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Hadron collider

A hadron collider is a type of particle accelerator that accelerates beams of hadrons—composite particles such as protons, antiprotons, or heavy ions—to near the speed of light and collides them head-on at extremely high energies to study the fundamental constituents of matter and the forces governing them.^[1]^[2] These machines operate by confining particle beams in circular rings using powerful superconducting magnets, achieving center-of-mass energies ranging from tens of GeV in early designs to 13.6 TeV in modern ones, with luminosities up to 10^{34} cm^{-2} s^{-1} to enable rare event detection.^[2] The concept of hadron colliders emerged in the mid-20th century as physicists sought higher energies than fixed-target experiments could provide, with the first operational example being CERN's Intersecting Storage Rings (ISR) in 1971, a proton-proton collider reaching √s ≈ 60 GeV.^[1] Subsequent advancements included the Super Proton Synchrotron (SPS) at CERN, repurposed in the 1980s for proton-antiproton collisions at √s = 540 GeV, and Fermilab's Tevatron, operational from 1983 to 2011 with energies up to √s = 1.96 TeV.^[1] The current flagship, the Large Hadron Collider (LHC) at CERN, a 27 km circumference proton-proton and heavy-ion collider, began operations in 2008 and, as of 2025, operates in Run 3 at collision energies of 13.6 TeV, including first oxygen-ion runs in July 2025.^[2]^[3] Hadron colliders have revolutionized particle physics by enabling discoveries of key Standard Model particles, including the W and Z bosons at the SPS in 1983,^[1] the top quark at the Tevatron in 1995,^[2] and the Higgs boson at the LHC in 2012.^[2] These facilities rely on sophisticated detectors, such as ATLAS and CMS at the LHC, to analyze collision debris for signatures of new physics, including potential supersymmetric particles or extra dimensions, while also providing precision measurements of known phenomena like quark distributions within hadrons via parton distribution functions.^[2] Ongoing upgrades, like the High-Luminosity LHC scheduled to begin in 2030, aim to increase data collection by a factor of ten to further explore the energy frontier.^[4]

Fundamentals

Definition and purpose

A hadron collider is a type of particle accelerator designed to accelerate beams of hadrons—composite particles such as protons or heavy ions—to speeds approaching that of light and collide them head-on or at angles to produce high-energy interactions.^[5]^[6] The fundamental purpose of these machines is to investigate the building blocks of matter and the forces governing them by simulating the extreme temperatures and densities of the early universe, particularly through the study of quark-gluon plasma in heavy-ion collisions; to examine the properties and interactions of subatomic particles; to verify predictions of quantum chromodynamics (QCD), the theory describing the strong nuclear force; and to seek evidence of new physics beyond the Standard Model, such as supersymmetric particles or extra dimensions.^[6]^[7]^[8] At a high level, hadron colliders incorporate linear accelerators to impart an initial energy boost to the hadron beams, large circular rings serving as synchrotrons for sustained acceleration and storage, superconducting magnets to focus the beams and guide their paths around the ring, and specific interaction points equipped with detectors to capture and analyze collision debris.^[2]^[9] Collision energies in hadron colliders are quantified in units of giga-electronvolts (GeV) or tera-electronvolts (TeV), referring to the total center-of-mass energy available; notable examples include the Fermilab Tevatron's peak of 1.96 TeV for proton-antiproton collisions and the CERN Large Hadron Collider's 13.6 TeV for proton-proton interactions (as of Run 3, 2022–present).^[5]^[3]

