles.^[1]^[2] Operated by an international collaboration of approximately 6,000 members from 248 institutions across 40 countries, ATLAS exemplifies global teamwork in high-energy physics, with contributions spanning detector construction, data analysis, and theoretical interpretation.^[1] The experiment's physics program encompasses precision measurements of known particles like the top quark and W/Z bosons, studies of quantum chromodynamics in high-density environments, and hunts for supersymmetric particles, dark matter candidates, and rare processes that could reveal cracks in the Standard Model.^[2] ATLAS shares the LHC interaction point with the complementary CMS detector, enabling cross-verification of results through diverse technological approaches.^[1] Among its landmark achievements, ATLAS played a pivotal role in the 2012 discovery of the Higgs boson, announced jointly with CMS on July 4 of that year, confirming the mechanism by which particles acquire mass as predicted by the Standard Model; this breakthrough earned the 2013 Nobel Prize in Physics for theorists Peter Higgs and François Englert.^[3] Since then, ATLAS has delivered increasingly precise Higgs property measurements, including its couplings to other particles and potential rare decays, while also observing phenomena like light-by-light scattering in heavy-ion collisions.^[4] Currently in LHC Run 3 (2022–2025), ATLAS is collecting vast datasets at 13.6 TeV center-of-mass energy to refine these studies and probe rarer events.^[5] Looking ahead, the experiment is undergoing upgrades for the High-Luminosity LHC phase starting around 2030, which will boost collision rates by a factor of 10, enabling the collection of exabyte-scale data for even deeper insights into the Universe's fundamental nature.^[2]

History

Origins and Planning

The ATLAS experiment was conceived in 1992 as a general-purpose proton-proton collider detector for the proposed Large Hadron Collider (LHC) at CERN, addressing the limitations of existing facilities like the Large Electron-Positron (LEP) collider, which operated at center-of-mass energies up to approximately 209 GeV but lacked the reach to fully probe TeV-scale physics. The initiative emerged from the merger of earlier concepts, including the ASCOT and EAGLE proposals, to create a versatile detector capable of exploiting the LHC's planned 14 TeV collision energy. This conception was formalized through the ATLAS Letter of Intent (LoI), submitted to the LHC Experiments Committee (LHCC) on October 1, 1992, marking the first official use of the ATLAS acronym—standing for A Toroidal LHC ApparatuS—and outlining a broad research program driven by the need to explore uncharted territory beyond the Standard Model.^[6]^[2] The primary scientific motivations centered on elucidating electroweak symmetry breaking, a cornerstone of the Standard Model that remained experimentally elusive. Central to this was the search for the Higgs boson, predicted to have masses between 80 and 1000 GeV, with key decay channels such as H → γγ and H → ZZ* → 4ℓ offering discovery potential at significances exceeding 5σ with 10^5 pb^{-1} of integrated luminosity. Equally compelling were investigations into supersymmetry (SUSY), an extension to the Standard Model that could stabilize the Higgs mass hierarchy and provide dark matter candidates, with ATLAS designed to detect signatures like multijet events plus missing transverse energy or same-sign dileptons, sensitive to gluinos and squarks up to 2.3 TeV. These goals underscored the experiment's role in testing mechanisms for mass generation and new physics at the electroweak scale, building on LEP's successes while targeting phenomena inaccessible at lower energies.^[6]^[7] Planning advanced with the submission of the ATLAS Technical Proposal on December 15, 1994, which detailed the detector's architecture, performance simulations, and resource needs, estimated at 370–450 million Swiss francs and 1500 person-years of effort. This led to a positive recommendation from the LHCC and CERN Research Board in 1995, culminating in official approval for construction by CERN Director-General Chris Llewellyn Smith, enabling initial funding commitments from CERN member states. Site selection focused on the LHC's interaction point 1, utilizing the existing underground cavern UX15—approximately 100 meters below ground near the Franco-Swiss border—with a design incorporating a 33-meter-diameter access shaft for lowering pre-assembled detector components, ensuring compatibility with ongoing LEP operations.^[8]^[9]^[2] A major planning challenge was adapting to the LHC's extreme environment, including instantaneous luminosities up to 10^{34} cm^{-2} s^{-1}, which would produce interaction rates of 20–40 events per crossing and necessitate precise event reconstruction amid pile-up. Radiation hardness emerged as a critical concern, with projected doses reaching 28 kGy per year and neutron fluences of 6.0 \times 10^{14} cm^{-2} per year (for E ≥ 100 keV, with moderator) in inner regions, driving early R&D (e.g., via CERN's RD29 project) for resilient materials, electronics, and calorimetry to maintain efficiency over a 10-year lifespan without significant degradation. These factors shaped a staged construction approach, prioritizing core components while deferring peripheral systems like end-cap toroids to optimize costs and timelines.^[6]

Construction and Commissioning

Construction of the ATLAS detector began in 1998 with the excavation of underground caverns at Point 1 along the Large Hadron Collider (LHC) ring in Meyrin, Switzerland, marking the start of the engineering and infrastructure setup phase.^[2] Civil engineering work included digging two shafts and two caverns to house the detector, a process that took five years and concluded in 2003, enabling the subsequent installation of detector components 90 meters underground.^[10] This initial phase involved international teams coordinating the preparation of surface buildings and access infrastructure, laying the foundation for the assembly of one of the largest particle detectors ever built.^[11] Major milestones in the construction included the assembly of the barrel toroid magnet in 2004, a critical step in integrating the superconducting magnet system. The first of eight massive coils, each 25 meters long and weighing 100 tonnes, was lowered into the cavern on 26 October 2004, with the full mechanical assembly completed by autumn 2005.^[12] By 2008, detector integration reached completion, with the final large component—the small wheel of the muon spectrometer—installed in early 2007, and the entire structure sealed on 4 October 2008 to celebrate the end of the construction phase.^[13] These achievements highlighted the project's scale, involving the precise positioning of over 7,000 tonnes of materials within a 44-meter-long, 25-meter-diameter cylinder.^[2] Key technologies developed during construction encompassed advanced superconducting magnets, including the central solenoid providing a 2 T field for the inner detector and the toroid system—comprising a barrel toroid and two end-cap toroids—for bending charged particle paths in the muon spectrometer. The solenoid and toroids utilized niobium-titanium coils cooled to 4.5 K via cryogenic systems, with the barrel toroid's eight coils interconnected in a rigid structure to withstand immense magnetic forces.^[14] Silicon pixel sensors formed the innermost layer of the tracking system, offering high-resolution vertex reconstruction, while liquid argon calorimeters provided precise energy measurements for electrons, photons, and hadrons across the detector's full coverage.^[2] The project relied heavily on international contributions, with over 20 countries funding and building major components through in-kind agreements. The United States led the design, construction, and testing of the inner detector, including its pixel, semiconductor tracker (SCT), and transition radiation tracker subsystems, contributing approximately $164 million in materials and expertise.^[15] Japan played a pivotal role in the SCT, producing a significant portion of its 4,088 silicon strip modules and associated hybrids, leveraging expertise in semiconductor fabrication. Other nations, such as those in Europe and Canada, handled calorimeters, muon systems, and magnet components, ensuring distributed responsibility across the collaboration.^[16] The total construction cost approximated 1 billion Swiss francs, supported by funding agencies from more than 20 member states and non-member countries, reflecting the global scale of the endeavor.^[17] Commissioning in 2008 addressed significant challenges, including the alignment of approximately 100 million electronic readout channels to achieve micrometer precision for particle tracking. Cryogenic systems for the magnets required stabilization at superconducting temperatures, with successful cool-down and powering tests ensuring operational integrity. First cosmic ray tests, conducted from spring through autumn 2008, validated subsystem performance and integration, using natural muons to calibrate detectors and identify issues like cooling leaks in end-cap regions, which affected about 2.5% of channels but were largely recoverable.^[18] These efforts confirmed the detector's readiness for LHC beams by late 2008.^[19]

Operations and Milestones

The ATLAS experiment commenced operations with the first circulation of LHC beams in September 2008, followed by the inaugural proton-proton collisions on November 23, 2009, at a center-of-mass energy of 900 GeV.^[20] These initial events marked the beginning of data collection, with the collision energy ramping up to 3.5 TeV per beam by December 2009 and further to 7 TeV in March 2010, initiating Run 1. During Run 1 (2010–2012), ATLAS recorded approximately 25 fb^{-1} of integrated luminosity at 7–8 TeV, enabling early precision measurements in particle physics.^[21] Following Run 1, the LHC entered Long Shutdown 1 (LS1) from February 2013 to April 2015, primarily to consolidate over 10,000 electrical splices between superconducting magnets, addressing vulnerabilities exposed by a 2008 incident and preparing for higher energies.^[22] Run 2 commenced in June 2015 at 13 TeV, with ATLAS collecting about 140 fb^{-1} of data through December 2018, including a 2016 milestone where the LHC exceeded its annual target by delivering over 40 fb^{-1} that year alone.^[23]^[24] Long Shutdown 2 (LS2), spanning December 2018 to April 2022, facilitated ATLAS Phase-1 upgrades, including a new Level-1 calorimeter trigger system and additional muon chambers in the New Small Wheels, to cope with increased pileup from higher luminosities in Run 3.^[25] Run 3 began on July 5, 2022, at 13.6 TeV and continues through 2026, with 2025 data contributing to advanced Higgs boson analyses amid record proton runs that year, delivering 125 fb^{-1}.^[26]^[27] By November 2025, ATLAS had amassed over 300 fb^{-1} of cumulative integrated luminosity across all runs, maintaining a data-recording efficiency exceeding 95%.^[28] In April 2025, the ATLAS collaboration, alongside CMS, ALICE, and LHCb, received the Breakthrough Prize in Fundamental Physics for transformative LHC discoveries.^[29]

