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ALICE experiment

The A Large Ion Collider Experiment (ALICE) is a major particle physics experiment at the CERN Large Hadron Collider (LHC), dedicated to studying the properties of quark-gluon plasma (QGP)—a hot, dense state of matter consisting of deconfined quarks and gluons that is thought to have filled the universe microseconds after the Big Bang—through high-energy collisions of heavy ions such as lead nuclei.^[1]^[2] Located in a vast underground cavern 56 meters below ground near Saint-Genis-Pouilly, France, on the Franco-Swiss border, the ALICE detector is a cylindrical apparatus measuring 16 meters in diameter and height and 26 meters in length, with a total mass of approximately 10,000 tonnes.^[1] It comprises 18 specialized subsystems, including a silicon inner tracking system (ITS) for vertex reconstruction, a time projection chamber (TPC) for momentum measurement and particle identification, a transition radiation detector (TRD) for electron identification, and electromagnetic and hadron calorimeters for energy measurements, all optimized to handle the extreme particle multiplicities—up to thousands of charged particles per event—in central heavy-ion collisions.^[2] The experiment operates within an international collaboration of nearly 2,000 scientists from 174 institutes across 40 countries, which has evolved from an initial group of over 1,000 members in 2008.^[1]^[2] ALICE's physics program centers on characterizing the thermodynamic, hydrodynamic, and transport properties of QGP in lead-lead (Pb-Pb) collisions at center-of-mass energies per nucleon pair up to √s_NN = 5.36 TeV, while using proton-proton (pp) and proton-lead (p-Pb) collisions at similar energies for baseline comparisons to isolate nuclear effects.^[3]^[4] Key capabilities include high-precision tracking over a pseudorapidity range of |η| < 0.9 and full azimuthal coverage, enabling detailed analyses of anisotropic flow, jet quenching, heavy-flavor production, and strangeness enhancement, which reveal QGP behaviors such as rapid thermalization and low shear viscosity-to-entropy density ratio (η/s) approaching the quantum lower bound of 1/(4π).^[2]^[3] Notable achievements from LHC Runs 1 and 2 (2009–2018) include measurements of initial energy densities reaching ~12 GeV/fm³ in central Pb-Pb collisions—over three times higher than at the Relativistic Heavy Ion Collider (RHIC)—and effective temperatures up to ~300 MeV, confirming the formation of a nearly perfect fluid-like QGP with strong collective effects even in smaller systems like high-multiplicity pp and p-Pb events; these findings have been further corroborated by Run 3 data at higher energies.^[3] In 2025, the ALICE collaboration shared the Breakthrough Prize in Fundamental Physics for contributions to particle physics at the LHC. The experiment has also quantified heavy-quark energy loss, charmonium suppression and regeneration, and the observation of QGP-like long-range correlations, providing insights into partonic interactions and the QCD phase diagram.^[3] Upgrades implemented for Run 3 (2022–2025), including a new ITS with improved readout rates up to 50 kHz and plans for ALICE 3 in the 2030s with enhanced resolution and forward coverage, aim to achieve tenfold precision improvements in key observables during LHC Runs 3–6.^[1]^[3]^[5]

Background and Development

Introduction

The ALICE (A Large Ion Collider Experiment) is one of the major detectors at CERN's Large Hadron Collider (LHC), specifically designed to investigate the behavior of strongly interacting matter under extreme conditions of temperature and density.^[1] Located in an underground cavern at Point 2 along the 27-kilometer LHC ring, near the France-Switzerland border, ALICE captures and analyzes particles produced in high-energy collisions to recreate conditions akin to those in the early universe.^[1] The primary objectives of ALICE include probing the properties of quark-gluon plasma (QGP), a state of deconfined quarks and gluons formed in heavy-ion collisions, as well as studying high-multiplicity events in proton-proton interactions to explore similar phenomena on smaller scales.^[6] These investigations aim to deepen understanding of quantum chromodynamics (QCD) in extreme environments, with the experiment's design optimized for tracking charged particles and identifying a wide range of particle species over a broad momentum range.^[1] As of 2025, the ALICE collaboration comprises 1,882 scientists from 165 institutes across 39 countries, reflecting its international scope and multidisciplinary expertise.^[7] In recognition of the LHC experiments' contributions to fundamental physics, including discoveries related to the Higgs boson and QGP, ALICE shared the 2025 Breakthrough Prize in Fundamental Physics with the ATLAS, CMS, and LHCb collaborations.^[8]

Historical Development

The ALICE (A Large Ion Collider Experiment) collaboration was formed in the early 1990s following discussions at an ECFA workshop on future heavy-ion facilities, with the group submitting a Letter of Intent to the CERN LHC Committee on 1 March 1993, proposing a dedicated detector for studying quark-gluon plasma in heavy-ion collisions at the LHC.^[9]^[10] This document outlined the initial physics motivations and conceptual design, emphasizing the need for high-multiplicity event reconstruction in ultrarelativistic nucleus-nucleus interactions. A more detailed Technical Proposal followed in December 1995, refining the detector layout and performance expectations based on simulations and prototype tests. The experiment received formal approval from the CERN Research Board on 14 February 1997, marking it as one of the four major LHC detectors alongside ATLAS, CMS, and LHCb.^[11] This approval initiated the full construction phase, which spanned from 1997 to 2008 and involved over 1,000 scientists and engineers from more than 100 institutes across 30 countries, coordinated through international funding agencies including CERN, the European Union, and national bodies like the NSF and BMBF. Key components, such as the Time Projection Chamber (TPC) installed in January 2007 and the Inner Tracking System (ITS) with its silicon layers achieving full yield, were assembled in the underground cavern at Point 2 of the LHC, 56 meters below ground near Saint-Genis-Pouilly, France. The 10,000-tonne detector integrated subsystems like the Transition Radiation Detector (TRD), Time-of-Flight (TOF), and muon spectrometer, with designs validated through Technical Design Reports (TDRs) published between 1999 and 2000. Commissioning began in earnest during the LHC's initial startup phase, with the ALICE detector recording its first cosmic-ray events for alignment and calibration in summer 2008, including approximately 10^5 charged tracks used to align the ITS to micrometer precision.^[12] These efforts addressed early data challenges, such as noise suppression in the TPC (achieving σ ~ 0.7 ADC counts) and integration of the trigger and data acquisition systems with cosmic muon triggers via the ACORDE detector.^[12] The first proton-proton (pp) collisions were observed on 23 November 2009 at √s = 900 GeV during LHC commissioning, allowing initial measurements of charged-particle pseudorapidity density and marking ALICE's seamless integration into LHC operations.^[13] This transitioned into Run 1 (2009–2013), a period of progressive luminosity increases and data collection in pp, proton-lead, and lead-lead modes, overcoming challenges like beam-induced background and high-multiplicity event processing.^[14] The first lead-lead (Pb-Pb) collisions occurred on 7 November 2010 at √s_NN = 2.76 TeV, enabling ALICE to record over 10 million minimum-bias events in the inaugural heavy-ion run and probe the hot, dense medium produced in these interactions.^[15] Run 1 concluded in early 2013 after accumulating datasets that confirmed the detector's performance across collision systems, with subsequent long shutdowns allowing minor optimizations before Run 2 (2015–2018). During Run 2, ALICE operated at higher luminosities, integrating further into the LHC's heavy-ion program while handling increased data rates up to 1 kHz for pp collisions and addressing challenges like pileup in Pb-Pb runs at √s_NN = 5.02 TeV.^[16] These phases established ALICE as a cornerstone for heavy-ion physics at the LHC, with the collaboration expanding to over 1,500 members by 2018.