Hadron types and properties

Hadrons are composite subatomic particles bound together by the strong nuclear force, as described by quantum chromodynamics (QCD), the theory of strong interactions. In QCD, hadrons consist of elementary quarks confined by gluons, which mediate the color force between quarks, ensuring that quarks do not exist in isolation due to color confinement.^[10]^[11] Hadrons are broadly classified into two categories based on their quark content: baryons and mesons. Baryons are fermions composed of three quarks, such as the proton (uud) and neutron (udd), where u and d denote up and down quarks, respectively. Mesons, in contrast, are bosons formed from a quark-antiquark pair, exemplified by the charged pions π⁺ (u¯d) and π⁻ (d¯u). This distinction arises from the baryon number conservation, with baryons having B = +1 and mesons B = 0.^[10] Key quantum properties of hadrons include mass, spin (total angular momentum), electric charge, and isospin, which reflects symmetry between up and down quarks. The proton has a rest mass of 938.272 MeV/c², spin 1/2, charge +1 e, and isospin I = 1/2. The neutron possesses a mass of 939.565 MeV/c², the same spin of 1/2, zero charge, and isospin I = 1/2. For mesons, charged pions have masses of approximately 139.57 MeV/c², spin 0, charges ±1 e, and isospin I = 1. These properties govern hadron interactions and are crucial for interpreting collider data.^[12]^[13] Regarding stability, protons are stable particles with no observed decay and a lower limit on their mean lifetime exceeding 10³⁴ years, making them ideal for long-term beam storage. Neutrons, however, are unstable when free from nuclear binding, undergoing beta decay (n → p + e⁻ + ¯ν_e) with a mean lifetime of 878.4 seconds, or about 14.6 minutes. Mesons like pions are highly unstable, decaying electromagnetically or weakly within nanoseconds.^[12]^[13] In the context of hadron colliders, protons serve as primary beam particles owing to their stability and abundance in nature, enabling high-luminosity proton-proton collisions. Antiprotons, the antiparticles of protons with opposite charge but identical mass and spin, must be artificially produced through high-energy proton interactions with targets and subsequently cooled for use in beams, as in historical proton-antiproton colliders. Heavy ions, such as lead nuclei (comprising multiple protons and neutrons), are utilized to recreate extreme conditions mimicking the early universe, probing quark-gluon plasma formation.

History

Early concepts and prototypes

Following World War II, the burgeoning field of particle physics, deeply intertwined with quantum field theory, demanded experimental tools capable of probing subatomic interactions at unprecedented energies to test theoretical predictions and explore phenomena like meson production and nuclear forces.^[14] This era saw accelerators evolve from wartime applications to dedicated high-energy instruments, with theoretical advancements in beam dynamics, such as phase stability proposed by Edwin McMillan and Vladimir Veksler in 1945, enabling the design of synchrotrons that maintained particle bunches during acceleration.^[14] Pioneering physicists like Enrico Fermi, who advanced understanding of particle interactions and contributed to early accelerator technologies including synchrocyclotrons at the University of Chicago, and Robert R. Wilson, who developed electron synchrotrons at Cornell and applied betatron principles to proton machines, laid crucial groundwork for these innovations.^[15]^[16] The first operational proton synchrotron, the Cosmotron at Brookhaven National Laboratory, came online in 1952, injecting protons from a Van de Graaff accelerator and boosting them to 3 GeV in a 75-foot-diameter ring using 288 electromagnets.^[17] This machine marked a leap from earlier cyclotrons by achieving relativistic energies while compensating for mass increase through frequency modulation, allowing systematic studies of particles previously observed only in cosmic rays.^[14] Just two years later, the Bevatron at Lawrence Berkeley National Laboratory activated in 1954, delivering 6.2 GeV protons via a larger 90-foot ring and enabling groundbreaking discoveries, including the production and identification of strange particles like the lambda baryon and kaons through high-energy collisions with targets.^[18]^[19] Building these prototypes presented formidable engineering hurdles in the 1950s and 1960s, including beam stability reliant on weak focusing that struggled against space charge effects and resonances, necessitating precise control of particle orbits to prevent losses.^[14] Vacuum systems had to reach ultra-low pressures around 10^{-6} Torr to avoid scattering from residual gas, while magnet technology was constrained by the need for high-field iron-core electromagnets—typically 10,000–15,000 gauss—that limited ring sizes and energy gains without excessive power consumption or hysteresis issues.^[14] Synchrotron radiation, an energy-sapping effect prominent at relativistic speeds, further complicated operations, particularly for electrons but increasingly relevant for protons in these machines.^[14] By the mid-1950s, physicists recognized the inefficiencies of fixed-target setups, where much of the beam energy was wasted on target motion, prompting a conceptual shift toward colliding beams to maximize effective center-of-mass energies.^[20] In 1956, Donald W. Kerst and collaborators proposed intersecting storage rings where counter-rotating particle beams would collide head-on, potentially achieving equivalent energies far beyond single-beam limits without proportionally larger accelerators, thus laying the foundation for modern hadron colliders.^[20] This idea, initially explored for electrons, soon extended to hadrons, addressing the era's energy frontiers through innovative beam storage and intersection geometries.^[14]