Collaboration

Formation and Membership

The ATLAS Collaboration was formally established on October 1, 1992, with the submission of its Letter of Intent to the LHC Experiments Committee, marking the unification of earlier proto-collaborations and involving approximately 850 scientists from 88 institutions across 25 founding countries.^[2]^[30] Initially comprising physicists and engineers focused on designing a general-purpose detector for the proposed Large Hadron Collider, the collaboration grew rapidly through the 1990s and 2000s, reaching nearly 3,000 members by 2008 as construction advanced and international participation expanded.^[31] As of 2025, the ATLAS Collaboration encompasses around 6,000 members, including approximately 3,000 scientific authors, from over 170 institutions in 40 countries, drawing in physicists, engineers, technicians, doctoral students (about 1,200), and support staff.^[32] Membership is structured into full institutional members, which hold complete rights and obligations including representation on the Collaboration Board; associate institutes, designed for universities or labs transitioning toward full membership with access to data and participation opportunities; and clustered institutions, allowing smaller groups to join collectively as a single voting entity to meet the same criteria as individual institutions.^[33]^[34]^[35] The collaboration's diversity is evident in its global reach, with members from over 100 nationalities, including significant representation from non-OECD countries, fostering broad international expertise.^[36] Key recruitment efforts have included international agreements, such as the US ATLAS program funded by the Department of Energy and National Science Foundation, which has integrated numerous American institutions since the early 2000s.^[37] The 2012 discovery of the Higgs boson spurred a notable influx of new members, attracting talent eager to contribute to precision measurements and beyond-Standard-Model searches, further diversifying the collaboration's composition.^[38] Contributions to the collaboration are divided between in-kind deliverables, such as hardware components, and cash support for common projects like computing and operations. Notable in-kind examples include the Russian Federation's provision of superconducting coils for the barrel toroid magnet system and contributions from European institutions to the Athena software framework for data simulation and reconstruction.^[39]^[40] Inclusivity initiatives emphasize gender balance and support for early-career researchers, with the collaboration actively tracking demographics to promote equity and providing programs like the ATLAS Early Career Scientists Board to mentor PhD students and young professionals.^[41]^[42]^[43]

Organization and Leadership

The ATLAS collaboration operates under a hierarchical governance structure designed to coordinate its international efforts in particle physics research. At the apex is the Spokesperson, elected every two years by the Collaboration Board with a maximum of two consecutive terms, who oversees all aspects of the experiment, represents ATLAS to CERN, funding agencies, and external bodies, and ensures alignment between scientific goals and operational needs.^[32] The current Spokesperson is Stéphane Willocq from the University of Massachusetts Amherst, serving from March 2025 to March 2027, supported by Deputy Spokespersons Anna Sfyrla from the University of Geneva and Guillaume Unal from CEA Saclay, who assist in management and share representational duties.^[44]^[45] The Executive Board, comprising the Spokesperson, Deputies, Technical Coordinator (Martin Aleksa from CERN), Resource Coordinator (David Francis from CERN), and Upgrade Coordinator (Benedetto Gorini from CERN), handles day-to-day coordination of technical integration, resource planning, and upgrade programs.^[46] Leadership transitions emphasize international rotation to reflect the collaboration's global composition, fostering diverse perspectives and equitable participation. Notable past Spokespersons include Fabiola Gianotti (Italy, 2009–2013), who led during the initial LHC data-taking phase; Karl Jakobs (Germany, 2017–2021), who guided upgrades for high-luminosity operations; and Andreas Hoecker (France/CERN, 2021–2025), who advanced data analysis and computing strategies.^[47]^[48]^[31] This rotation, drawn from institutions across Europe, North America, and beyond, ensures continuity while promoting broad institutional involvement in decision-making.^[32] Key committees support specialized functions and contribute to governance. The Physics Committee reviews and approves scientific publications and analyses, ensuring rigorous standards; the Operations Committee manages detector performance and data-taking coordination; the Detector and Performance Committee oversees hardware maintenance and upgrades; the Trigger Committee handles event selection systems; and the Computing Committee addresses software development, data storage, and processing infrastructure.^[49]^[50] These committees play critical roles in approving upgrades, such as those for the High-Luminosity LHC, by evaluating proposals and integrating feedback from subsystems.^[46] Decision-making within ATLAS is consensus-based, prioritizing broad agreement while allowing voting by member institutions when necessary, particularly through the Collaboration Board, which serves as the primary policy-setting body and convenes annual general meetings to address strategic issues.^[51]^[52] The Collaboration Board, composed of representatives from all institutions, endorses major initiatives like new memberships or policy changes via institutional votes.^[53] Complementing this, the Resource Review Board (RRB), involving funding agencies, allocates resources by reviewing and approving budgets for construction, operations, and upgrades proposed by the collaboration.^[54]^[16] In high-stakes scenarios, such as coordinating responses to LHC operational challenges during the 2012 proton-lead collision preparations, leadership and committees facilitate rapid, unified action across the collaboration.^[55]

Physics Objectives

Standard Model Investigations

The ATLAS experiment at the Large Hadron Collider (LHC) plays a central role in verifying the predictions of the Standard Model (SM) through high-precision measurements, which are essential for establishing a robust baseline before pursuing searches for physics beyond the SM. These investigations focus on electroweak and quantum chromodynamics (QCD) processes, where deviations from SM expectations could signal new physics, often interpreted within the framework of effective field theory (EFT). By achieving sensitivities to anomalies at the percent level or better, ATLAS measurements constrain potential extensions of the SM, such as those involving higher-dimensional operators in the Standard Model Effective Field Theory (SMEFT).^[56] Precision tests of electroweak parameters include detailed studies of W and Z boson properties. For instance, using LHC Run 1 data at 7 TeV with an integrated luminosity of 4.6 fb^{-1}, ATLAS measured the W boson mass to be 80,360 ± 16 MeV, achieving an uncertainty of 16 MeV and confirming consistency with SM predictions. This result refines global electroweak fits and probes the gauge structure of the SM. Similarly, measurements of Z boson properties, such as decay widths and angular distributions, provide stringent tests of electroweak unification.^[57] QCD studies at ATLAS validate perturbative QCD at high energies through measurements of jet production cross-sections. Inclusive jet cross-sections at 13 TeV have been measured differentially in jet rapidity and transverse momentum up to several TeV, showing agreement with next-to-next-to-leading-order predictions within uncertainties of 5-10% at high p_T. These results constrain parton distribution functions (PDFs) and test the strong coupling constant α_s, demonstrating the SM's predictive power in high-multiplicity environments.^[58] Investigations of electroweak symmetry breaking involve diboson production processes, such as WW and ZZ, whose rates probe unitarity and the stability of gauge boson self-interactions. ATLAS has measured fiducial cross-sections for ZZ production at 13.6 TeV, yielding values consistent with SM expectations to within 7%, and used these to set limits on anomalous quartic gauge couplings that could arise from EWSB mechanisms. Such analyses ensure the SM's longitudinal gauge boson scattering remains unitary at TeV scales. Initial Run 3 analyses at 13.6 TeV continue these investigations with higher precision.^[59]^[60] Although flavor physics is primarily the domain of dedicated experiments like LHCb, ATLAS contributes measurements of B-hadron decays to complement SM tests. Recent analyses include the B^0 meson lifetime, measured as 1.5053 ± 0.0012 (stat.) ± 0.0035 (syst.) ps using 140 fb^{-1} of 13 TeV data from the full Run 2 dataset in decays to J/ψ K^{*0}, aligning with SM predictions and aiding in the validation of heavy-quark effective theory. These results help constrain CKM matrix elements and search for subtle flavor anomalies.^[61] Theoretically, ATLAS frames SM investigations using EFT to quantify potential deviations, parameterizing new physics via Wilson coefficients in the SMEFT Lagrangian. Production cross-sections for SM processes, such as pp → X, are computed as

\sigma(pp \to X) = \int L(x_1, x_2) \, f(x_1) \, f(x_2) \, dx_1 \, dx_2,

where L(x_1, x_2) represents the parton luminosity, and f(x_i) are the PDFs, allowing EFT corrections to be incorporated for sensitivity to dimension-6 operators. This approach motivates precision measurements by highlighting how SM confirmation underpins BSM sensitivities, with ATLAS results excluding certain EFT parameters at 95% confidence levels.^[56]

Beyond Standard Model Searches

The ATLAS experiment at the Large Hadron Collider (LHC) conducts extensive searches for physics beyond the Standard Model (BSM) to probe theoretical extensions that address shortcomings in the Standard Model, such as the hierarchy problem, the nature of dark matter, and the unification of forces. These searches target specific signatures predicted by models like supersymmetry (SUSY), extra dimensions, and dark matter candidates, often characterized by events with high missing transverse energy (MET), multiple leptons, or unusual jet topologies. By analyzing proton-proton collision data, ATLAS employs signature-based approaches that are sensitive to a broad range of BSM scenarios, integrating model-dependent interpretations with model-agnostic techniques using effective field theory operators to explore deviations from Standard Model expectations. Initial Run 3 data at 13.6 TeV enhance sensitivities to these signatures.^[62]^[60] In supersymmetry searches, ATLAS focuses on the production of strongly interacting superpartners such as squarks and gluinos, which are expected to decay into final states with significant MET arising from the lightest supersymmetric particle, often a stable neutralino. These investigations utilize simplified models that parameterize the masses and decay modes of superpartners, allowing for targeted exclusions in parameter spaces where squarks or gluinos cascade to lighter particles plus MET. Complementary searches target electroweak production of sleptons, charginos, and neutralinos in multi-lepton channels, enhancing sensitivity to compressed mass spectra. Standard Model backgrounds, such as those from W/Z boson decays and QCD multijet events, are estimated using data-driven methods to isolate potential BSM signals.^[63]^[64] Searches for extra dimensions, particularly in Randall-Sundrum models, aim to detect the production of Kaluza-Klein gravitons through resonances in di-lepton, di-photon, or jet+MET final states, where the warped geometry of extra dimensions could explain the weakness of gravity. ATLAS probes these signatures by reconstructing high-mass invariant systems and applying cuts on angular distributions to distinguish them from Standard Model processes like Drell-Yan or diboson production. For dark matter candidates, mono-jet events with large MET provide a key channel, where dark matter particles are produced in association with a quark-initiated jet, evading detection and manifesting as an imbalance in transverse momentum; these analyses often interpret results in terms of simplified dark matter models or effective operators coupling dark matter to quarks via mediators.^[65] Additional BSM probes include leptoquarks, hypothetical particles mediating quark-lepton interactions, searched for in final states like single or pair-produced leptoquarks decaying to a lepton and jet, with sensitivities to scalar or vector types across generations. ATLAS also investigates axion-like particles (ALPs), light pseudoscalars that could couple to photons or gluons, via channels such as light-by-light scattering in heavy-ion collisions or ALP production in Higgs decays to diphotons. In the flavor sector, measurements of CP violation in B meson decays, such as time-dependent asymmetries in B^0 → J/ψ K_S, test for deviations beyond Standard Model predictions, potentially indicating new sources of CP violation from BSM contributions to mixing or decay amplitudes. These diverse searches employ statistical methods like the CL_s approach to set limits at 95% confidence level, combining signal regions with control samples to quantify excesses over background-only hypotheses. Model-agnostic strategies further allow ATLAS to parameterize BSM effects using dimension-6 effective operators, providing inclusive constraints on new physics scales without assuming specific particle spectra.^[66]^[67]^[68]