Physics Programme

Heavy-Ion Collision Studies

The ALICE experiment primarily investigates heavy-ion collisions to recreate and study the quark-gluon plasma (QGP), a deconfined state of quarks and gluons that forms under extreme conditions of temperature and density. In these collisions, heavy nuclei such as lead (Pb) are accelerated to ultrarelativistic speeds, generating temperatures on the order of 10^{12} K, where the strong nuclear force is overcome, allowing quarks and gluons to exist freely rather than being confined within hadrons. This state mimics the early universe shortly after the Big Bang and provides a laboratory for probing quantum chromodynamics (QCD) in regimes inaccessible to other collision types. The core goals of ALICE's heavy-ion program are to characterize key QGP properties, including its density, temperature, and viscosity, through precise measurements of collective and hard-probe observables. Elliptic flow, quantified by the second-order Fourier coefficient v_2, reveals the anisotropic expansion of the QGP, indicating its near-perfect fluidity with low shear viscosity. Jet quenching, observed as the suppression of high-transverse-momentum particle yields (e.g., nuclear modification factor R_{AA} reduced by up to a factor of five), demonstrates energy loss of partons traversing the dense medium, offering insights into its opacity and interaction strength. These studies are conducted in lead-lead (Pb-Pb) collisions at center-of-mass energies per nucleon pair up to \sqrt{s_{NN}} = 5.02 TeV, enabling the production of a larger and hotter QGP volume compared to lower-energy facilities. The theoretical framework relies on relativistic hydrodynamic models, such as IP-Glasma+MUSIC or TRENTo+VISHNU, which simulate the QGP's evolution from initial conditions to hadronization. These models incorporate an equation of state for the QGP as a nearly ideal fluid, characterized by a shear viscosity-to-entropy density ratio \eta/s \approx 1/(4\pi), consistent with AdS/CFT predictions and lattice QCD calculations. Relative to results from the Relativistic Heavy Ion Collider (RHIC), ALICE achieves higher precision in QGP characterization due to the elevated collision energies at the LHC, which produce a longer-lived medium (lifetime ~10–13 fm/c) with greater particle multiplicity and flow signals. This allows for more detailed investigations of QGP transport coefficients and phase transition dynamics, building on RHIC's confirmation of QGP formation while extending the parameter space.

Proton-Proton and Ultrarelativistic Collision Studies

Proton-proton (pp) collisions at the Large Hadron Collider (LHC), conducted at a center-of-mass energy of \sqrt{s} = 13.6 TeV during Run 3, serve as a fundamental baseline for ALICE's heavy-ion physics program by enabling precise measurements of parton distribution functions (PDFs) and the underlying strong interactions in elementary QCD processes. These collisions allow for the isolation of perturbative and non-perturbative QCD effects without the complications of nuclear geometry or medium-induced modifications, providing reference cross sections for particle production that are crucial for quantifying enhancements or suppressions in heavy-ion environments. For instance, hard probes such as isolated photon production in pp collisions at LHC energies constrain gluon and quark PDFs at high momentum fractions, offering insights into the proton's internal structure relevant to high-energy strong interactions.^[17] In high-multiplicity pp events—selected to mimic the density of larger collision systems—ALICE investigates collective phenomena, including radial and elliptic flow, to determine if hydrodynamic-like behavior emerges in small systems.^[18] Measurements reveal non-zero elliptic flow coefficients (v_2) that increase with multiplicity, indicating azimuthal anisotropies potentially driven by initial-state geometry or final-state interactions, challenging traditional models of pp dynamics.^[19] These findings suggest that collective effects, such as long-range di-hadron correlations resembling ridge structures, can arise even without a large medium, serving as benchmarks for quark-gluon plasma (QGP) signatures in heavy-ion collisions. Ultrarelativistic proton-nucleus (p-A) collisions, such as p-Pb at \sqrt{s_{NN}} = 5.02 TeV, are key to probing initial-state effects like gluon saturation, where the high density of gluons in the nucleus leads to nonlinear QCD evolution. ALICE data on forward particle production and di-hadron correlations in these systems reveal asymmetries and suppression patterns consistent with color glass condensate predictions, highlighting saturation scales beyond those in pp collisions. Furthermore, observations of enhanced multi-strange particle yields and flow-like anisotropies in high-multiplicity p-A events point toward the formation of a mini-QGP, a compact deconfined state that bridges small- and large-system physics. Central to these studies are specific observables in pp collisions, including strangeness enhancement, where the yield of strange hadrons like \Xi and \Omega rises anomalously with charged-particle multiplicity, deviating from baseline expectations and suggesting thermal-like production mechanisms.^[20] Di-hadron correlations, particularly in high-multiplicity triggers, exhibit near- and away-side structures that probe parton energy loss and fragmentation, providing tests of jet quenching models adapted to small systems. Electroweak probes, such as W and Z boson decays, complement these by offering clean accesses to weak interactions and further PDF constraints, isolated from strong final-state effects.

Collision Campaigns at the LHC

Lead-Lead and Other Ion-Ion Collisions

The first lead-lead (Pb-Pb) collisions at the Large Hadron Collider (LHC) were delivered to the ALICE experiment in November 2010, marking the inaugural heavy-ion run at a centre-of-mass energy per nucleon pair of \sqrt{s_{NN}} = 2.76 TeV, with an instantaneous luminosity up to approximately $2 \times 10^{25} cm^{-2} s^{-1}. These initial collisions involved beams with up to 114 bunches per beam, each containing about $7 \times 10^7 lead ions, allowing ALICE to collect a commissioning dataset of roughly 30 million minimum-bias events over several days.^[21] Subsequent Pb-Pb runs expanded the dataset significantly, with operations in 2011 at the same energy of \sqrt{s_{NN}} = 2.76 TeV achieving higher luminosities up to $10^{25} cm^{-2} s^{-1} using more bunches and optimized beam intensities. The energy was increased to \sqrt{s_{NN}} = 5.02 TeV for the 2015 and 2018 runs during LHC Run 2, delivering integrated luminosities of about 0.02 nb^{-1} and 0.5 nb^{-1}, respectively (total ~0.75 nb^{-1}), which enabled detailed studies of quark-gluon plasma properties.^[22]^[23] In Run 3, which began in 2022, Pb-Pb collisions reached a record \sqrt{s_{NN}} = 5.36 TeV starting from the 2023 campaign, with the first long run from September to October 2023 collecting ~2 nb^{-1} of data at interaction rates up to 50 kHz. By November 2025, Run 3 Pb-Pb data total over 4 nb^{-1}, approaching the 10 nb^{-1} goal for precision measurements.^[24]^[25] Beyond lead ions, ALICE has recorded collisions with lighter species to probe system-size dependencies in heavy-ion physics. Xenon-xenon (Xe-Xe) collisions occurred in late 2017 at \sqrt{s_{NN}} = 5.44 TeV, yielding an integrated luminosity of about 0.34 \mub^{-1} over three days, providing a benchmark between smaller and larger systems. In July 2025, during a special LHC operation, oxygen-oxygen (O-O) collisions were delivered for the first time at \sqrt{s_{NN}} = 5.36 TeV, followed immediately by neon-neon (Ne-Ne) collisions at the same energy; initial analyses from these datasets, collected over a few days, reveal evidence of anisotropic flow driven by nuclear geometry.^[26]^[27] For Run 3 Pb-Pb operations, the LHC aims to deliver an integrated luminosity of approximately 10 nb^{-1} per year, supporting ALICE's goal of accumulating over 10 nb^{-1} total for precision measurements. Beam parameters include up to 1248 bunches per beam, each with around $1.15 \times 10^8 ions, and a beta function at the ALICE interaction point of \beta^* = 0.5 m to optimize luminosity while managing long bunch lengths inherent to ion beams.^[28]^[4]^[29]