Development of high-energy facilities

The development of high-energy hadron colliders accelerated in the 1970s with the commissioning of CERN's Intersecting Storage Rings (ISR), the world's first dedicated hadron collider. Operational from 1971 to 1984, the ISR achieved proton-proton collisions at center-of-mass energies up to 62 GeV, demonstrating the practical feasibility of storing and colliding high-intensity hadron beams in intersecting rings fed by the CERN Proton Synchrotron. This breakthrough shifted particle physics from fixed-target experiments to collider-based studies, enabling higher effective energies and luminosity for exploring strong interactions.^[21] The 1980s and 1990s brought significant scaling through proton-antiproton colliders, leveraging antiproton production and cooling techniques. CERN's Super Proton Synchrotron (SPS), originally a proton accelerator completed in 1976, was converted into a collider in 1981, achieving first proton-antiproton collisions at energies up to 315 GeV per beam (center-of-mass 630 GeV) and operating until 1991 before transitioning to other roles. In parallel, the Tevatron at Fermilab began collider operations in 1983, becoming the highest-energy facility at the time with proton and antiproton beams accelerated to 980 GeV each, yielding a center-of-mass energy of 1.96 TeV; it ran until 2011, pioneering large-scale superconducting acceleration. These projects, supported by stochastic cooling to manage beam quality, established the infrastructure for TeV-scale physics.^[22] Entering the 2000s, the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory marked a milestone in heavy-ion collision capabilities, with first gold-gold collisions in June 2000 at center-of-mass energies up to 200 GeV per nucleon pair. Unlike prior proton-focused machines, RHIC's 3.8 km dual rings enabled symmetric heavy-ion beams, facilitating investigations into nuclear matter under extreme conditions while also supporting polarized proton runs. This facility underscored the diversification of collider types beyond light hadrons.^[23]^[24] Engineering innovations were pivotal to these advances, particularly the adoption of superconducting magnets using niobium-titanium (Nb-Ti) alloys, which generate strong dipole fields (up to 4.4 T in the Tevatron) while minimizing energy losses. Cooled via liquid helium to 4.5 K in early systems like the Tevatron and to superfluid 1.9 K in subsequent designs, these magnets enabled compact, high-field rings essential for TeV energies. Complementary technologies included high-gradient radio-frequency (RF) cavities for beam acceleration—such as the 200 MHz systems in the ISR and upgraded superconducting niobium variants in RHIC—and robust cryogenic infrastructures to maintain vacuum and thermal stability across kilometer-scale tunnels.^[25]^[26]^[27] International collaboration drove these facilities' success, with CERN coordinating European resources and expertise for the ISR and SPS through its member states. In the United States, the Department of Energy provided core funding for the Tevatron (over $1 billion across its lifetime) and RHIC (initial construction ~$500 million), fostering transatlantic knowledge exchange. This culminated in global partnerships for the LHC era, exemplified by the ATLAS and CMS experiments, which involved over 10,000 scientists from 100+ countries and drew $531 million in direct US contributions plus in-kind support for detectors.^[28]^[29]

Operating principles

Particle acceleration and storage

In hadron colliders, the acceleration of hadrons to relativistic speeds typically commences in linear accelerators (linacs), which provide an initial low-energy boost using oscillating electric fields.^[30] These devices operate by propelling charged particles, such as protons or ions, through a series of gaps between conductive drift tubes within a resonant radiofrequency cavity, where the electric field accelerates the particles during the favorable phase of its oscillation.^[31] The drift tubes shield the beam from the reversing field, allowing particles to coast between acceleration gaps, with tube lengths progressively increasing to match the rising particle velocity and maintain synchronism.^[30] The Alvarez design, featuring a chain of coupled cylindrical resonators, remains the foundational structure for such linacs, enabling efficient injection energies up to tens of MeV for hadrons.^[31] For achieving higher energies required in colliders, hadrons are transferred to synchrotrons, circular accelerators that employ time-varying magnetic fields to incrementally increase the beam's momentum while guiding it along a fixed orbit radius.^[32] In these machines, radiofrequency cavities provide the longitudinal acceleration, synchronized with the magnetic ramping to keep particles on the design trajectory as their rigidity grows.^[32] A key challenge in synchrotron acceleration of hadrons is the emission of synchrotron radiation, an electromagnetic loss mechanism arising from the centripetal acceleration in the bending magnets, which becomes more pronounced at ultra-relativistic energies.^[33] The power radiated by a single relativistic particle is given by