The ATLAS Detector

Design Principles

The ATLAS detector is engineered as a general-purpose instrument for proton-proton collisions at the Large Hadron Collider (LHC), optimized to probe a wide spectrum of physics phenomena, from Standard Model precision measurements to searches for new particles and interactions.^[69] Its design emphasizes comprehensive solid-angle coverage, achieving |\eta| < 2.5 for charged-particle tracking in the central region, extending to |\eta| < 3.2 for electromagnetic calorimetry and |\eta| < 4.9 for forward hadronic calorimetry, alongside full $2\pi azimuthal coverage to ensure nearly hermetic detection of particles across all directions.^[69] This hermeticity is crucial for reconstructing missing transverse energy (MET), enabling sensitive analyses of processes involving neutrinos or hypothetical invisible particles.^[69] The detector adopts a layered, cylindrical architecture with forward-backward symmetry, comprising three primary concentric systems: the inner detector for precise tracking of charged particles, the calorimeter system for energy measurements of electrons, photons, and hadrons, and the outer muon spectrometer for identifying and momentum-analyzing muons.^[69] This modular structure facilitates particle identification through successive interactions—tracks originate in the inner volume, energy deposits occur in the calorimeters, and penetrating muons reach the outermost layers—allowing robust event reconstruction in complex collision environments.^[69] Key design requirements address the LHC's high-luminosity challenges, including high spatial and temporal granularity to reject overlapping pile-up events (up to 200 simultaneous interactions per bunch crossing) and radiation hardness tolerant to integrated fluences of $10^{15} \, n_\mathrm{eq}/\mathrm{cm}^2 over the experiment's lifetime.^[69] ATLAS employs a cylindrical coordinate system aligned with the LHC beam axis, where pseudorapidity \eta = -\ln\left(\tan(\theta/2)\right) quantifies the polar angle \theta from the beam direction, providing a rapidity-like measure invariant under Lorentz boosts along the beam; transverse momentum p_T is defined in the plane perpendicular to the beam.^[69] Physically, the detector spans 46 meters in length and 25 meters in diameter, with a total mass of approximately 7,000 tonnes and a power consumption of around 1 MW to operate its vast array of sensors and electronics.^[69] Detector response and performance are validated through detailed simulations based on the GEANT4 toolkit, which models particle interactions within the full geometry to predict signatures and optimize reconstruction algorithms.^[69]

Inner Detector

The Inner Detector (ID) of the ATLAS experiment is the innermost component of the detector system, designed to reconstruct the trajectories of charged particles produced in proton-proton collisions at the Large Hadron Collider (LHC). Operating within a 2 tesla solenoidal magnetic field, it provides precise tracking and vertexing capabilities essential for identifying decay vertices, measuring transverse momentum, and enabling b-tagging for heavy-flavor identification. The ID consists of three primary subsystems: the Pixel Detector, the Semiconductor Tracker (SCT), and the Transition Radiation Tracker (TRT), which together offer high granularity and robustness against the high radiation environment of the LHC.^[70] The Pixel Detector is positioned closest to the interaction point, starting at a radial distance of approximately 3.3 cm from the beam axis. It comprises four barrel layers and three disks in each endcap, totaling around 1,736 sensor modules with 92 million readout channels and a silicon sensing area of about 1.9 m². Each pixel measures 50 × 250–400 μm², providing a spatial resolution of roughly 10 μm, which is crucial for reconstructing primary and secondary vertices with high precision. This subsystem excels in b-tagging by identifying displaced vertices from b-hadron decays, leveraging its fine granularity to resolve tracks separated by as little as 100 μm.^[70]^[71] Surrounding the Pixel Detector is the SCT, which uses silicon microstrip sensors arranged in four barrel layers and nine disks per endcap (18 disks total). It features 4,088 double-sided modules with 6 million readout channels and a total silicon area of 60 m², where strips are spaced at 80 μm on each side, oriented at ±40 mrad stereo angles for 3D position measurement. The SCT delivers a resolution of about 17 μm in the radial direction and 580 μm along the beam axis (from stereo strips), contributing significantly to track momentum measurement by providing up to eight measurement points per track.^[72]^[73] The outermost subsystem, the TRT, consists of approximately 300,000 straw tubes filled with a xenon-based gas mixture, with 50,640 long (144 cm) straws in the barrel and shorter (37 cm) straws in the endcaps, covering a volume of 12 m³. Each 4 mm diameter straw contains a 30 μm gold-plated tungsten sense wire, providing up to 36 hits per track and a single-hit resolution of 170 μm. Beyond tracking, the TRT enables electron-pion discrimination through transition radiation detection and energy loss measurements (dE/dx), achieving particle identification efficiency above 90% for electrons with transverse momentum above 2 GeV.^[70]^[74] The entire Inner Detector provides azimuthal coverage over 2π and pseudorapidity coverage up to |η| < 2.5, ensuring comprehensive reconstruction of charged particles from LHC collisions. Its transverse momentum resolution is given by

\frac{\sigma(p_T)}{p_T} \approx 0.05\% \cdot p_T \oplus 1.0\%,

where p_T is in GeV, achieving better than 1% resolution for tracks with p_T > 20 GeV; this performance relies on the combined hits from all subsystems in the magnetic field. To minimize multiple scattering, the ID employs low-mass materials, with the Pixel Detector contributing about 0.14 radiation lengths (X_0) and the total ID material budget kept below 0.4 X_0 in the central region.^[71]^[75]^[76] Readout for the Pixel and SCT subsystems uses binary hit registration via hybrid silicon sensors connected through bump bonding, which enhances radiation tolerance by separating sensitive silicon from radiation-hardened readout ASICs (e.g., FE-I3 for pixels and ABCD3T for SCT). The TRT employs analog readout via time-over-threshold measurements in straws for precise dE/dx estimation. Overall, the ID achieves transverse impact parameter resolution better than 10 μm for high-momentum tracks and primary vertex resolution of ~10–20 μm, enabling efficient reconstruction of up to 2000 tracks per event with efficiencies exceeding 99% for isolated tracks. Radiation damage mitigation includes cooling to -7°C for SCT endcaps and defect monitoring via bump-bonded structures.^[77]^[78]^[79]

Calorimeters

The ATLAS calorimeters are designed to measure the energy of particles produced in proton-proton collisions, providing essential data for identifying electrons, photons, hadrons, and jets while compensating for the non-compensating response to electromagnetic and hadronic showers. These systems surround the inner detector and extend to high pseudorapidity regions, absorbing most of the energy from particles originating at the interaction point. The electromagnetic calorimeter (EMC) uses liquid argon (LAr) as the active medium in an accordion geometry to ensure uniform and gapless coverage, while the hadronic calorimeter (HCAL) combines a central tile scintillator barrel with LAr endcaps for robust hadron energy measurement. The forward calorimeter (FCal) extends coverage to very high rapidities using LAr with dense absorbers.^[80]^[81] The electromagnetic calorimeter consists of a barrel covering |η| < 1.475 and endcaps extending to |η| < 3.2, employing lead absorbers interleaved with LAr gaps of 2 mm in the barrel and varying thicknesses in the endcaps to achieve a total radiation length of at least 24 X₀. Its accordion structure, formed by folded lead plates and copper readout electrodes, eliminates azimuthal cracks and provides fast signal collection via capacitive charge readout. Granularity is finely tuned for precise shower shape analysis: the presampler and first sampling layer offer Δη × Δφ = 0.025 × 0.1 and 0.003 × 0.1, respectively, while the middle layer uses 0.025 × 0.025 cells for optimal electron/photon separation, and the back layer has 0.05 × 0.025; trigger towers aggregate to 0.1 × 0.1. The energy resolution is given by

\sigma_E / E \approx 10\% / \sqrt{E} \oplus 0.7\%

where E is in GeV, achieving high precision for Higgs boson decays to photons, for example. This sampling fraction captures electromagnetic showers efficiently, with the LAr operated at -184°C to maintain purity and stability.^[80] The hadronic calorimeter measures jet energies and missing transverse energy, featuring a steel-tile barrel for |η| < 1.7 (with 64 wedge-shaped modules containing 420,000 plastic scintillator tiles read out by wavelength-shifting fibers to photomultiplier tubes) and LAr endcaps (HEC) for 1.5 < |η| < 3.2 using copper absorbers for compensation of the e/h response ratio. The tile barrel, the heaviest component at 2,900 tonnes, provides non-compensating sampling with three longitudinal layers, while the HEC adds depth for better containment. Overall HCAL coverage reaches |η| < 4.9 when including the FCal, with granularity of Δη × Δφ ≈ 0.1 × 0.1 in the central regions (coarser to 0.2 × 0.1 in outer layers). The energy resolution is parameterized as

\sigma_E / E \approx 50\% / \sqrt{E} \oplus 3\%

(E in GeV), sufficient for top quark studies and electroweak precision measurements, though it degrades slightly in the endcaps due to increased sampling fluctuations.^[80]^[81] The forward liquid argon calorimeter (FCal) covers 3.2 < |η| < 4.9 with three modules of copper (EM section) and tungsten (hadronic sections) absorbers in a matrix of LAr-filled tubes, providing about 10 interaction lengths for containment despite the high radiation environment. Its rod-and-tube design offers Δη × Δφ ≈ 0.2 × 0.2 granularity in hexagonal cells, optimized for forward jet reconstruction and diffraction physics. Jets are reconstructed from calorimeter deposits using the anti-k_t clustering algorithm with radius R = 0.4, seeded by tracks from the inner detector for charged particle subtraction, and calibrated to account for pile-up contamination via area-based methods. This forward coverage ensures comprehensive event characterization, including for beyond-Standard-Model searches involving high-rapidity particles.^[80]^[80]