Proton-Ion and Proton-Proton Collisions

Proton-proton (pp) collisions at the LHC provide essential reference data for ALICE's heavy-ion program, enabling precise studies of quantum chromodynamics (QCD) in elementary hadronic interactions. These collisions began during Run 1 in 2009, initially at a center-of-mass energy of 0.9 TeV, progressing to 2.36 TeV in 2011 and up to 7 TeV in 2010–2012, with a brief period at 8 TeV in 2012. To mitigate pileup effects in the high-multiplicity environment of the ALICE detector, the instantaneous luminosity was typically limited to around $10^{30} cm^{-2} s^{-1}, resulting in integrated luminosities up to approximately 100 nb^{-1} per energy campaign, supplemented by high-multiplicity triggers to select rare events with enhanced particle production.^[30]^[31] In Run 2 (2015–2018), pp collisions reached a center-of-mass energy of 13 TeV, with dedicated runs at intermediate energies such as 5.02 TeV and 2.76 TeV to align with heavy-ion benchmarks. ALICE recorded an integrated luminosity of ~32 nb^{-1} at 13 TeV across 2016-2018, plus additional at lower energies, benefiting from improved beam stability and triggering strategies that prioritized high-multiplicity events.^[32] Run 3, commencing in 2022, elevated pp collisions to 13.6 TeV, leveraging ALICE's upgrades—including continuous readout and a new inner tracking system—to handle higher interaction rates. By late 2024, ~82 pb^{-1} had been recorded for proton physics (19.3 pb^{-1} in 2022, 9.7 pb^{-1} in 2023, 53.1 pb^{-1} in 2024), with the 2025 campaign starting in May at 13.6 TeV aiming for a total integrated luminosity exceeding 200 pb^{-1} by the end of Run 3 in 2025.^[25]^[33]^[34] Proton-lead (p-Pb) collisions, which probe cold nuclear matter effects, were introduced in Run 1 with the first campaign in 2013 at a nucleon-nucleon center-of-mass energy \sqrt{s_{NN}} = 5.02 TeV. Asymmetric beam energies were employed—protons at 4 TeV and lead ions at 1.23 TeV per nucleon—to achieve the target \sqrt{s_{NN}} while introducing a rapidity shift of approximately 0.47 units for forward-backward asymmetry studies. ALICE collected up to 100 nb^{-1} in this campaign. Run 2 featured p-Pb collisions at \sqrt{s_{NN}} = 8.16 TeV in 2016, followed by a reversed Pb-p configuration to symmetrize rapidity coverage, yielding a combined integrated luminosity of about 200 nb^{-1} for the two p-Pb campaigns in Runs 1 and 2. In Run 3, p-Pb operations are planned for 2025 at 8.16 TeV with similar asymmetric setup (protons at 6.5 TeV and lead at 2.58 TeV per nucleon), targeting up to 200 nb^{-1} to enhance precision in nuclear modification factor measurements.^[35]^[36]^[37]

Detector System

Central Barrel Trackers

The central barrel trackers in the ALICE experiment form the core system for reconstructing the trajectories of charged particles produced in the central rapidity region, enabling precise momentum measurements, vertex reconstruction, and contributions to particle identification. Comprising the Inner Tracking System (ITS), Time Projection Chamber (TPC), and Transition Radiation Detector (TRD), these detectors surround the beam pipe and operate within the 0.5 T solenoidal magnetic field of the L3 magnet, covering the full azimuthal angle and pseudorapidity range |η| < 0.9. The Inner Tracking System (ITS) is a low-mass silicon pixel tracker positioned closest to the interaction point, consisting of seven cylindrical layers at radii between 2.3 cm and 39.3 cm. All layers employ CMOS Monolithic Active Pixel Sensors (MAPS) with a 30 μm pixel pitch and over 12 billion pixels in total, providing a transverse point resolution of approximately 5 μm. This design achieves an impact parameter resolution of better than 30 μm at p_T ≈ 0.5 GeV/c, allowing tracking of charged particles down to p_T > 0.1 GeV/c over |η| < 0.9, with a material budget of approximately 0.35% radiation length (X_0) for the inner layers and up to 0.8% X_0 for the outer layers to minimize multiple scattering.^[38] The Time Projection Chamber (TPC) serves as the primary tracking device, a large cylindrical drift volume of 90 m³ filled with a Ne–CO_2–N_2 gas mixture (90:10:5 by volume) that enables three-dimensional reconstruction of particle trajectories. Electrons liberated by ionizing particles drift up to 2.5 m toward the endcaps under a 400 V/cm electric field, where they are amplified and read out by 557,000 multichannel pads, providing up to 159 space points per track. The TPC delivers a momentum resolution of σ(p_T)/p_T < 5% at 10 GeV/c (extending to < 8% at 20 GeV/c), with transverse and longitudinal point resolutions of 800–1100 μm and 1100–1250 μm, respectively, covering charged particles from p_T ≈ 0.1 GeV/c up to 100 GeV/c in |η| < 0.9. Additionally, energy loss (dE/dx) measurements in the TPC support particle identification, complementing dedicated systems. The Transition Radiation Detector (TRD) enhances tracking for high-momentum particles and provides electron identification through transition radiation produced when relativistic charged particles traverse its multilayer structure. It comprises six layers of radiator and drift chamber modules arranged radially from 2.9 m to 3.7 m, with each supermodule containing 30 modules (six layers by five longitudinal stacks) across 18 azimuthal sectors, totaling 540 chambers with 1.18 million readout pads. The radiators, made of polypropylene fiber and Rohacell foam (4.8 cm thick, 0.93% X_0), are followed by Xe–CO_2 (85:15) drift chambers (3 cm drift gap) that detect both ionization and transition radiation photons, achieving a position resolution of 200–600 μm and pion rejection factor of ≈100 at 90% electron efficiency for p_T > 2 GeV/c over |η| < 0.8. The TRD's material budget is about 23% X_0 overall, supporting tracking up to p_T ≈ 10 GeV/c per layer. Together, the ITS, TPC, and TRD enable global track reconstruction by seeding tracks in the ITS or TPC and matching them across all three detectors, yielding high-purity trajectories with momentum measurements up to p_T = 100 GeV/c and pointing resolution dominated by the ITS for low-p_T particles. This integrated system is essential for studying the dense medium formed in heavy-ion collisions, with the TPC providing the bulk of the tracking points and the ITS and TRD refining precision and identification.

Particle Identification Systems

The Time-of-Flight (TOF) detector in the ALICE experiment utilizes Multi-gap Resistive Plate Chamber (MRPC) technology to perform charged particle identification in the central barrel region through precise timing measurements. Comprising 1593 MRPC chambers with a total active area of 141 m², the TOF system covers the pseudorapidity range |η| < 0.9 and the full azimuthal angle, enabling velocity determination for particles traversing approximately 3.7 m from the interaction point. The intrinsic time resolution of the MRPCs is better than 50 ps, resulting in an overall detector resolution of about 80 ps, which supports efficient identification (>95%) of pions, kaons, and protons up to momenta of around 5 GeV/c.^[39]^[40] This timing performance allows for 3σ separation between pion and kaon signals at 1 GeV/c, extending to 2.5 GeV/c for π/K and up to 4 GeV/c for K/p, with stable operation demonstrated across LHC runs without performance degradation after 15 years. The TOF detector builds on tracking information from the central barrel trackers to associate time measurements with reconstructed trajectories, enhancing species identification in high-multiplicity environments typical of heavy-ion collisions.^[41]^[40] The High Momentum Particle Identification Detector (HMPID) employs a Ring Imaging Cherenkov (RICH) technique with CsI photocathodes to extend identification capabilities to higher momenta, focusing on charged hadrons in a limited acceptance. Consisting of seven proximity-focusing RICH counters, each measuring 1.4 m × 1.3 m and equipped with multi-wire proportional chambers for photon detection, the HMPID covers approximately 5% of the central barrel phase space in a single-arm configuration at radii of 4.7–5.3 m from the beam axis. It identifies pions and kaons up to 3 GeV/c and protons up to 5 GeV/c with 3σ separation, while also enabling electron-pion discrimination leveraging the Cherenkov threshold differences in the liquid perfluorohexane radiator.^[42]^[43] Particle identification in ALICE benefits from combining TOF and HMPID signals with specific energy loss (dE/dx) measurements from the Time Projection Chamber (TPC), providing multi-GeV/c resolution for hadrons and extending lepton-hadron separation to tens of GeV/c via the relativistic rise in dE/dx. This integrated approach achieves track-by-track identification with high purity in the intermediate momentum regime, crucial for studies of quark-gluon plasma properties and hadron production spectra.^[44]^[45]