P = \frac{2}{3} \frac{q^2 a^2 \gamma^4}{4\pi \epsilon_0 c^3},

where q is the particle charge, a is the acceleration, \gamma is the Lorentz factor, \epsilon_0 is the vacuum permittivity, and c is the speed of light; for hadrons in strong bending fields, this loss is mitigated by their higher mass compared to electrons, allowing larger ring circumferences without prohibitive energy dissipation.^[33] Once accelerated, hadron beams are stored in the collider rings, where stability is maintained through careful control of the betatron tune, defined as the number of transverse oscillations per revolution, which must be tuned to irrational values to avoid resonances that could amplify instabilities.^[34] The fractional part of the tune is precisely adjusted to optimize the dynamic aperture and prevent emittance growth from collective effects like beam-beam interactions.^[32] To counteract emittance—the phase space volume characterizing beam quality—cooling techniques are employed, such as stochastic cooling, which uses noise detectors and kickers to damp random particle motions by correlating position and momentum fluctuations across the bunch.^[35] Electron cooling complements this by merging the hadron beam with a co-propagating electron beam of matched velocity, transferring thermal energy from hadrons to electrons, which radiate efficiently due to their low mass, thereby reducing transverse and longitudinal emittances.^[32] The process of populating the main collider ring involves multi-stage injection from booster synchrotrons, where lower-energy beams are transferred in batches to accumulate the required number of bunches, followed by ramping of the magnetic fields to elevate the energy from injection levels (typically hundreds of GeV) to operational values, such as 7 TeV per beam.^[36] During ramping, the beam orbit and optics are dynamically adjusted to accommodate the increasing rigidity, ensuring stable transport over the acceleration cycle lasting tens of minutes.^[36] Key beam parameters in stored hadron bunches determine the collider's performance, particularly the luminosity L, which governs the expected collision rate and is expressed as

L = \frac{N_b f_\mathrm{rev} n_b}{4\pi \sigma_x \sigma_y},

where N_b is the number of particles per bunch, f_\mathrm{rev} is the revolution frequency, n_b is the number of colliding bunch pairs per crossing, and \sigma_x, \sigma_y are the horizontal and vertical RMS beam sizes at the interaction point.^[37] Optimizing these parameters through cooling and precise optics minimizes \sigma_x and \sigma_y while maximizing N_b and n_b, thereby enhancing the probabilistic overlap of counter-rotating beams.^[37]

Collision dynamics

In hadron colliders, the kinematics of collisions are determined by the center-of-mass energy, denoted as \sqrt{s}, which for counter-rotating beams of equal energy E is given by \sqrt{s} = 2E.^[38] This energy scale sets the available energy for particle production and governs the overall collision dynamics. However, since hadrons are composite objects, the actual hard interactions occur between their constituent partons (quarks and gluons), with the parton-parton center-of-mass energy \hat{s} related to the hadron-hadron \sqrt{s} by \hat{s} = x_1 x_2 s, where x_1 and x_2 are the longitudinal momentum fractions carried by the interacting partons.^[39] The primary processes in hadron collisions are described by quantum chromodynamics (QCD), dividing into hard and soft components. Hard scattering involves high-momentum-transfer quark-gluon interactions, calculable perturbatively, where the partonic cross-section scales as \hat{\sigma} \sim \alpha_s^2 / \hat{s} with \alpha_s the strong coupling constant.^[38] In contrast, soft processes encompass non-perturbative effects like hadronization, where quarks and gluons combine into color-neutral hadrons through mechanisms modeled by Monte Carlo simulations.^[38] These processes contribute to the underlying event surrounding hard scatters, influencing the overall energy flow. High-transverse-momentum (p_T) partons produced in hard scatters fragment into jets, which appear as collimated sprays of particles in the final state. Jet formation arises from the perturbative splitting of partons followed by non-perturbative hadronization, quantified by fragmentation functions D(z), where z is the fraction of the parton's momentum carried by the resulting hadron.^[40] These functions evolve with the factorization scale according to DGLAP equations and are universal across collision types, enabling predictions of jet substructure in proton-proton environments.^[40] Hadron collisions produce diverse multi-particle final states, ranging from elastic scattering, where hadrons emerge intact, to highly inelastic events resembling deep-inelastic scattering with significant energy transfer. Minimum-bias events, which capture generic inelastic interactions without selection on specific hard processes, dominate the total cross-section and provide insights into soft QCD dynamics, often involving multiple parton interactions.^[41] In heavy-ion collisions, event topology is characterized by centrality, a measure of collision overlap defined by the impact parameter; central events (low impact parameter) yield higher particle multiplicities and more isotropic final states compared to peripheral ones.^[42] This parameterizes the geometry and probes collective effects in the produced medium, though detailed medium evolution is addressed elsewhere.^[42]