Muon Spectrometer

The ATLAS Muon Spectrometer is the outermost component of the detector, designed to identify and precisely measure the momentum of muons that penetrate the inner detector and calorimeters. It operates within the air-core toroid superconducting magnet system, which provides a bending field for track curvature measurement. The spectrometer covers the pseudorapidity range |η| < 2.7, enabling high-efficiency muon detection across a wide angular acceptance. Standalone momentum resolution is approximately σ(p_T)/p_T ~ 10% at 1 TeV, achieved through multi-layer tracking chambers.^[82]^[83] The spectrometer's structure comprises precision tracking chambers and dedicated trigger detectors. Monitored Drift Tubes (MDTs) form the primary precision tracking system, consisting of over 1,000 multilayer chambers with tubes of 30 mm diameter and lengths up to 6.5 m, providing position resolution better than 80 μm per tube. In the forward regions (2.0 < |η| < 2.7), Cathode Strip Chambers (CSCs) supplement the MDTs for high-rate environments, offering two-dimensional readout with strip cathodes for η-coordinate measurement. Resistive Plate Chambers (RPCs) in the barrel (|η| < 1.05) serve as the trigger system, featuring parallel-plate detectors with 2 mm gas gaps operated at high voltage for fast response and coarse φ-coordinate determination.^[82]^[83] For triggering, the RPCs and complementary Thin Gap Chambers (TGCs) in the endcaps enable fast readout with a latency of about 1 μs, allowing Level-1 muon trigger decisions within the 2.5 μs bunch-crossing window. This low-latency system selects candidate muons based on track segments, rejecting most background particles. Alignment of the chambers is maintained by an optical system using laser beams, alignment bars, and sensors to monitor relative positions with 30 μm precision over baselines up to 100 m, ensuring long-term stability against mechanical deformations.^[82]^[84]^[83] Background rejection is enhanced through η-strip coincidence logic in the trigger chambers, which requires matching hits in η-oriented strips across layers to suppress fake triggers from cavern background or decays, achieving rejection factors that maintain high purity at luminosities up to 10^34 cm^{-2}s^{-1}. This approach, particularly refined in forward upgrades, reduces random coincidences while preserving efficiency for genuine muons.^[85]^[83]

Magnet and Forward Systems

The ATLAS solenoid magnet generates a uniform axial magnetic field of 2 tesla within the inner detector volume, enabling precise momentum measurements of charged particles by bending their trajectories.^[86] This superconducting magnet, constructed from niobium-titanium (NbTi) coils stabilized in aluminum, measures 5.8 meters in length and 2.56 meters in outer diameter, with a total weight of approximately 5 tonnes.^[86] The design minimizes material to preserve tracking efficiency, while the field uniformity is maintained below 0.1% variation across the sensitive volume to ensure consistent particle bending.^[87] The toroid magnet system provides azimuthal bending fields for muon momentum reconstruction, consisting of a barrel toroid with 8 coils and two endcap toroids each with 8 coils, totaling 24 superconducting units.^[86] These NbTi-based coils operate at fields up to 4 tesla on the superconductors, producing an effective bending field of 0.5 to 1 tesla for muons traversing the spectrometer.^[88] The entire system stores approximately 1.6 gigajoules of energy, necessitating advanced quench protection mechanisms, including fast dump resistors and energy extraction systems, to safely dissipate magnetic energy during superconducting transitions.^[87] Field uniformity in the toroid is optimized to less than 0.1% to support accurate muon tracking, with the barrel toroid spanning 25.3 meters in length and 20.1 meters in diameter.^[87] The forward detector systems in ATLAS monitor luminosity and probe diffractive processes at very high rapidities. The LUCID detector, positioned at approximately 17 meters from the interaction point, uses Cherenkov light from quartz rods to detect charged particles, achieving a detection efficiency of about 99% for inelastic proton-proton collisions and serving as the primary luminometer.^[89] The Zero Degree Calorimeter (ZDC), located at 140 meters along the beamline, captures neutral particles like spectator neutrons in heavy-ion collisions at pseudorapidities beyond 8.5, providing centrality and reaction plane information.^[90] Complementing these, the ATLAS Forward Proton (AFP) detectors, installed in Roman pots at 205 and 217 meters, tag intact diffractive protons with silicon trackers and time-of-flight systems to study central diffractive events.^[91] Luminosity in ATLAS is measured using LUCID via bunch-integrated event counting, where the instantaneous luminosity L is determined from the formula L = \frac{N_{\text{coll}}}{\sigma f}, with N_{\text{coll}} as the number of collisions per bunch crossing, \sigma the inelastic cross section (approximately 80 millibarns at 13 TeV), and f the bunch revolution frequency.^[92] This method, calibrated through van der Meer beam scans, achieves uncertainties below 2.5% per data-taking period, ensuring reliable normalization for physics analyses. The toroid fields briefly support muon identification in forward regions, but primary forward monitoring relies on these specialized detectors.^[86]

Trigger and Data Acquisition

The ATLAS Trigger and Data Acquisition (TDAQ) system selects a small fraction of the approximately 1 billion proton-proton collisions occurring every second at the LHC during Run 3, reducing the data volume from a potential full readout of over 100 TB/s to a sustainable rate for storage and analysis.^[93]^[94] This real-time selection ensures that events with potentially interesting physics signatures, such as those involving high-energy particles, are prioritized while discarding the majority of uninformative minimum-bias interactions. The system integrates hardware and software components to achieve this filtering with minimal latency, relying on inputs from the calorimeter and muon spectrometer subsystems.^[95] The trigger operates in two main stages: the Level-1 (L1) hardware trigger and the High-Level Trigger (HLT). The L1 trigger processes coarse-grained data from the calorimeters and muon spectrometer using custom electronics, making a decision within 2.5 μs to accept events containing objects like electrons, photons, jets, taus, muons, or significant missing transverse energy above predefined thresholds.^[94] This stage reduces the 40 MHz bunch crossing rate to up to 100 kHz, defining regions of interest (ROIs) for further scrutiny. Trigger algorithms focus on single- or multi-object signatures, such as isolated muons with transverse momentum p_T > 20 GeV or electron-photon pairs, to capture potential signals from processes like electroweak boson decays.^[94] As part of the Phase-1 upgrade, field-programmable gate arrays (FPGAs) were implemented in the L1 calorimeter trigger to handle increased input rates and improved object identification precision.^[96] The HLT stage employs software algorithms running on approximately 50,000 CPU cores to perform detailed reconstruction within the L1-defined ROIs and, for high-priority events, a full-event analysis similar to offline processing.^[94] This reduces the rate further to an average of 1 kHz (with peaks up to 3 kHz during Run 3), using refined criteria that incorporate particle identification and kinematic refinements, such as machine learning-based improvements in tau energy resolution via boosted decision trees.^[94] The HLT decision time is typically 200–400 ms per event, balancing computational resources with the need for rapid filtering.^[94] Data flow through the TDAQ begins with L1-accepted events triggering the readout of relevant subdetector fragments, aggregating to an input bandwidth of about 0.2 TB/s for the DAQ system during Run 3.^[97] After HLT selection, the output rate is approximately 1.5 GB/s of raw event data, which is buffered and streamed to the CERN Tier-0 facility via a high-bandwidth network.^[98] From Tier-0, data is promptly replicated to multiple Tier-1 centers worldwide and further distributed to Tier-2 sites through the Worldwide LHC Computing Grid (WLCG), enabling global access for processing.^[99] Offline reconstruction and analysis utilize the Athena framework, a modular software environment that performs full event processing, including track finding, calorimeter clustering, and particle identification, with reconstruction efficiencies exceeding 97% for electrons in the central rapidity region.^[100]^[101] Athena integrates simulation, reconstruction, and analysis tools, ensuring consistency between online HLT selections and offline workflows. The resulting raw data volume totals over 10 PB per year, stored durably across the WLCG infrastructure to support long-term physics studies.^[102]

Key Results and Discoveries

Higgs Boson Research

The ATLAS experiment contributed decisively to the discovery of the Higgs boson on July 4, 2012, through the analysis of proton-proton collision data collected at center-of-mass energies of 7 TeV and 8 TeV. The observation was established in the diphoton (H → γγ) and four-lepton (H → ZZ*) decay channels, yielding a local significance of 5.9σ in the mass region around 126 GeV, consistent with Standard Model expectations.^[103] Subsequent analyses refined this mass measurement to 125.11 ± 0.11 GeV using the full Run 2 dataset.^[104] Post-discovery, ATLAS has measured the Higgs boson's properties with increasing precision, confirming its consistency with the Standard Model scalar particle. The spin and parity were determined to be 0^{++} through analyses of angular distributions in H → ZZ* and H → γγ decays, showing no evidence for alternative spin-2 or non-zero parity hypotheses at high confidence levels. Charge-parity (CP) properties have also been probed, with measurements of CP-odd components in H → VV (V = W, Z) decays revealing no deviations from the Standard Model CP-even prediction, with limits on the CP-mixing parameter \tilde{\kappa}_V below 0.25 at 68% confidence level using Run 2 data extended into Run 3.^[105] Higgs boson production and decay have been characterized across dominant modes, with gluon fusion (ggF) accounting for approximately 90% of the cross-section at the LHC, while vector boson fusion (VBF) and vector boson associated production (VH) provide tagged signatures for enhanced sensitivity. The signal strength μ, defined as

\mu = \frac{\sigma_\text{obs}}{\sigma_\text{SM}}

where σ_obs is the observed production cross-section times branching ratio and σ_SM is the Standard Model prediction, has been measured to be μ = 1.06^{+0.06}{-0.05} overall, with ggF-tagged μ ≈ 1.10 and VBF/VH-tagged μ ≈ 0.90, demonstrating compatibility within uncertainties. Couplings to vector bosons (λ{HVV}) align with Standard Model values to about 10% precision in combined fits, κ_V = 1.01^{+0.08}_{-0.07}.^[106] The trilinear Higgs self-coupling λ_{HHH} has been constrained using vector boson associated production channels VH → VV in Run 3 data up to 2025, with limits of -1.6 < λ_{HHH}/λ_{SM} < 6.6 at 95% confidence level from analyses sensitive to off-shell contributions and di-Higgs final states.^[107] Rare decay modes have been explored, including the 2024 evidence for H → μμ with 3.4σ significance and a branching ratio of (0.30 ± 0.09) × 10^{-3}, consistent with the Standard Model prediction of 0.22 × 10^{-3}, achieved through dimuon invariant mass searches in ggF-enriched categories.^[108] These results underscore ATLAS's role in validating the Higgs sector while probing for subtle deviations.