Calorimetry and Multiplicity Detectors

The calorimetry and multiplicity detectors in the ALICE experiment provide essential measurements of electromagnetic energy deposits and particle multiplicities, enabling studies of photon production, jet structures, and event characteristics in high-multiplicity collisions. These systems include high-resolution electromagnetic calorimeters for precise photon and electron detection at mid-rapidity, as well as forward detectors for characterizing charged and neutral particle densities. Together, they support key analyses such as direct photon identification, neutral meson reconstruction, and centrality estimation, complementing the central barrel trackers by quantifying neutral components of events.^[46]^[47] The Photon Spectrometer (PHOS) is a high-granularity electromagnetic calorimeter designed for high-precision measurements of photons and neutral mesons in heavy-ion collisions. It consists of lead tungstate (PbWO₄) crystals, each with dimensions of 2.2 × 2.2 × 18 cm³, arranged in a matrix of 17,280 channels across four modules. The detector covers a pseudorapidity range of |η| < 0.12 and an azimuthal extent of φ = 260°, providing a compact acceptance at mid-rapidity optimized for detecting photons with transverse energies from 0.5 to 10 GeV. Its energy resolution is σ(E)/E ≈ 2%/√E (GeV), achieved through cryogenic operation at -25°C and coupling to large-area PIN photodiodes with low-noise preamplifiers, enabling discrimination against charged particles and efficient π⁰ reconstruction via two-photon decays. PHOS plays a critical role in probing quark-gluon plasma properties through direct photon and jet quenching observables.^[46]^[46]^[46] The Electromagnetic Calorimeter (EMCal) complements PHOS with a larger acceptance for inclusive electromagnetic measurements and jet-related studies. It employs a lead-scintillator sampling technique, featuring alternating layers of 1.44 mm lead absorbers and 1.76 mm polystyrene scintillator tiles, read out via wavelength-shifting fibers and avalanche photodiodes. Covering |η| < 0.7 and Δφ = 107°, the EMCal spans approximately 10,000 towers with a cell size of 6 × 6 cm², enhancing ALICE's ability to reconstruct full jets by capturing neutral energy fractions missed by tracking detectors. Its energy resolution is approximately 11%/√E ⊕ 1.7% for electrons, supporting γ-jet correlation analyses and neutral pion measurements up to high transverse momenta. Additionally, the EMCal provides Level-0 and Level-1 triggers for high-p_T jets, improving event selection efficiency in proton-proton and heavy-ion runs by identifying energetic electromagnetic clusters in real time.^[47]^[47]^[47] In the forward region, the Photon Multiplicity Detector (PMD) measures the spatial distribution and multiplicity of photons to characterize electromagnetic activity beyond mid-rapidity. The PMD features a preshower plane with a 3 radiation length thick lead converter followed by a plane of gas-filled honeycomb proportional chambers for shower detection, paired with a veto electromagnetic multiplicity (VEM) layer using identical chambers without converter to reject charged particles. Positioned at 367 cm from the interaction point, it covers the pseudorapidity range 2.3 < η < 3.9 with full azimuthal acceptance and a granularity of 24 × 48 cells per module across 24 modules, achieving about 73% photon detection efficiency. This setup allows event-by-event assessment of neutral transverse energy and fluctuations, aiding studies of initial-state geometry and isospin dynamics in heavy-ion collisions.^[48]^[48]^[49] The Forward Multiplicity Detector (FMD) extends multiplicity measurements to charged particles in the very forward directions, using silicon strip technology for high precision. Comprising five rings of double-sided silicon sensors with 51,200 strips (250 μm pitch for inner rings, 500 μm for outer), the FMD is divided into forward (1.7 < η < 5.0) and backward (-5.0 < η < -1.7) sides, with specific ranges of 3.68 < η < 5.03 (FMD1), 1.70 < η < 3.68 (FMD2), and symmetric backward coverage (FMD3). Each ring provides 20 radial sectors for azimuthal segmentation, enabling event-plane reconstruction and asymmetry studies. The detector estimates collision centrality from integrated charged particle yields and supports fluctuation analyses, with low material budget ensuring minimal multiple scattering.^[50]^[50]^[50]

Forward and Trigger Detectors

The forward and trigger detectors in the ALICE experiment play a crucial role in characterizing collision events, providing precise timing information, and enabling efficient event selection through triggers based on centrality and minimum bias criteria. These detectors, positioned in the forward regions, complement the central barrel systems by covering high pseudorapidity ranges and detecting spectator particles, which are essential for determining collision geometry in heavy-ion runs. The V0 detector consists of two arrays of scintillator counters, V0-A and V0-C, located at approximately 3.3 m and 0.9 m from the interaction point along the beam axis, respectively. Each array comprises 32 segments of BC404 plastic scintillator tiles coupled to wavelength-shifting fibers and read out by photomultiplier tubes, covering pseudorapidity ranges of 2.8 < η < 5.1 for V0-A and -3.7 < η < -1.7 for V0-C. It measures charged-particle multiplicity in the forward regions to estimate centrality and provides signals for minimum-bias triggers by requiring coincident hits in both arrays, achieving an efficiency of about 83% in proton-proton collisions. Additionally, its timing resolution of better than 1 ns allows rejection of beam-gas background events.^[51]^[50] The T0 detector, comprising two Cherenkov radiator arrays at positions similar to V0 (around 3.3 m and 0.9 m from the interaction point), uses fused silica radiators viewed by fine-mesh photomultiplier tubes to generate fast timing signals. It covers narrower pseudorapidity intervals of approximately 4.6 < η < 4.9 (T0-A) and -3.3 < η < -3.0 (T0-C), with 12 modules per side. The primary function is to provide the start time for the Time-of-Flight (TOF) detector with a resolution of about 40 ps, enabling precise event timing and vertex position reconstruction to within ±1.5 cm. It also contributes to level-0 (L0) triggers for minimum bias and multiplicity, with near-100% efficiency in heavy-ion collisions.^[50] The Zero Degree Calorimeter (ZDC) is situated at ±112.5 m from the interaction point, consisting of two neutron (ZN) and two proton (ZP) calorimeters on opposite sides of the beam pipe. Each ZN module is a tungsten-quartz sampling calorimeter with quartz fibers embedded in tungsten plates to detect Cherenkov light from spectator neutrons via hadronic showers, while ZP uses brass-quartz for protons. Operating at beam rapidity (|η| ≈ 8.8 for neutrons), it measures the energy carried by spectator nucleons to determine centrality in ion-ion collisions, independent of vertex position, with resolutions of around 11-13% at 2.76 TeV per nucleon. This spectator signal correlates with the number of participating nucleons, aiding in impact parameter estimation. The trigger logic integrates signals from V0 and ZDC to classify events efficiently at the L0 and level-1 (L1) stages. Minimum-bias triggers require coincident activity in V0-A and V0-C, while centrality classes (e.g., 0-10% central or 0-50% semi-central) are defined by programmable thresholds on V0 multiplicity or ZDC energy sums, allowing selective readout of rare central events in heavy-ion runs. This combination ensures high selectivity for physics-relevant events, with V0 providing rapidity coverage and ZDC offering spectator-based centrality orthogonal to charged-particle multiplicity.^[50]^[51]

Muon Spectrometer

The Muon Spectrometer in the ALICE experiment is a dedicated forward detector that measures muons originating from heavy-flavor hadron decays and quarkonia production, covering the pseudorapidity range -4 < η < -2.5 with full azimuthal acceptance.^[52] This forward arm enables the study of phenomena such as quarkonium suppression and heavy-quark energy loss in the hot, dense medium created in heavy-ion collisions at the LHC.^[53] The spectrometer's design prioritizes high-purity muon identification and precise momentum reconstruction for particles with transverse momentum p_T > 4 GeV/c, filtering out low-momentum muons and background hadrons effectively.^[54] The layout begins with a front absorber, a passive 10 m long structure composed of layered high-density materials including carbon (graphite), concrete, and steel, providing approximately 10 nuclear interaction lengths (λ_I) to suppress charged hadrons and photons from the collision vertex while minimizing multiple scattering for penetrating muons.^[53] Following the absorber, five tracking stations—each equipped with two multi-wire proportional cathode pad chambers—measure track positions with a spatial resolution better than 100 μm, enabling momentum determination via curvature in the 3 T·m integrated field of a large dipole magnet positioned between stations 3 and 5.^[52] The transverse momentum resolution is approximately 10% at 20 GeV/c, sufficient for resolving key heavy-flavor signals amid high-multiplicity environments.^[55] Downstream of the tracking system lies a passive muon filter consisting of iron walls (∼7 λ_I thick) to further attenuate residual hadrons, followed by the muon identifier comprising two stations of resistive plate chambers (RPCs) with four detection planes for fast triggering, timing (∼2 ns resolution), and single-muon p_T selection above 0.5-1 GeV/c.^[52] This configuration yields an invariant mass resolution of ∼70-100 MeV/c² for J/ψ → μ⁺μ⁻ decays, allowing clear separation of charmonium and bottomonium states.^[52] Overall, the spectrometer facilitates exclusive reconstruction of dimuon pairs from quarkonia (e.g., J/ψ, ψ(2S), Υ) and inclusive single-muon spectra from open heavy-flavor decays (e.g., D and B hadrons), providing critical probes of quark-gluon plasma properties without reliance on central barrel particle identification for forward muons.^[56]