Event detection and analysis

In hadron colliders, event detection begins with specialized detector components designed to capture the diverse products of particle collisions. Tracking detectors, typically employing silicon pixel or strip sensors, reconstruct the trajectories of charged particles by measuring their positions as they traverse layers of material within a magnetic field, enabling momentum determination through curvature analysis. Calorimeters complement this by absorbing particles and quantifying their energy deposits: electromagnetic calorimeters, often using lead-scintillator or crystal technologies, target photons and electrons, while hadronic calorimeters, constructed with steel or copper absorbers interleaved with active media, measure the energy of strongly interacting hadrons. Muon systems, positioned outermost and shielded by the calorimeter and magnet yoke, identify penetrating muons using drift tubes, resistive plate chambers, or cathode strip chambers, as muons are the only charged particles that traverse the full detector volume without significant energy loss.^[43] To manage the enormous data rates from collisions—up to 40 million per second—trigger systems selectively filter events in real time. The Level-1 trigger, implemented in custom hardware like field-programmable gate arrays, processes raw data from calorimeters, muon detectors, and minimum-bias triggers within microseconds, reducing the rate to about 100 kHz by identifying high-transverse-momentum objects such as electrons, photons, taus, muons, and jets based on simple topological criteria. Subsequent software-based high-level triggers (HLT) then apply more sophisticated algorithms on partially reconstructed data, refining selections for specific physics signatures like b-jets or global event shapes, achieving a final rate of around 1 kHz for full event readout and storage. Data reconstruction transforms raw detector signals into physical objects through algorithmic pipelines. Particle flow (PF) algorithms integrate information from tracking and calorimetry to form clusters and tracks, associating charged particle tracks with calorimeter deposits while correcting for neutral particles and pile-up contamination, thereby improving jet and missing transverse energy resolution.^[44] Jets, representing collimated sprays of hadrons, are reconstructed by clustering these PF candidates using the anti-kT algorithm, which sequentially merges protojets based on distance measures favoring collinear emissions and yielding infrared- and collinear-safe cones. Analysis pipelines rely on Monte Carlo simulations to model event generation and detector response. Tools like PYTHIA generate hard scattering processes, parton showers, and hadronization for proton-proton collisions, providing simulated datasets that are passed through full detector simulations (e.g., GEANT4) and reconstruction chains to mimic real data. Statistical methods then discriminate signal from background using techniques such as likelihood fits, multivariate classifiers (e.g., boosted decision trees), and profile likelihood scans, quantifying excesses or limits on new physics through hypothesis testing that accounts for systematic uncertainties.^[45] High-luminosity operations, targeting up to $10^{34} cm^{-2}s^{-1}, introduce significant challenges including pile-up, where multiple interactions per bunch crossing (up to 200) overlap, complicating reconstruction and necessitating advanced mitigation like charged-hadron subtraction or data-driven pile-up modeling. Radiation hardness is another critical issue, as detectors must withstand fluences exceeding $10^{16} n_{\rm eq}/cm² over years of operation, requiring robust materials and designs to maintain efficiency against displacement damage and ionization effects.^[46]