Top Quark and Electroweak Studies

The ATLAS experiment has conducted precise measurements of the top quark mass using direct reconstruction techniques in top-antitop (tt̄) pair production events, yielding a value of m_t = 172.56 ± 0.31 GeV (PDG 2024 average including ATLAS results). This determination relies on kinematic reconstructions in leptonic and hadronic decay channels, incorporating advanced modeling of jet energy scales and b-tagging to minimize systematic uncertainties from detector resolution and parton shower effects. Complementing this, the inclusive tt̄ production cross-section at √s = 13.6 TeV has been measured as σ = 850 ± 21 pb, consistent with next-to-next-to-leading-order (NNLO) quantum chromodynamics predictions.^[109] The theoretical framework for tt̄ production at leading order scales as

\sigma_{tt} = \frac{\alpha_s^2}{m_t^2} \times \text{(phase space factors)},

with NNLO corrections enhancing accuracy by including higher-order strong coupling (α_s) terms and resummation of soft gluon emissions, achieving agreement within 5% of experimental values.^[109] In single top quark production, ATLAS has focused on the dominant t-channel process, measuring the production rate to probe the Cabibbo-Kobayashi-Maskawa matrix element V_tb in the Wtb vertex.^[110] The observed cross-section aligns with Standard Model expectations, yielding |V_tb| ≈ 1.0 with uncertainties below 5%, indicating no significant deviations in the left-handed coupling strength.^[110] These analyses employ multivariate discriminants to isolate signal from background, leveraging the distinctive single b-jet signature and forward light-quark jet in t-channel events. Electroweak precision studies by ATLAS include an updated measurement of the W boson mass from 2023, m_W = 80.360 ± 0.016 GeV, derived from transverse mass distributions in W → eν and W → μν decays using a reanalysis of 7 TeV data.^[111] This result refines earlier determinations by incorporating improved lepton momentum calibrations and electroweak corrections, providing a stringent test of the Standard Model's gauge sector. For the Z boson, the invisible width has been measured as 501 ± 11 MeV using 37 fb^{-1} of 13 TeV data, consistent with three light neutrino species and limits on additional invisible decays.^[112] Searches for anomalous triple gauge couplings in diboson processes, such as WZ and ZZ production, have set limits on parameters like κ_γZ, with observed deviations constrained to |Δκ_γZ| < 0.10 at 95% confidence level using angular distributions and invariant masses.^[113] These bounds arise from effective field theory interpretations, assuming linear realizations of new physics operators. Supporting these precision efforts, ATLAS employs the DL1r deep learning-based flavor tagging algorithm, achieving b-jet identification efficiencies of 70-80% across transverse momentum ranges up to 1 TeV, with 2025 updates incorporating recurrent neural networks for enhanced track and vertex resolution in high-pileup environments.^[114]

New Physics Constraints

The ATLAS experiment has utilized data from LHC Run 3 (up to an integrated luminosity of approximately 100 fb⁻¹ by mid-2025 at √s = 13.6 TeV) to set stringent limits on various beyond-Standard-Model (BSM) scenarios, focusing on null results from dedicated searches for new phenomena. These constraints probe supersymmetry (SUSY), extra dimensions, dark matter candidates, microscopic black holes, CP violation in beauty meson decays, and effective field theory (EFT) operators, often employing advanced machine learning techniques for signal reconstruction and background rejection. No evidence for BSM physics has been observed, pushing parameter spaces to higher scales and informing global interpretations of LHC data.^[115] In SUSY searches, ATLAS excludes gluino masses above 2.25 TeV in simplified models where gluinos decay to quarks and the lightest neutralino (assumed stable), based on analyses targeting events with jets, missing transverse energy (E_T^miss), and τ-leptons using combined Run 2 and partial Run 3 datasets. For compressed spectra, stop (top-squark) searches limit stop masses up to 1.23 TeV assuming a massless neutralino, with sensitivities extending to 600 GeV in scenarios where the stop-neutralino mass difference is small (~10-20 GeV), exploiting final states like top-antitop pairs plus E_T^miss. These results represent improvements over Run 2 limits by incorporating higher center-of-mass energy and refined reconstruction algorithms.^[116] Searches for extra dimensions in the Randall-Sundrum model constrain the mass of the first Kaluza-Klein (KK) graviton excitation to above 6 TeV, derived from analyses of diphoton, dilepton, and diboson final states with high invariant masses, using Run 2 data extended with early Run 3 contributions. These limits arise from the absence of resonant enhancements in high-mass spectra, with sensitivities enhanced by improved jet and photon calibration in the higher-luminosity environment.^[117] For dark matter, ATLAS sets upper limits on spin-dependent weakly interacting massive particle (WIMP)-nucleon cross-sections below 10^{-47} cm² for mediators around 100 GeV, from mono-jet and vector-boson-plus-missing-energy searches incorporating Run 3 data up to 2023. These constraints complement direct detection experiments by probing axial-vector mediators in quark-initiated processes, with no excess observed beyond Standard Model backgrounds like Z + jets.^[118] No evidence for microscopic black holes has been found in multi-jet events with or without leptons, leading to model-independent limits on the fundamental Planck scale M_* exceeding 10 TeV in scenarios with large extra dimensions, based on Run 2 analyses projected to Run 3 with increased luminosity. These searches target semi-classical black hole production and rapid evaporation, setting bounds via shape analyses of event kinematics.^[115] In the realm of flavor physics, ATLAS constrains CP violation through measurements of the asymmetry ΔA_CP in B_s → J/ψ φ decays to below 0.1, using time-dependent analyses of Run 2 data with prospects for Run 3 enhancements in beauty tagging efficiency. This limit tests new physics contributions to B_s mixing, consistent with Standard Model expectations within uncertainties.^[119] Global fits to dimension-6 EFT operators from combined ATLAS electroweak, Higgs, and top-quark measurements exclude coefficients like C_{ll} (lepton-lepton contact interactions) below 0.1 TeV^{-2} at 95% confidence level, incorporating Run 3 data for improved precision on anomalous triple-gauge couplings and Yukawa modifications. These interpretations use principal component analyses to reduce operator degeneracies, highlighting sensitivities to new physics scales around 10 TeV.^[120] By 2025, improvements in b-jet triggers, including the deployment of the transformer-based GN3 algorithm with neutral particle flow integration and looser track selections, have enhanced flavor-sensitive BSM searches by up to 40% in efficiency for processes like Higgs-to-bottom pairs or supersymmetric squark production. New triggers for b + τ combinations further boost acceptance in multi-flavor final states, enabling deeper probes into flavor-violating new physics.^[121]

Upgrades and Future Outlook

Long Shutdown Modifications

During the first Long Shutdown (LS1) from 2013 to 2015, the ATLAS experiment implemented key upgrades to enhance its tracking capabilities and prepare for higher luminosity operations. A major addition was the Insertable B-Layer (IBL), a new innermost pixel layer installed inside the existing Pixel Detector to maintain b-tagging performance and vertex resolution amid increasing radiation damage and pile-up effects expected in subsequent LHC runs.^[122]^[123] The IBL utilized novel 3D pixel sensors and FE-I4 readout chips, marking the first ATLAS application of CO2 evaporative cooling for improved thermal management in a high-radiation environment.^[124] Additionally, consolidation efforts included refreshing the electronics of the Monitored Drift Tube (MDT) chambers in the Muon Spectrometer to improve trigger reliability and reduce background noise from beam-induced events.^[125] These modifications also encompassed a new cooling system for the Inner Detector to support the upgraded pixel components.^[125] The second Long Shutdown (LS2) from 2019 to 2022 focused on Phase-I upgrades to the trigger and muon systems, enabling ATLAS to handle the increased collision rates of LHC Run 3 starting in 2022. The Level-1 (L1) calorimeter trigger was enhanced with new electronics providing finer granularity for electron, photon, and tau identification, allowing topological clustering and improved rejection of pile-up jets.^[126] In the muon sector, the forward region saw the installation of the New Small Wheels (NSW), replacing the innermost Small Wheel stations with Micromegas (MM) detectors for precision tracking and small-strip Thin Gap Chambers (sTGC) for fast triggering, achieving a position resolution of about 100 μm and timing precision under 25 ns.^[127]^[128] Complementary to this, Gas Electron Multiplier (GEM) detectors were integrated into the endcap muon stations to suppress fake triggers from background muons and low-pT particles, particularly in the 1.5 < |η| < 2.5 region.^[129] These upgrades collectively reduced muon fake trigger rates by approximately 50% while maintaining high efficiency for physics signals.^[130] Computing infrastructure upgrades during LS1 and LS2 shifted ATLAS toward more scalable data management and processing. The experiment transitioned to the EOS distributed storage system for handling petabyte-scale datasets, with dedicated instances at CERN and Tier-1 sites optimizing access patterns for reconstruction and analysis workflows.^[131] In parallel, advancements in AI-based algorithms were integrated into event reconstruction pipelines, leveraging machine learning for faster jet and track finding to cope with higher event rates post-shutdown.^[132] The Simulation at Point 1 (Sim@P1) facility was also upgraded during LS2 to utilize the enhanced High-Level Trigger hardware for Monte Carlo simulations, improving throughput for Run 3 preparations.^[133] To address radiation-induced degradation observed after Run 2, LS2 included targeted replacements of damaged sensors in the Inner Detector, particularly in the pixel layers closest to the interaction point, where fluences exceeded 10^15 n_eq/cm². These replacements involved swapping out modules with radiation-hardened spares, restoring full operational efficiency and extending the detector's lifespan without full-scale redesign.^[134] The Phase-I upgrades, encompassing the L1 trigger and muon enhancements completed in LS2, enabled an increase in the L1 trigger acceptance rate to 300 kHz while preserving data quality under luminosities up to 2 × 10^34 cm^{-2} s^{-1}.^[135]

High-Luminosity LHC Preparations

The High-Luminosity Large Hadron Collider (HL-LHC), planned to commence operations in 2030 following Long Shutdown 3 (LS3) from July 2026 to mid-2029, will deliver proton-proton collisions at a center-of-mass energy of 14 TeV and an instantaneous luminosity reaching a baseline of $5 \times 10^{34} \ \mathrm{cm}^{-2} \ \mathrm{s}^{-1}, with the goal of accumulating an integrated luminosity of 3000 \mathrm{fb}^{-1} by approximately 2040.^[136]^[137] This upgrade, enabled by the recent extension of LHC Run 3 to end June 2026, will dramatically increase the number of simultaneous collisions per bunch crossing, up to 140 on average, posing severe challenges from radiation damage and pile-up. To fully exploit these conditions, the ATLAS experiment is implementing Phase-II upgrades, focusing on enhanced tracking precision, timing capabilities, calorimetry readout, and triggering efficiency to maintain high physics reach while handling rates exceeding current capabilities.^[138] Central to these preparations is the Inner Tracker (ITk), which will replace the existing Inner Detector with an all-silicon system comprising a five-layer pixel detector in the barrel and endcaps, complemented by a four-layer microstrip detector, providing at least nine measurement points per track across |\eta| < 4.^[139]^[140] The ITk modules incorporate radiation-hardened sensors and electronics, designed to endure fluences up to $2 \times 10^{16} \ n_{\mathrm{eq}} / \mathrm{cm}^2 at the innermost layers, ensuring robust performance over the HL-LHC lifetime.^[141] By 2025, substantial R&D advancements have been achieved, particularly on the strip detector, with full-size prototypes demonstrating efficient assembly, low noise operation, and integration with the serial powering scheme to manage power distribution for over 10,000 modules.^[142]^[143] Pile-up mitigation is further addressed by the High-Granularity Timing Detector (HGTD), positioned between the ITk and electromagnetic calorimeter in the forward regions ($2.4 < |\eta| < 4.0). This system employs Low-Gain Avalanche Diode (LGAD) sensors in a double-sided configuration, delivering a per-track timing resolution of 30 ps at the start of operations, degrading to 50 ps after full radiation exposure.^[144]^[145] The HGTD's high granularity—over 3,000 sensors per side—enables association of tracks to the correct vertex, reducing combinatorial backgrounds in dense events. The Liquid Argon (LAr) calorimeter upgrade for Phase-II introduces new front-end electronics and off-detector processing, allowing full-granularity readout of all cells without presumming, which provides finer spatial and energy resolution for triggering and reconstruction under high pile-up.^[146] This enables the calorimeter to contribute detailed topological information to the trigger, improving electron, photon, and jet identification. Complementing these, the trigger and data acquisition system will feature a hardware-based Level-0 trigger accepting events at 1 MHz latency, followed by software processing on a large computing farm to select the final dataset at ~10 kHz.^[138]^[147] These enhancements collectively ensure ATLAS's sensitivity to rare processes.^[142]