Specialized Detectors

The ALICE Cosmic Ray Detector (ACORDE) is a specialized subdetector designed to detect cosmic rays interacting with the ALICE apparatus at CERN's Large Hadron Collider (LHC).^[57] It consists of an array of 60 scintillator modules, each comprising two superimposed plastic scintillator paddles with an effective area of approximately 0.37 m² per module, positioned on the three upper octants of the ALICE magnet yoke.^[58] This configuration provides an overall effective detection coverage of about 17 m², enabling the capture of high-energy particles traversing the detector from above.^[59] ACORDE's primary goals include measuring high-energy cosmic muons produced in the atmosphere and searching for rare exotic cosmic ray events, such as those involving unusual particle compositions or decay signatures.^[60] These muons, typically with energies exceeding 10 GeV, reach the underground ALICE cavern at a measured rate of approximately 6.3 Hz per m², corresponding to a total single-muon trigger rate of around 100 Hz across the detector's area.^[58] By triggering on multiple coincidences (e.g., n=2 or higher within a 100 ns window), ACORDE facilitates studies of muon bundles from extensive air showers, probing primary cosmic ray energies in the range of 10¹⁵ to 10¹⁷ eV.^[61] In addition to its scientific objectives, ACORDE plays a crucial role in the calibration and alignment of ALICE's tracking detectors. It generates reference tracks from cosmic muons that pass through the central barrel, allowing precise determination of geometrical positions and timing parameters for detectors such as the Time Projection Chamber (TPC) and Inner Tracking System (ITS).^[62] These tracks, collected during LHC downtime, enable alignment resolutions down to tens of micrometers in the ITS, independent of collision data.^[63] ACORDE's signals are integrated into ALICE's Level-0 trigger system to select cosmic events efficiently.^[64] Key results from ACORDE-enabled measurements include determinations of the cosmic muon flux and charge ratio. The atmospheric muon flux at 1 TeV has been quantified as 0.225 m⁻² s⁻¹ sr⁻¹ TeV⁻¹, based on data from LHC Run 2 (2015–2018), with over 165 million single-muon events and 15,702 multimuon events (N_μ > 4) recorded over 62.5 days of live time.^[65] The muon charge ratio, R = μ⁺/μ⁻, has been measured using ACORDE-triggered tracks in the TPC for momenta from 20 GeV/c to 300 GeV/c, yielding values consistent with theoretical predictions and indicating a slight excess of positive muons due to pion and kaon production asymmetries in the atmosphere.^[58] These analyses have confirmed that high-multiplicity muon events (N_μ > 100) arise from primary cosmic rays dominated by heavy nuclei like iron, with event rates around 2.4 × 10⁻⁶ Hz, aligning with air-shower simulations such as QGSJET-II-04.^[65]

Data Acquisition and Operations

Trigger and Readout Systems

The ALICE experiment utilizes a multi-level trigger architecture to manage the high interaction rates at the LHC, selectively filtering events for readout and storage while preserving those relevant to heavy-ion physics. The system comprises three hardware-based trigger levels: Level 0 (L0), Level 1 (L1), and Level 2 (L2). The L0 trigger, executed in dedicated hardware within approximately 1.2 μs of the collision, processes inputs from fast detectors to issue an accept decision at rates up to several hundred kHz, depending on the collision type—such as 8 kHz for Pb-Pb and up to 300 kHz for proton-proton interactions.^[66]^[67] This level primarily relies on simple thresholds from detectors like the V0 for centrality and multiplicity estimation, and the EMCal for high-energy jet and photon triggers.^[68] Following L0 acceptance, the L1 trigger incorporates additional data from the Time Projection Chamber (TPC) after about 6.5 μs, reducing the rate to around 10-20 kHz by applying more refined selection criteria on track multiplicity and transverse energy. The L2 trigger, operating with a latency of up to 100 μs, uses software-based algorithms on partial event reconstructions to further suppress the rate to approximately 1 kHz, focusing on physics signatures like rare particle decays or high-multiplicity events.^[69]^[70] These levels collectively enable efficient data sparsification in the dense collision environment, where detector inputs from the central barrel and forward systems provide the necessary signals without full event reconstruction at this stage.^[67] The readout system transfers accepted event data from detectors to the data acquisition framework via approximately 1,200 optical fiber links, handling a raw data volume of about 1 TB/s in high-rate scenarios. Raw events, particularly central Pb-Pb collisions, can reach sizes up to 100 MB before compression, which reduces them to manageable levels for storage—typically 3-4 MB per event post-processing.^[68]^[71] The architecture employs Common Readout Units (CRUs) with 10 Gbps optical links to aggregate and stream data synchronously with trigger decisions.^[68] For LHC Run 3, starting in 2022, ALICE transitioned to a continuous readout mode, eliminating traditional hardware triggers for most detectors to capture all interactions at elevated rates: up to 50 kHz for Pb-Pb collisions and higher for proton-proton (up to 400-500 kHz minimum-bias). This upgrade, supported by the new O2 framework, enables real-time data reduction through on-the-fly compression and software filtering, sustaining the ~1 TB/s throughput while prioritizing centrality and jet triggers from V0 and EMCal for event classification.^[68]^[72]

Data Processing and Analysis

The ALICE experiment utilizes the Worldwide LHC Computing Grid (WLCG), a distributed computing infrastructure, to manage the storage, processing, and analysis of its extensive datasets generated from LHC collisions. This grid integrates resources from over 170 computing centers worldwide, enabling ALICE to handle peak data rates of up to 770 GB/s during heavy-ion runs and forward approximately 50 PB of processed data annually to the WLCG for long-term storage and analysis.^[73]^[74] The system supports both synchronous reconstruction immediately after data taking and asynchronous processing on the grid, with about 56% of 2023 Pb-Pb data reconstructed on the Event Processing Nodes (EPNs) before distribution. In 2024, ALICE recorded another high-luminosity Pb-Pb run, accumulating comparable data volumes to 2023, with continued use of EPNs for reconstruction.^[74]^[33] Event reconstruction in ALICE begins with cluster finding in the detector layers, particularly in the Time Projection Chamber (TPC), where ionization clusters are identified from raw signals. This is followed by track seeding and fitting using a Kalman filter algorithm, which iteratively refines track parameters by propagating hypotheses through the detector geometry while accounting for multiple scattering and energy loss.^[75]^[76] Particle identification (PID) integrates measurements from the TPC (via specific energy loss, dE/dx), Inner Tracking System (ITS), Time-Of-Flight (TOF) detector, and other subsystems, often employing Bayesian methods to assign probability densities for particle species during or after track fitting. In the upgraded O² framework for Run 3 and beyond, GPU-accelerated reconstruction on the EPNs achieves up to 55 times faster TPC processing compared to CPUs, enabling real-time handling of continuous readout data at 50 kHz for Pb-Pb collisions.^[74]^[77] The core analysis workflow is supported by the AliRoot and AliPhysics software frameworks, built on the ROOT system, which provide object-oriented tools for detector simulation, reconstruction, calibration, and physics analysis. AliRoot handles low-level simulation and reconstruction tasks using GEANT4 for particle transport and FLUKA for hadronic interactions, while AliPhysics offers high-level analysis classes for tasks like event selection and histogram filling. To address complex environments, machine learning techniques have been integrated, such as deep neural networks for jet momentum reconstruction and background subtraction in heavy-ion collisions, improving accuracy in estimating underlying event contributions to jet measurements.^[78]^[74]^[79] These frameworks also facilitate Monte Carlo simulations of billions of events per production campaign to model physics processes and validate reconstruction efficiencies.^[74] Processing high-multiplicity events poses significant challenges, as central Pb-Pb collisions can produce up to 4,000 charged particles per event, resulting in over 1,000 reconstructed tracks and complex overlap in the TPC. Algorithms must correct for distortions like space charge effects in the TPC, achieving residual precisions of about 100 μm, while managing computational demands through parallelization on multi-core CPUs and GPUs. Data from reconstruction is stored in formats such as Compressed Time Frames (CTF) for raw-like data and Analysis Object Data (AOD) for reduced, analysis-ready outputs, with AOD files being 7-10 times smaller than CTFs to optimize grid storage and access. The legacy Event Summary Data (ESD) format, containing full reconstruction details, has largely been phased out in favor of these for efficiency in Run 3 operations.^[74]^[74]