References

[1]
ATLAS
### Key Facts About ATLAS
[2]
About - ATLAS Experiment
ATLAS is a general-purpose particle physics experiment at the Large Hadron Collider (LHC) at CERN. It is designed to exploit the full discovery potential of ...
[3]
The Higgs boson - CERN
On 4 July 2012, the ATLAS and CMS collaborations announced the discovery of a new particle to a packed auditorium at CERN. This particle had no electrical ...What's so special about the... · How does the Higgs boson...
[4]
The Higgs boson: a landmark discovery - ATLAS Experiment
On 4 July 2012, the ATLAS and CMS experiments at CERN announced that they had independently observed a new particle in the mass region of around 125 GeV: a ...
[5]
ATLAS Experiment at CERN | ATLAS Experiment at CERN
This unprecedented dataset will enable precision measurements of the Higgs boson's properties and enhance sensitivity to a wide range of new-physics scenarios.About · The Collaboration · News · The Physics
[6]
[PDF] atlas - CERN Document Server
Oct 1, 1992 · The CERN group eekuowledges the important contributions made to this Letter of Intent by: ... The ATLAS Collaboration proposes in this Letter of.
[7]
25 years of the LHC experimental programme - CERN
Dec 12, 2017 · ATLAS was formed from the merger of two proposals, ASCOT and EAGLE, whereas CMS had evolved from a single proposal presented at Evian. In 1993, ...Missing: origins | Show results with:origins
[8]
https://cds.cern.ch/record/290968
[9]
Archives of ATLAS Collaboration records - CERN Library
... CERN Annual Reports from 1994 onwards. 1992. The ATLAS collaboration writes a Letter of Intent (CERN/LHCC/92-4) in which the building of a general purpose ...
[10]
1998 - LHC Milestones - CERN
1998: APRIL Civil engineering work begins for the ATLAS experiment. Projects include: the excavation of two shafts, the digging of two caverns.
[11]
[PDF] Status of ATLAS
Jan 12, 2009 · The excavation and completion of the underground cavern took five years, from 1998 to. 2003. In summer 2003 began the installation of the ...<|separator|>
[12]
ATLAS On the Move - CERN Document Server
Nov 11, 2004 · On 26 October, cranes lowered the first of ATLAS's eight Barrel Toroid coils into the cavern. Because it's so long, 25 metres, the cranes had to ...
[13]
All events | timeline.web.cern.ch
ATLAS records first collisions at 0.9 TeV. Timelines. The ATLAS experiment ... End of ATLAS construction. 04 October 2008. To celebrate the end of the ATLAS ...
[14]
[PDF] ATLAS Superconducting Toroids and Solenoid
Oct 5, 2004 · ATLAS optimized the configuration and decided to construct a hybrid system of 4 superconducting magnets: a Central. Solenoid (CS) providing 2 T ...
[15]
[PDF] The US and the ATLAS Experiment
Cost of US construction project: $164 million. Construction contributions: Leading role in design, construction and testing for charged particle tracking ...
[16]
https://www.worldscientific.com/doi/pdf/10.1142/9789813271807_0003
[17]
[DOC] Draft 1 - ATLAS Canada
No single country could afford to build the $8 billion LHC project on its own. ATLAS has been built by researchers from more than 150 universities and ...<|separator|>
[18]
[PDF] Commissioning of the ATLAS Experiment
Dec 22, 2008 · Much of the 2.5% of channels lost are due to cooling leaks and heater problems in the end-cap and can be recovered during the 2008-9 winter ...Missing: challenges | Show results with:challenges
[19]
[PDF] ATLAS RPC commissioning status and cosmic ray test results
Nov 2, 2008 · We present the status of ATLAS RPC detector, the problems met during the commissioning ... Since spring 2008, several high rate tests were ...Missing: challenges | Show results with:challenges
[20]
First collisions in ATLAS - ATLAS Experiment at CERN
Nov 23, 2009 · The very first collisions at 0.9 TeV total energy have been observed in ATLAS, greeted by a less explosive joy but by a deeper emotion.<|control11|><|separator|>
[21]
[PDF] Recent results from the ATLAS experiment
Dec 17, 2021 · – Integrated luminosity during Run1 (2010-2012): ~25fb-1 at ECM 7-8TeV. – Integrated luminosity during Run2 (2015-2018) : ~150fb-1 at ECM 13 TeV.
[22]
Long Shutdown 1: Exciting times ahead - CERN
Feb 8, 2013 · Over 10,000 high-current splices between LHC magnets will be opened and consolidated during the first Long Shutdown of the LHC. This image ...
[23]
ATLAS delivers most precise luminosity measurement at the LHC
Jan 24, 2023 · With its uncertainty of 0.83%, the result represents the most precise luminosity measurement at a hadron collider to date. It improves upon ...
[24]
2016: an exceptional year for the LHC - CERN
Dec 6, 2016 · In technical terms, the integrated luminosity received by ATLAS and CMS reached 40 inverse femtobarns (fb−1), compared with the 25fb−1 ...
[25]
[PDF] The Phase-1 Upgrade of the ATLAS Level-1 Calorimeter Trigger
Oct 22, 2025 · During the Long Shutdown 2 of the LHC, the ATLAS detector received its Phase-1 Upgrade in order to better handle the increased luminosity in ...
[26]
Run 3: an opportunity to expand the LHC physics programme - CERN
Run 3, a new period of data taking, begins in July 2022 for the experiments at the Large Hadron Collider (LHC), after more than three years of upgrade and ...
[27]
https://videos.cern.ch/record/2302300
[28]
Ensuring high-quality data at ATLAS
Nov 13, 2019 · ATLAS achieved an exceptionally high data-quality efficiency for a hadron collider, with over 95% of the 13 TeV proton–proton collision data certified for ...
[29]
The LHC experiment collaborations at CERN receive Breakthrough ...
Apr 7, 2025 · The ALICE, ATLAS, CMS and LHCb collaborations at the Large Hadron Collider at CERN were honoured with the Breakthrough Prize in Fundamental Physics.
[30]
The ATLAS experiment - Scholarpedia
Oct 14, 2014 · ATLAS submitted in October 1992 a Letter of Intent (LoI, a 100-page document) to the new CERN LHC Experiments Committee (LHCC) proposing a ...Missing: planning | Show results with:planning
[31]
ATLAS makes a smooth changeover at the top - CERN Courier
Originally “only” about 800-strong, the ATLAS collaboration today has almost 3000 members from all over the world. “Keeping the groups united, inviting new ...
[32]
The Collaboration - ATLAS Experiment
It is one of the largest collaborative efforts ever attempted in science, with approximately 6000 members and 3000 scientific authors.Join as a Technical Associate... · Join as a Clustered InstitutionMissing: current | Show results with:current
[33]
Join as an Institution - ATLAS Experiment
Full Institutional Members (Institutions) have full rights and obligations in the Collaboration. In particular, they are represented in the Collaboration ...
[34]
Join as an Associate Institute - ATLAS Experiment
The Associate Institute status is intended for universities or research labs that are on the path to joining ATLAS as a full institutional member.
[35]
Join as a Clustered Institution - atlas . CERN
Each Cluster will have one vote in the ATLAS Collaboration Board. The requirements for a new Cluster to join ATLAS are the same as for single Institutions. The ...
[36]
https://cds.cern.ch/record/2880107/files/ATL-OREACH-PUB-2023-001.pdf
[37]
U.S. ATLAS Operations Program: Empowering University Physicists ...
The ATLAS collaboration, consisting of 174 institutions from 38 countries, completed construction of the large multi-purpose ATLAS detector at the Large Hadron ...<|separator|>
[38]
ATLAS and CMS celebrate their 25th anniversaries - CERN
Oct 2, 2017 · On 1 October 1992, the two collaborations each submitted a letter of intent for the construction of a detector to be installed at the proposed ...
[39]
[PDF] Status of ATLAS Common Projects & Proposals for In-Kind ...
Apr 15, 2003 · Memorandum of Understanding, ATLAS RRB-D 98-44 rev.). Contributions to the ATLAS Common Projects are made either in kind or by cash.
[40]
9 From Russia with Love: A Contributing Country Perspective
What exactly did the Russian contribution consist of? Given the architecture of the ATLAS detector,11 a structure was needed to support the entire barrel toroid ...
[41]
Diversity & Inclusion - ATLAS Experiment
As a collaboration of over 5000 scientists, students, engineers, technicians, and administrators, ATLAS is made up of diverse members from around the globe with ...Missing: OECD percentage
[42]
[PDF] Evolution of Regional, Age and Gender Demographics in the ATLAS ...
Feb 28, 2025 · The ATLAS collaboration tracks gender, age, and geographic diversity. ... Unlike the above categories they are not picked by the ATLAS experiment.Missing: OECD | Show results with:OECD
[43]
ATLAS Early Career Scientists Board | ATLAS Experiment at CERN
Official public website for the ATLAS Experiment at CERN.Missing: inclusivity gender balance programs
[44]
Steering ATLAS forward with new management
Mar 10, 2025 · The ATLAS Collaboration embarks on a new chapter under the leadership of Spokesperson Stéphane Willocq (University of Massachusetts Amherst).Missing: organization | Show results with:organization
[45]
Steering ATLAS forward with new management - CERN
Mar 11, 2025 · Spokesperson Stéphane Willocq and Deputy Spokespersons Anna Sfyrla and Guillaume Unal will lead the collaboration for the next two years.Missing: organization | Show results with:organization
[46]
Management - ATLAS Experiment
The Spokesperson and Deputy Spokespersons oversee all aspects of the ATLAS project. The Spokesperson represents ATLAS to CERN, funding agencies, and other ...Missing: organization committees
[47]
ATLAS Management enters new term - CERN
Apr 29, 2019 · The management of the ATLAS experiment begins a new term this Spring, with Spokesperson Karl Jakobs (University of Freiburg) continuing to steer the ...
[48]
ATLAS: now under new management
Mar 2, 2021 · The ATLAS Collaboration welcomes new management this month, with Andreas Hoecker (CERN) taking the lead as Spokesperson for the experiment.Missing: list | Show results with:list
[49]
How the ATLAS Collaboration Works - Twiki - CERN
Nov 3, 2022 · It consists of Activity Areas (Data Preparation, Operations , Physics, Software & Computing, Trigger), Projects (Forward , HGTD, ID (Pixel, SCT ...
[50]
ATLAS Organisation - CERN
ATLAS has a Spokesperson (Andreas Hoecker), Deputy Spokesperson (Stephane Willocq), Technical Coordinator (Martin Aleksa), and Collaboration Board Chair (Lucia ...
[51]
Justification and Interlaced Knowledge at ATLAS, CERN - jstor
Apr 2, 2014 · emphasis on consensus in decision making. To facilitate the involvement of all interested partic- ipants in seeking solutions and arriving ...
[52]
[PDF] Memorandum of Understanding - CERN Indico
May 14, 2002 · The spokesperson brings the EoI to the CB for decision when the future ATLAS activities of the Institution are clarified. The CB is notified ...
[53]
Joining the ATLAS Collaboration
Full Institutional Membership can either be obtained by single institutes or by several smaller institutes that work together and join the ATLAS Collaboration ...
[54]
[PDF] Description of the ATLAS Organisation
Dec 16, 1996 · The Resource Review Board, RRB, is the body where the ATLAS resources are approved upon proposals from the collaboration, and monitored. It ...
[55]
Coordination for collisions | ATLAS Experiment at CERN
May 14, 2015 · When coordinating with the LHC, a "handshake" must take place between the computers in the ATLAS Control Room in Geneva and the LHC Control Room ...Missing: crisis 2012 incident
[56]
[1508.02507] Constraints on non-Standard Model Higgs boson ...
Aug 11, 2015 · No significant deviations from the Standard Model are observed and limits on the effective field theory parameters are derived. The statistical ...
[57]
New ATLAS result weighs in on the W boson
Mar 23, 2023 · The ATLAS Collaboration has measured the W boson mass to be 80360 MeV with an uncertainty of 16 MeV – in agreement with the Standard Model.
[58]
[1711.02692] Measurement of inclusive jet and dijet cross-sections ...
Nov 7, 2017 · Inclusive jet and dijet cross-sections are measured in proton-proton collisions at a centre-of-mass energy of 13 TeV.
[59]
[1305.3773] Diboson production cross section at LHC - arXiv
May 16, 2013 · This paper presents an overview of the diboson production cross-section measurements and constraints on anomalous triple-gauge boson couplings ...
[60]
[2411.09962] Precision measurement of the $B^{0}$ meson lifetime ...
Nov 15, 2024 · A measurement of the B^{0} meson lifetime and related properties using B^0 \to J/\psi K^{*0} decays in data from 13 TeV proton-proton collisions.Missing: hadron | Show results with:hadron
[61]
The quest to discover supersymmetry at the ATLAS experiment - arXiv
Mar 4, 2024 · This report summarises the supersymmetry searches at ATLAS using up to 140 fb^{-1} of pp collisions at \sqrt{s} = 13 TeV.
[62]
[2506.06839] Supersymmetry searches at ATLAS and CMS - arXiv
Jun 7, 2025 · Abstract:This document presents recent results from the ATLAS and CMS experiments at the CERN Large Hadron Collider (LHC) on supersymmetry ...
[63]
Searches for supersymmetric particles with prompt decays ... - arXiv
Jun 26, 2023 · The latest results from electroweak and strong SUSY searches are reported here, conducted by the ATLAS experiment at the CERN LHC.
[64]
[1709.10440] Search for new phenomena in high-mass final states ...
Sep 29, 2017 · ... (Randall-Sundrum) model with six (one) extra dimensions. Comments: 38 pages in total, author list starting page 22, 6 figures, 3 tables ...
[65]
[2107.10094] Searches for Leptoquarks with the ATLAS Detector
The ATLAS experiment has a broad program of direct searches for Leptoquarks, coupling to the first-, second- or third-generation particles.
[66]
[2304.10953] Search for an axion-like particle with forward proton ...
Apr 21, 2023 · A search for forward proton scattering in association with light-by-light scattering mediated by an axion-like particle is presented
[67]
[1906.05056] ATLAS Measurements of CP Violation and Rare ...
Jun 12, 2019 · The ATLAS experiment at the Large Hadron Collider (LHC) has performed accurate measurements of mixing and CP violation in the neutral B mesons, ...Missing: SM | Show results with:SM
[68]
[PDF] Technical Design Report - CERN Document Server
May 25, 1999 · This is a Technical Design Report for the ATLAS detector and physics performance, created by the ATLAS Collaboration on May 25, 1999.
[69]
The Inner Detector - ATLAS Experiment
It is very compact and highly sensitive. It consists of three different systems of sensors all immersed in a magnetic field parallel to the beam axis.Missing: principles | Show results with:principles
[70]
[PDF] The ATLAS Pixel Detector
Sep 26, 2011 · The Inner Detector has been designed to achieve a momentum resolution of σ(pT )/pT = 0.05%pT [GeV/c]⊕1% and an impact parameter resolution.Missing: p_T
[71]
[PDF] ATLAS-TDR-4-Volume-I.pdf - Inner Detector - CERN Document Server
Apr 30, 1997 · This is the Technical Design Report for the Inner Detector, including Forward, SCT, Barrel, SCT, TRT, and Pixel Detectors.
[72]
[PDF] Commissioning and performance of the ATLAS Inner Detector with ...
Jul 7, 2009 · The SCT [4] consists of 4 barrel layers and 9 disks in each end-cap. Modules are built from two wafers of 80 µm pitch strips at a 40 mrad ...
[73]
[PDF] Particle Identification with the ATLAS Transition Radiation Tracker
Sep 14, 2011 · In addition to its tracking capabilities, the TRT provides particle identification (PID) ability through the detection of transition radiation X ...
[74]
[PDF] A TL-GEN-PROC-2011-003 ATLAS DETECTOR OVERVIEW
Aug 12, 2011 · This whole system was designed to achieve precise tracking and vertexing with transverse momentum σ/ pT = 0.038% pT (GeV) ⊕ 1.5% (< 2% pT.Missing: p_T
[75]
[PDF] The material budget of the ATLAS Inner Detector - CERN
May 31, 1998 · Figure 8-5 Total pixel material in radiation lengths in the current simulation and at the time of the Inner Detector. TDR within the pixel ...
[76]
[PDF] The ATLAS Pixel Detector
The space resolutions measured for one hit and two hit clusters for different incident particle angles in binary readout mode, i.e. approx. 12 µm in rφ and 110 ...
[77]
[PDF] Operation and Performance of the ATLAS Semiconductor Tracker
Jun 4, 2013 · SCT uses binary readout, where a “hit” is registered if the charge on a strip exceeds a configurable threshold. (nominally 1fC for LHC Run 1).
[78]
[PDF] The ATLAS Inner Detector commissioning and calibration
– Combined ATLAS cosmic-ray run. Data were taken by the full ATLAS ... 2008 cosmic-ray data-taking periods is shown in Table 1. Similar data-taking ...Missing: challenges | Show results with:challenges
[79]
ATLAS detector and physics performance - CERN Document Server
Geneva : CERN, 1999 - 460. Series, (Technical design report. ATLAS ; 14). Subject code, 539.1.072. Subject category, Detectors and Experimental Techniques.
[80]
Calorimeter - ATLAS Experiment
They are designed to absorb most of the particles coming from a collision, forcing them to deposit all of their energy and stop within the detector.
[81]
Muon Spectrometer - ATLAS Experiment at CERN
Monitored Drift Tubes (MDTs) are used to detect muons. These modules are placed throughout the Barrel Toroid Magnet structure, and make up the MDT Big Wheels.
[82]
[PDF] Overview of ATLAS muon detectors: status and performance
Oct 27, 2025 · The MS detector system consists of Resistive Plate Chambers (RPC) and Thin Gap Chambers (TGC) for the trigger and coarse measurement of the 𝜙 ...
[83]
[PDF] First Results from the ATLAS Muon Spectrometer Optical Alignment ...
Apr 16, 2009 · The ATLAS muon spectrometer optical alignment sys- tem has been designed to provide muon chamber positions with a sagitta accuracy of 30–40 μm.
[84]
[PDF] Muon Spectrometer Phase-I Upgrade for the ATLAS Experiment
Oct 1, 2018 · To maintain an excellent detection and background rejection ... at an angle of ±1.5◦ with respect to the η-strips. This strip ...
[85]
Magnet System
### ATLAS Solenoid Magnet
[86]
https://atlas.cern/Discover/Detector/Magnet-System
[87]
[PDF] Superconducting magnet system for the ATLAS detector at CERN
solenoid is magnetically de-coupled from the toroids. Each of the three toroids consists of 8 coils (flat pancake type) assembled radially and symmetrically ...
[88]
[PDF] The new LUCID-2 detector for luminosity measurement and ...
Jul 23, 2018 · The photomultipliers were tested for up to 200kGray without any adverse effects [5]. The LUCID-1 detector performance was affected by the stray ...
[89]
[PDF] Performance of the ATLAS Zero Degree Calorimeter
Nov 15, 2013 · Each ZDC module individually provides energy and trigger information using a dedicated set of quartz rods that are grouped together and ...
[90]
[PDF] ATLAS Forward Proton Technical Design Report
May 21, 2015 · The official representative of AFP (and of the other ATLAS Forward Detectors ALFA, LUCID,. ZDC) to the ATLAS Executive Board is the FWD PL.
[91]
[PDF] ATLAS NOTE
Mar 7, 2011 · In 2010, ATLAS has used event counting algorithms to measure the delivered luminosity, where a bunch crossing is said to contain an 'event' if ...
[92]
https://cds.cern.ch/record/1334563/files/ATLAS-CONF-2011-011.pdf
[93]
ATLAS event selection system readies for LHC Run 3
Feb 28, 2022 · Every second, the first-level trigger selects just 100,000 of the 40 million collision events generated by the LHC for further study. During ...
[94]
Trigger and Data Acquisition System - ATLAS Experiment