Key Scientific Results

Quark-Gluon Plasma Formation and Properties

The formation of quark-gluon plasma (QGP) in heavy-ion collisions at the LHC is evidenced by the collective behavior of produced particles, particularly through anisotropic flow measurements that indicate a strongly interacting medium behaving as a near-perfect fluid. Measurements of the elliptic flow coefficient v_2(p_T) for charged particles in Pb-Pb collisions at \sqrt{s_{NN}} = 2.76 TeV by the ALICE experiment demonstrate a strong centrality dependence, consistent with pressure gradients driving the expansion of a hot, dense medium.^[80] Similarly, the triangular flow v_3, arising from initial-state density fluctuations, exhibits similar hydrodynamic response patterns, further supporting the creation of a deconfined QGP phase. These flow harmonics are well-reproduced by viscous hydrodynamic models with a shear viscosity to entropy density ratio \eta/s \lesssim 0.2, approaching the conjectured lower bound of $1/(4\pi) for a strongly coupled plasma, thus highlighting the perfect fluidity of the QGP. The highest temperatures achieved in the QGP are inferred from probes sensitive to the early evolution, such as hadron multiplicity yields and low-mass dilepton spectra. Statistical model fits to identified hadron yields in central Pb-Pb collisions at \sqrt{s_{NN}} = 2.76 TeV yield a chemical freeze-out temperature of approximately 156 MeV, close to the pseudocritical temperature T_c \approx 154 MeV from lattice QCD, indicating rapid hadronization near the phase boundary. Low-mass dilepton excess in the intermediate invariant-mass region (1-3 GeV/c^2) points to thermal radiation from the QGP, with an effective temperature around 250 MeV, but hydrodynamic extrapolations to initial times suggest peak temperatures of about 500 MeV, or roughly 3-4 times T_c, reflecting the extreme conditions at formation. The expansion dynamics of the QGP are described by boost-invariant hydrodynamic models, which assume longitudinal invariance and capture the rapid transverse and longitudinal evolution observed in ALICE data. These models, incorporating initial conditions from gluon saturation, predict a system lifetime of approximately 10 fm/c before hadronization, consistent with the duration required for flow observables to develop. The inferred expansion velocity approaches the speed of light, with radial and elliptic flows aligning with experimental transverse momentum spectra. Comparisons of hydrodynamic simulations with ALICE measurements validate an equation of state (EoS) for the QGP that is nearly ideal, approaching the Stefan-Boltzmann limit at high temperatures as predicted by lattice QCD calculations. The lattice-derived EoS, featuring a crossover transition at T_c and weak deviations from conformality above 2T_c, reproduces the observed particle ratios and flow coefficients when implemented in viscous hydrodynamics for Pb-Pb collisions.^[81] This agreement underscores the weakly interacting nature of the QGP at LHC energies, with trace effects from non-perturbative QCD dynamics.

Jet Quenching and Energy Loss

Jet quenching refers to the phenomenon where high-transverse momentum (high-p_T) partons traversing the quark-gluon plasma (QGP) lose significant energy through interactions with the medium, leading to suppression of high-p_T hadrons and jets in heavy-ion collisions compared to proton-proton references. In ALICE, this energy loss is quantified using the nuclear modification factor R_{AA}, which compares the yield of particles in nucleus-nucleus collisions to scaled proton-proton yields. For light hadrons in central Pb-Pb collisions at \sqrt{s_{NN}} = 5.02 TeV, R_{AA}(p_T) drops below 0.2 for p_T > 10 GeV/c in the 0-10% centrality class, indicating strong suppression due to parton energy loss in the QGP. This suppression persists up to p_T \approx 20 GeV/c, providing direct evidence of the medium's opacity to hard probes. Further insight into the energy loss mechanism comes from measurements of photon-tagged jets, where the photon's lack of strong interactions tags the initial parton energy, allowing isolation of medium-induced modifications. ALICE has measured the jet asymmetry A_J, defined as the difference in away-side jet p_T relative to inclusive jets, normalized by their sum, showing significant imbalance in central Pb-Pb collisions at \sqrt{s_{NN}} = 5.02 TeV. Models incorporating medium response and radiative energy loss, fitted to these A_J data, yield a jet quenching parameter \hat{q} \approx 1-2 GeV^2/fm, representing the transverse momentum squared imparted per unit length to the parton by the medium. This value aligns with expectations for a hot, dense QGP and highlights the role of gluon radiation in the quenching process. The QGP's response to the traversing parton includes excitation of the medium, modeled as wake effects and Mach cones in hydrodynamic simulations. These simulations predict a conical shock wave (Mach cone) forming when the parton propagates supersonically relative to the medium's speed of sound, accompanied by a diffusion wake of depleted particle density behind the jet.^[82] ALICE data on jet-hadron correlations exhibit enhancements in soft hadron yields on the away side, consistent with hydrodynamic medium response incorporating these wake structures.^[82] Such features underscore the QGP's collective behavior under jet perturbation. Recent ALICE measurements in oxygen-oxygen (O-O) collisions at \sqrt{s_{NN}} = 5.36 TeV, recorded in July 2025, provide the first hints of jet quenching in small collision systems. Preliminary analysis of high-p_T hadron and jet yields shows suppression patterns analogous to larger systems, suggesting QGP formation even in these lighter nuclei.^[83] This extends the study of medium-induced energy loss to regimes with smaller system sizes, bridging insights from proton-nucleus to heavy-ion collisions.^[83]

Quarkonium Production and Suppression

Quarkonium states, such as J/ψ and Υ, serve as sensitive probes of the quark-gluon plasma (QGP) formed in heavy-ion collisions, where color screening in the deconfined medium leads to sequential suppression of these bound states based on their binding energies and sizes. In the QGP, the color Debye screening radius decreases with temperature, reaching approximately 0.2 fm at T = 300 MeV, which is smaller than the size of excited states like ψ(2S) and Υ(2S), causing their dissociation, while ground states like J/ψ and Υ(1S) may partially survive or reform. This suppression is quantified by the nuclear modification factor R_AA, defined as the ratio of quarkonium yields in Pb-Pb collisions to those in proton-proton (pp) collisions scaled by the number of binary nucleon-nucleon collisions. Measurements by ALICE in Pb-Pb collisions at √s_NN = 2.76 TeV and 5.02 TeV reveal a centrality-dependent suppression, with stronger effects in central collisions due to higher medium density and temperature. For J/ψ production, ALICE measurements indicate an average R_AA ≈ 0.6 in central Pb-Pb collisions at mid-rapidity (|y| < 0.9) for low transverse momentum (p_T < 5 GeV/c), reflecting a balance between initial suppression from color screening and regeneration through charm quark recombination in the QGP. This regeneration mechanism dominates at low p_T, where the high density of charm quarks from initial hard scatterings allows statistical recombination, partially offsetting the dissociation. At higher p_T, suppression increases due to reduced recombination efficiency and additional partonic energy loss, with R_AA dropping below 0.5. These observations are consistent across forward (2.5 < y < 4) and mid-rapidity, highlighting the QGP's role in modifying charmonium production compared to cold nuclear matter effects. Upsilon suppression is more pronounced owing to its larger binding energy and smaller size, making it a cleaner probe of high-temperature QGP phases. ALICE data show R_AA ≈ 0.4 for Υ(1S) in central Pb-Pb collisions at √s_NN = 2.76 TeV, integrated over 0 < p_T < 15 GeV/c and across rapidity ranges, with even stronger suppression for excited states like Υ(2S) and Υ(3S), where R_AA approaches 0.2-0.3 due to sequential melting as the screening radius falls below their Bohr radii. Invariant yield measurements confirm this hierarchy, while the elliptic flow coefficient v_2 for Υ(1S) at forward rapidity (2.5 < y < 4) in 5-60% central Pb-Pb collisions at √s_NN = 5.02 TeV is consistent with zero (v_2 ≈ 0 within uncertainties), indicating minimal thermalization or regeneration contributions compared to J/ψ. To disentangle hot medium effects from cold nuclear matter (CNM) phenomena, ALICE establishes baselines using p-Pb collisions, where no QGP forms. In p-Pb at √s_NN = 5.02 and 8.16 TeV, the nuclear modification factor R_pPb for J/ψ and Υ shows suppression at forward rapidity due to CNM effects like shadowing, well-described by EPS09 nuclear parton distribution functions (nPDFs) at next-to-leading order, with R_pPb ≈ 0.7-0.9 at mid-rapidity and lower values (~0.6) at forward y, without centrality dependence. For Υ(1S), similar shadowing leads to mild suppression (R_pPb ≈ 0.8), increasing toward backward rapidity due to anti-shadowing, providing essential normalization for Pb-Pb R_AA interpretations.

Collective Flow and Correlation Structures

In heavy-ion collisions studied by the ALICE experiment, collective flow manifests as anisotropic azimuthal distributions of particles, characterized by flow harmonics v_n, which provide insights into the medium's response to initial geometric asymmetries.^[84] The second-order harmonic, elliptic flow v_2, arises primarily from the almond-shaped overlap region in non-central Pb-Pb collisions and reaches values around 10% for charged hadrons in mid-central events at \sqrt{s_{NN}} = 2.76 TeV.^[84] Measurements of v_2 for identified particles reveal a characteristic mass ordering at low transverse momenta (p_T < 2 GeV/c), where lighter particles like pions exhibit higher v_2 than heavier ones such as protons or hyperons, consistent with hydrodynamic models incorporating radial expansion.190) Higher-order harmonics, such as the triangular flow v_3 and quadrangular flow v_4, originate from event-by-event fluctuations in the initial energy density distribution, leading to non-boost-invariant triangular and quadrilateral asymmetries.^[85] In central Pb-Pb collisions at \sqrt{s_{NN}} = 2.76 TeV, v_3 for charged particles achieves magnitudes comparable to v_2, while v_4 is smaller but significant, with both harmonics increasing toward mid-centralities before decreasing in peripheral events.^[85] These are quantified using multi-particle cumulants, such as c_n\{2\} and c_n\{4\}, which isolate genuine collective contributions by suppressing short-range non-flow effects like resonances and jets.^[85] In proton-lead (p-Pb) collisions, ALICE observations of long-range correlations in pseudorapidity (\Delta\eta) and azimuthal angle (\Delta\phi) reveal a double-ridge structure in high-multiplicity events at \sqrt{s_{NN}} = 5.02 TeV, featuring symmetric peaks on the near- and away-side over |\Delta\eta| > 1. This pattern, extending up to \Delta\eta \approx 4, suggests a possible hydrodynamic-like response in small collision systems, where the ridge amplitude scales with event multiplicity and resembles flow-driven correlations seen in Pb-Pb. To measure these harmonics accurately, ALICE employs the event-plane method, which estimates the reaction-plane angle from subevent multiplicities and corrects for resolution, alongside the Q-cumulant technique that leverages multi-particle correlations to subtract non-flow contributions effectively. These approaches confirm that the observed flow patterns align with quark-gluon plasma properties, such as near-ideal hydrodynamic behavior in larger systems.^[84]

Recent Advances in Small Systems and Exotic Collisions

In the 2020s, the ALICE experiment has extended investigations of quark-gluon plasma (QGP)-like effects to smaller collision systems, including proton-proton (pp), proton-lead (p-Pb), and light-ion collisions, revealing signatures of collectivity and medium formation that challenge traditional views of QGP emergence solely in large heavy-ion systems.^[86] These studies build on baseline observations from lead-lead (Pb-Pb) collisions, where collective flow and parton energy loss establish QGP properties, but focus on how similar phenomena scale to reduced system sizes.^[87] In high-multiplicity pp collisions at √s = 13 TeV, ALICE has observed azimuthal anisotropies indicative of collectivity, with the second-order elliptic flow coefficient v₂ exhibiting values comparable to those in Pb-Pb collisions when scaled to equivalent charged-particle multiplicity densities dN_ch/dη ~20–30.^[86] This multiplicity scaling suggests hydrodynamic-like evolution even in small systems, supported by long-range two-particle correlations and enhanced strange particle production, though the underlying mechanism—whether initial-state geometry or final-state interactions—remains under debate.^[87] Similarly, in p-Pb collisions at √s_NN = 5.02 TeV, the nuclear modification factor for charged jets R_pPb approximates unity across a broad transverse momentum range, indicating minimal overall suppression consistent with cold nuclear matter effects rather than strong jet quenching.^[88] However, suppression in the associated ridge structure—long-range azimuthal correlations—points to localized medium formation influencing high-p_T particle yields.^[89] The 2025 oxygen-oxygen (O-O) collision campaign at √s_NN = 5.36 TeV provided the first direct probe of flow in this light-ion system, with measured elliptic (v₂) and triangular (v₃) flow harmonics displaying centrality dependence that scales with system size relative to heavier ions like Pb-Pb and neon-neon (Ne-Ne).^[90] These harmonics, extracted via multi-particle cumulants, align with viscous hydrodynamic predictions incorporating nuclear geometry, showing reduced values compared to Pb-Pb but coherent patterns consistent with QGP-like expansion in smaller volumes.^[27] Ne-Ne collisions, also recorded in 2025 at the same energy, extend this scaling, with v₂ increasing in central events due to prolate nuclear shape effects, enabling future comparative studies of geometry-driven collectivity across light systems.^[27] Exotic near-miss collisions in ALICE have further illuminated nuclear transmutation processes, with 2025 analyses of lead-ion interactions revealing the production of approximately 86 billion gold-197 (from lead-208 via proton knockout) nuclei during LHC Run 2, equivalent to a fleeting mass of about 28 picograms.^[91] This electromagnetic dissociation mechanism, occurring in ultraperipheral Pb-Pb events, demonstrates precise control over nuclear reactions in high-energy environments. The ALICE Collaboration's contributions to small-system QGP evidence, including these flow and quenching observables, earned recognition in the 2025 Breakthrough Prize in Fundamental Physics, shared among LHC experiments for advancing QCD matter studies.^[8]

Upgrades and Future Plans

Long Shutdown 1 Upgrades (2009-2010)

Following the initial proton-proton collisions recorded by the ALICE experiment in late 2009, the winter shutdown period from December 2009 to March 2010 provided an opportunity for post-commissioning upgrades to enhance detector performance ahead of higher-energy runs and the first lead-lead collisions. These modifications focused on improving tracking precision, particle identification, and triggering efficiency to better probe quark-gluon plasma signatures in heavy-ion collisions. The upgrades were completed in time for the resumption of data taking in spring 2010, culminating in readiness for the inaugural Pb-Pb run at √s_NN = 2.76 TeV in November 2010.^[92] A key enhancement involved the Inner Tracking System (ITS), particularly its innermost silicon pixel detector layers (SPD), which were fine-tuned and verified during the shutdown to achieve a transverse impact parameter (DCA) resolution better than 50 μm for low-p_T tracks (p_T < 1 GeV/c). This precision, enabled by the hybrid pixel technology with 50 × 425 μm² cell size, significantly improved the reconstruction of secondary vertices from heavy-flavor decays, such as D and B mesons, essential for studying charm and beauty production in the medium. The pixel layers' low material budget (∼0.8% X_0 per layer) minimized multiple scattering, contributing to the overall ITS-TPC tracking resolution of approximately 100 μm in the transverse plane at higher p_T.^[93]^[94] The Time Projection Chamber (TPC), ALICE's main tracking device, underwent gas system optimization by adopting a neon-carbon dioxide (Ne-CO₂) mixture in a 90:10 ratio, which reduced electron attachment rates compared to alternative gases like Ar-based mixtures and enhanced drift velocity stability (∼2.5 cm/μs). This choice minimized signal losses due to attachments (attachment coefficient < 0.1% per cm) and supported high track densities in central Pb-Pb events, with diffusion limited to ∼400 μm at the outer radius. The optimization ensured reliable operation at the nominal gain of ∼2000 while maintaining low ion backflow, crucial for preserving electric field uniformity in dense collision environments.^[95]^[96] Trigger capabilities were bolstered by advancing the installation of the Electromagnetic Calorimeter (EMCal), with four supermodules (∼10% coverage) already operational in 2009 and additional modules integrated during the shutdown to reach ∼20% azimuthal acceptance. The EMCal, comprising lead-scintillator sampling layers with high granularity (Δη × Δφ ≈ 0.014 × 0.014), enabled level-1 triggers for jets and high-p_T photons/electrons (E_T > 5 GeV), suppressing minimum-bias rates by factors of 100-1000 and allowing efficient selection of hard probes for jet quenching studies. This upgrade complemented the central barrel's photon identification and was pivotal for the first measurements of neutral meson production in Pb-Pb collisions.^[97]^[92]

Long Shutdown 2 Upgrades (2018-2021)