The Trigger and Data Acquisition system ensures optimal data-taking conditions and selects the most interesting collision events for study.
[95]
[PDF] ATLAS TDAQ upgrades for Phase-2 - CERN Document Server
Aug 16, 2024 · The total DAQ throughput will increase from 0.2 TB/s to 4.6 TB/s ... and Data Acquisition System - Event Filter Tracking Amendment”, ATLAS ...
[96]
https://cds.cern.ch/record/2907275/files/ATL-DAQ-PROC-2024-008.pdf
[97]
[PDF] The ATLAS Data Acquisition System in LHC Run 2
Feb 5, 2017 · The Trigger and Data Acquisition system of the ATLAS experiment has been upgraded to deal with the increased performance required by this new ...
[98]
[PDF] Overview of the ATLAS distributed computing system
In total more than 370 PB of data is distributed over more than 150 sites in the WLCG and handled by the ATLAS data management system Rucio. To prepare for the ...
[99]
Electron efficiency in LHC Run-2 with the ATLAS experiment - arXiv
Dec 26, 2024 · The electron reconstruction efficiency is higher than 97%, and the ratio of data to Monte Carlo simulation efficiency is close to unity, with ...Missing: Athena | Show results with:Athena
[100]
Athena - ATLAS Software Documentation
Athena is the name of the ATLAS software framework that manages almost all ATLAS production workflows: event generation, simulation, reconstruction and ...Missing: efficiency | Show results with:efficiency
[101]
[PDF] The data management lifecycle at the Large Hadron Collider
Aug 31, 2015 · Over 10 PB/year of raw data stored for analysis. Data volume is ... • ~200 PB produced per year. More resources needed on a flat budget ...<|separator|>
[102]
[1207.7214] Observation of a new particle in the search for ... - arXiv
Jul 31, 2012 · A search for the Standard Model Higgs boson in proton-proton collisions with the ATLAS detector at the LHC is presented.
[103]
[PDF] Measurements of the CP structure of Higgs boson couplings with the ...
Oct 24, 2025 · The Standard Model predicts the Higgs boson to be a CP-even scalar, but CP-odd contributions to its interactions with vector bosons and ...
[104]
A detailed map of Higgs boson interactions by the ATLAS ... - Nature
Jul 4, 2022 · About 9 million Higgs bosons are predicted to have been produced in the ATLAS detector during this period, of which only about 0.3% are ...
[105]
ATLAS sets record limits on Higgs self-interaction using Run 3 data
May 22, 2025 · This new result is the first ATLAS measurement based on over 300 fb⁻¹ of proton–proton collision data, combining the full LHC Run 2 (2015–2018) ...
[106]
ATLAS closes in on rare Higgs decays
Jul 7, 2025 · The ATLAS Collaboration finds evidence of Higgs-boson decays to muons and improves sensitivity to Higgs-boson decays to a Z boson and a photon.
[107]
ATLAS probes uncharted territory with LHC Run 3 data - CERN
Jul 26, 2024 · The ATLAS collaboration has released its first results from searches for new physics phenomena conducted with data from Run 3 of the LHC.Missing: microscopic black holes
[108]
None
### Summary of ATLAS SUSY Limits (Run 3 Data up to 2025)
[109]
[PDF] 85. Extra Dimensions
May 31, 2024 · Less stringent limits on the KK graviton mass can be derived from analyses of the dijet [35, 36, 89, 90], HH [91–99], and V V [100–104] final ...
[110]
[PDF] Recent BSM results from ATLAS and CMS
Oct 22, 2025 · ○ Includes Run 2+partial Run 3 (2022-2023). ○ Use PNN to separate ... Dark Matter: Spin-Dependent. ○ Collider experiments set powerful ...
[111]
CP violation in Bs to J/psi phi - ATLAS - CERN Document Server
Oct 5, 2020 · In this talk, the measurement of the Bs → J/psi phi decay parameters using 80.5 fb−1 of integrated luminosity collected with the ATLAS detector ...Missing: Run 3 2024 2025
[112]
None
### Summary of EFT Operator Limits from ATLAS Global Fits (Run 3 Data)
[113]
[PDF] Recent developments in flavour tagging and b-jet triggers in the ...
Sep 30, 2025 · Improves on GN2 by (ATLAS-FTAG-2025-01):. * using looser track cuts,. * using neutral particle flow information along with the usual charged ...
[114]
[PDF] The ATLAS upgrade program
Sep 17, 2014 · IBL will help to preserve the tracking performance at high luminosity when the B-layer will suffer from radiation damage and high pile-up ...
[115]
[PDF] The ATLAS experiment
May 31, 2016 · − LS1: A new innermost pixel layer the Insertable B-Layer (IBL) has been added. This is the first ATLAS tracker using CO2 cooling, which is the ...
[116]
[PDF] ATLAS Pixel Detector and readout upgrades for the improved LHC ...
To solve the limitation of the current detector, an ulterior pixel layer, called. Insertable B-Layer (IBL), was installed during LHC Long Shutdown 1 (LS1, 2013-.
[117]
[PDF] arXiv:1409.5002v1 [physics.ins-det] 17 Sep 2014
Sep 18, 2014 · ATLAS has used the shutdown period (Phase-0) for detector consolidation works, including a new Inner. Detector (ID) cooling system, a new ...<|separator|>
[118]
[PDF] The ATLAS experiment Phase-I upgrades for the LHC Run-3
Jul 18, 2023 · The primary goal of these upgrades is to be able to operate Atlas at a luminosity of. 2.0x1034 cm-2 s-1 and it has been achieved. ▫ To be ...Missing: GEM | Show results with:GEM
[119]
ATLAS upgrades during LS2 - CERN
The new small wheels will improve ATLAS's triggering capabilities and will be able to cope with the higher muon rates expected from the High-Luminosity LHC (HL ...Missing: Phase- GEM
[120]
[PDF] New Small Wheel Technical Design Report - CERN Indico
The New Small Wheel (NSW) replaces the first station of the ATLAS muon end-cap system, operating in high radiation, using sTGC for trigger and MM for precision ...
[121]
ATLAS upgrades in LS2 - CERN Courier
Jan 24, 2019 · The main ATLAS upgrade activities during LS2 are aimed at increasing the trigger efficiency for leptonic and hadronic signatures, especially for electrons and ...Missing: Phase- GEM
[122]
[PDF] Upgrade project and plans for the ATLAS detector and first level ...
Nov 8, 2012 · ... 1 muon system has shown unexpectedly high rates of fake triggers in the ... 1 trigger upgrade addresses both the suppression of the fake triggers ...
[123]
[PDF] Data access optimisation at CERN and in the Worldwide LHC ...
Jun 21, 2024 · Currently, CERN operates numerous EOS instances, including ones corresponding to specific LHC experiments: LHCb, CMS, ATLAS, and ALICE. Each EOS ...
[124]
[PDF] D projects for the High Luminosity LHC under CERN openlab
By integrating these models into the data processing pipeline, AI has the potential to enable faster and more accurate event reconstruction, significantly ...
[125]
[PDF] ATLAS Sim@P1 upgrades during long shutdown two
Mar 23, 2020 · In this paper, we describe how the system was modified for operation with the upgraded HLT hardware. 2 The Sim@P1 infrastructure. During the ...<|separator|>
[126]
[PDF] The ATLAS experiment at the CERN Large Hadron Collider
May 23, 2024 · This paper describes the Run 3 configuration of the ATLAS detector. Keywords: Calorimeter methods; Large detector systems for particle and ...
[127]
[PDF] Overview of the ATLAS Phase-I Upgrade Plans: - CERN Indico
• Reduce drastically “fake” rates. Preliminary. Page 11. Key questions to establish: • What are the uncertainties on the parameters which determine the ...
[128]
[PDF] Report on the Physics at the HL-LHC and Perspectives for the HE-LHC
Jan 30, 2019 · ... 14 TeV with a baseline instantaneous luminosity of 5 · 1034 cm−2s−1 and an ultimate achievable instantaneous luminosity of 7.5 · 1034 cm−2s−1.
[129]
ATLAS prepares for High-Luminosity LHC
Apr 2, 2025 · ... total delivered integrated luminosity of 3,000 fb–1 by 2041 – including about 520 fb–1 recorded during the LHC Runs 1–3. With this ...
[130]
[PDF] The ATLAS Upgrade for the HL-LHC - CERN Document Server
The ATLAS experiment [1, 2] continues to extensively test the Standard Model (SM) and conduct a comprehensive search programme for physics beyond the SM during ...
[131]
[PDF] Status of the New Phase-2 ATLAS ITk Pixel Detector
May 31, 2025 · The upgrade of the LHC to the High Luminosity LHC (HL-LHC) by the end of this decade will impose significant challenges on the detectors of the ...<|control11|><|separator|>
[132]
[PDF] The ATLAS ITk Pixel and Strip Detector System for the Phase-II LHC ...
May 4, 2025 · ABCStar: ATLAS Binary Chip. - Readout analogue signals from sensors and provide a binary readout per strip. - Encode hit patterns in clusters.
[133]
[PDF] ATLAS ITk Pixel Detector Overview
✓ higher radiation hardness – up to 2 x 1016 neq/cm2. ✓ reduced material ... The ITk pixel detector will consist of 5 silicon tracking layers split into 3 regions.Missing: cm² | Show results with:cm²
[134]
[PDF] ATLAS ITk Strip Detector for the Phase-II LHC Upgrade
Apr 4, 2024 · The sensors closest to the beam pipe will face radiation levels over 1016 neq/cm2 fluence and around ... order to power 10 times more strip ...Missing: cm² | Show results with:cm²
[135]
https://indico.cern.ch/event/147780/contribution/4/attachments/143946/204180/FL-PhISubCRecDraft.pdf
[136]
A High-Granularity Timing Detector for the ATLAS Phase-II Upgrade
Dec 13, 2021 · The HGTD uses LGAD silicon sensors to provide timing information, with 6x better precision than collision time, and has double-sided layers for ...
[137]
[PDF] ATLAS LAr Phase-II Upgrade TDR - CERN Document Server
Jun 15, 2018 · 0.2%. ⊕. E/GeV. Intrinsic calo resolution: 10%/. 14 bit range and 12 bit resolution; high gain x30. 14 bit range and 12 bit resolution; low gain ...
[138]
[PDF] The upgrade of the ATLAS Trigger and Data Acquisition system for ...
Mar 28, 2025 · - DAQ throughout of 4.6 TB/s (200 GB/s in Run 3). Software Based Event Filter: - 1 MHz processing with regional and full tracking. - Reduction ...