During the Long Shutdown 2 (LS2) of the Large Hadron Collider, spanning from December 2018 to May 2022, the ALICE experiment underwent extensive upgrades to prepare for the high-luminosity conditions of Run 3, enabling continuous readout at interaction rates up to 50 kHz for lead-lead collisions. These modifications focused on enhancing tracking precision, reducing material budgets, and improving data handling capabilities, with major installations occurring between 2019 and 2021 and commissioning completed in 2022. Key hardware and software advancements included the replacement of core detectors and the introduction of a unified computing framework to manage the increased data volume. The Inner Tracking System (ITS) was upgraded to ITS2, a seven-layer silicon pixel detector utilizing ALPIDE Monolithic Active Pixel Sensor (MAPS) chips, covering approximately 10 m² with 12.5 billion pixels. To minimize multiple scattering and radiation length (X₀), the inner layers feature bent staves formed into cylindrical geometries, reducing the material budget to 0.35% X₀ per layer while maintaining structural integrity. This design achieves a spatial resolution of about 5 μm in both rφ and z directions, improving impact parameter resolution by a factor of 3 transversely and 6–10 longitudinally compared to the previous ITS, which enhances vertex reconstruction for heavy-flavor decays and low-p_T particle tracking. Installation of ITS2 occurred from January to May 2021, integrated within a new carbon fiber support cage that also houses the beam pipe. The Time Projection Chamber (TPC) received a major overhaul with the replacement of its Multi-Wire Proportional Chamber (MWPC) readout by Gas Electron Multiplier (GEM) chambers, comprising four GEM stacks per endplate in an S-LP-LP-S configuration to amplify signals efficiently. This upgrade eliminates the need for a gating grid, allowing continuous readout mode without ion backflow exceeding 2%, and supports particle identification via dE/dx measurements with 5% resolution. The GEM-based system facilitates online track reconstruction and handles the high data rates from Run 3, with the 15-month installation completed by August 2020 and initial testing during pilot beam runs in October 2021. Additionally, the Muon Forward Tracker (MFT) was newly installed as a five-disk silicon pixel detector using 936 ALPIDE sensors arranged in 500 ladders, positioned in the forward pseudorapidity region (–4 < η < –2.5) to complement the muon spectrometer. It provides ~100 μm pointing resolution and low noise occupancy below 10⁻⁷ hits per pixel per event, enabling precise tracking of low-p_T muons from heavy-flavor decays and improving mass resolution for forward particle identification. Installation of the MFT, integrated into the ITS2 support structure and cooled via water-based heat exchangers, was finalized in December 2020, with data readiness achieved by October 2021. The O₂ computing framework was developed and deployed during LS2 to unify synchronous online reconstruction—aligned with detector readout for calibration and compression—with asynchronous offline processing, handling raw data rates up to 3.4 TB/s (primarily from the TPC) and compressed outputs at 130 GB/s. Built on a multi-process, message-passing architecture with GPU acceleration, O₂ integrates First-Level Processors (FLPs) and Event Processing Nodes (EPNs) for real-time event building and analysis, scalable to Run 3 demands. Implementation spanned 2019–2021, with full commissioning by October 2021, enabling ALICE's first proton-proton collisions at 13.6 TeV in July 2022.

Run 3 Enhancements and Operations (2022-2025)

Run 3 of the LHC, commencing in July 2022, marked a significant phase for the ALICE experiment, leveraging upgrades from Long Shutdown 2 to enable high-rate data collection focused on heavy-ion and proton-ion collisions. The period from 2022 to 2025 emphasized operational stability, enhanced data throughput, and advanced analysis techniques to probe quark-gluon plasma properties at unprecedented scales. ALICE transitioned to a triggerless, continuous readout mode, allowing the capture of all collision events without hardware triggers, which dramatically increased the recorded luminosity compared to previous runs.^[98] A key enhancement was the implementation of continuous readout capabilities, supported by the upgraded Time Projection Chamber (TPC) and Inner Tracking System (ITS) from LS2, enabling interaction rates of up to 50 kHz for Pb-Pb collisions and 400 kHz for pp collisions. This resulted in a raw data rate of 3.5 Tb/s from the detectors, processed in real-time using the O^2 framework on GPU clusters for synchronous reconstruction and compression to manage the volume. To address high pileup in dense environments, machine learning algorithms were integrated into the track reconstruction pipeline within O^2, improving pattern recognition and efficiency in identifying tracks amid overlapping events, particularly in high-multiplicity pp and p-Pb collisions.^[99] Luminosity levelling techniques were employed to maintain stable interaction rates, targeting consistent conditions for Pb-Pb runs at approximately 50 kHz, which supported the collection of integrated luminosities exceeding previous campaigns. Key data-taking periods included Pb–Pb collisions in 2023 and 2024, extensive pp collisions in 2024 at 13.6 TeV to study baseline QCD processes, and novel O–O collisions alongside pp in 2025 at 5.36 TeV per nucleon pair to explore small-system dynamics and geometry effects in light-ion interactions. During a short O–O run in July 2025, ALICE recorded the first oxygen-oxygen collisions at the LHC, collecting data to study quark-gluon plasma formation in small systems. These runs yielded datasets enabling detailed studies of collective phenomena and parton energy loss.^[100]^[101]^[102] Operational challenges during Run 3 included mitigating radiation damage to detector components, addressed through regular monitoring, recalibration, and material replacements based on fluence predictions post-LS2. The ALICE collaboration held dedicated ALICE Upgrade Week meetings annually to review performance, troubleshoot issues like background noise and pileup, and plan incremental improvements for sustained operations through 2025. These efforts ensured robust data quality and maximized physics output amid increasing LHC intensities.^[103]

Long Shutdown 3 and Beyond (ITS3, FoCal, ALICE3)

The Long Shutdown 3 (LS3) of the LHC, scheduled from 2026 to 2028, will enable key upgrades to the ALICE experiment to enhance its performance for Run 4, focusing on improved tracking precision and forward physics capabilities. These include the replacement of the innermost three layers of the Inner Tracking System (ITS3) and the addition of the Forward Calorimeter (FoCal). These modifications aim to support higher interaction rates, up to 100 kHz for Pb-Pb collisions, while advancing studies of quark-gluon plasma properties through better vertex reconstruction and photon identification at forward rapidities.^[104]^[105]^[106] The ITS3 upgrade replaces the innermost layers of the current ITS with a novel three-layer system based on curved, wafer-scale Monolithic Active Pixel Sensors (MAPS) fabricated in a 65 nm CMOS process. These sensors, with a thickness of 50 μm and dimensions up to 26 × 10 cm², are stitched together to form truly cylindrical layers with a reduced material budget and closer proximity to the interaction point, minimizing multiple scattering. This design achieves a transverse pointing resolution better than 10 μm and a spatial resolution of approximately 5 μm, representing a factor-of-two improvement over the existing ITS and enabling more precise heavy-flavor hadron tracking at low transverse momenta. Installation is planned for LS3, with operations starting in Run 4 around 2029.^[104]^[107]^[108]^[109] FoCal introduces a high-granularity forward calorimeter to measure electromagnetic and hadronic showers, specifically for neutral pion (π⁰) and direct photon (γ) reconstruction in the pseudorapidity range |η| ≈ 3–5. The detector features a silicon-tungsten sampling electromagnetic calorimeter with longitudinal and transverse segmentation, paired with a hadronic calorimeter using steel and scintillator, providing π⁰/γ separation via shower shape analysis. Approved by the LHC Experiments Committee in March 2024 following its Technical Design Report, FoCal will extend ALICE's sensitivity to forward particle production, aiding investigations of initial-state effects and gluon saturation in heavy-ion collisions. It will be installed during LS3 for data-taking in Run 4.^[110]^[111]^[104]^[112] Looking beyond LS3, the ALICE3 project proposes a comprehensive replacement of the central barrel detectors post-2030, targeting Runs 5 and 6 to cope with even higher luminosities and precision requirements. Central to this is a new silicon-pixel tracker covering |η| < 4 with pointing resolution below 10 μm, complemented by an upgraded Time Projection Chamber (TPC) using Gas Electron Multiplier (GEM) readout in a neon-based gas mixture for continuous, high-rate operation and improved momentum resolution. This configuration, with enhanced granularity in both tracking and particle identification systems, will enable detailed mapping of quark-gluon plasma evolution over a broader rapidity range. The initiative stems from a Letter of Intent submitted in 2022 and a Scoping Document reviewed in 2025, emphasizing advancements in forward physics and rare probe measurements.^[113]^[114]^[115]

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