Types of colliders

Proton-proton colliders

Proton-proton colliders accelerate beams of protons in opposite directions within a single ring, utilizing separate beam pipes to enable counter-rotating circulation without the requirement for antiproton production.^[47] This configuration simplifies operations compared to proton-antiproton systems, as protons can be sourced directly from accelerators like synchrotrons, and collisions occur at intersection points where detectors capture the resulting particles.^[48] The Intersecting Storage Rings (ISR) at CERN, operational from 1971 to 1984, represented the first such collider, achieving center-of-mass energies up to 62 GeV through two intersecting rings fed by the Proton Synchrotron.^[49] A key advantage of proton-proton colliders is their potential for higher luminosity, which measures the rate of particle interactions and can exceed 10^{34} cm^{-2} s^{-1} due to the ability to store more bunches without production bottlenecks.^[47] This setup also provides a cleaner initial state for experiments, as the identical proton beams minimize uncertainties from differing particle properties, facilitating precision measurements in quantum chromodynamics and electroweak interactions.^[48] For instance, the Large Hadron Collider (LHC) at CERN exemplifies this by delivering peak luminosities over 2 \times 10^{34} cm^{-2} s^{-1} at 13 TeV center-of-mass energy, enabling detailed studies of rare processes.^[47] These colliders excel in probing fundamental aspects of particle physics, particularly direct investigations of electroweak symmetry breaking through Higgs boson production and decays, as well as comprehensive analyses of top quark properties, including its mass, couplings, and decay channels.^[50] The ISR pioneered observations of proton substructure and jet production, laying groundwork for these pursuits, while the LHC has confirmed the Standard Model's predictions in high-energy proton collisions.^[49] However, beam lifetimes in these systems are constrained by intra-beam scattering, where protons within the same bunch interact and cause emittance growth, necessitating advanced cooling techniques like stochastic cooling.^[47] Despite these strengths, proton-proton colliders face limitations arising from the composite nature of protons, composed of valence quarks and gluons, which results in broader initial parton distribution functions compared to point-like electrons in lepton colliders.^[50] This structure smears the effective collision energy, introducing uncertainties in reconstructing parton-level kinematics and reducing precision for certain electroweak observables.^[47]

Proton-antiproton colliders

Proton-antiproton colliders accelerate beams of protons and their antiparticles, antiprotons, to high energies for head-on collisions, enabling studies of fundamental particle interactions through quark-antiquark annihilation processes.^[51] Unlike proton-proton setups, these colliders exploit the distinct charge and matter-antimatter properties of the beams, often circulating them in opposite directions within a single storage ring or separate rings before intersection at interaction points.^[52] Antiprotons are generated by directing a high-energy proton beam, typically at energies around 120 GeV or 26 GeV, onto a fixed target such as Inconel or copper, producing antiprotons via particle collisions with momenta near 8-11 GeV/c.^[53] The resulting antiprotons, constituting a small fraction of the debris, are collected using focusing devices like lithium lenses to capture a broad momentum spread, then injected into dedicated cooling rings. Stochastic cooling reduces the beam's emittance from initial values around 330 eV·s to about 15 eV·s by detecting and correcting particle deviations, while electron cooling further stabilizes the beam for accumulation and transfer to the collider ring.^[52] Protons, by contrast, are accelerated directly from lower-energy sources and stored separately until collision.^[51] A key advantage lies in the valence quark-antiquark annihilation at the collision point, yielding cleaner hard scattering events that are more centrally produced compared to proton-proton interactions, allowing for compact detectors and reduced background noise.^[52] This configuration facilitated landmark discoveries, including the W and Z bosons in the early 1980s, by providing up to 10 times higher cross-sections for electroweak processes involving high-mass states.^[53] Additionally, lower synchrotron radiation losses—about 13 times less than in equivalent proton-proton rings—enable higher beam energies with less power dissipation.^[52] Challenges primarily stem from antiproton production and accumulation inefficiencies, as only a tiny yield emerges from target interactions, necessitating extensive cooling cycles that limit overall luminosity to levels below those of proton-proton colliders, often requiring multiple parallel cooling systems to stack sufficient antiprotons.^[51] Beam-beam interactions over numerous encounters per turn further complicate stability, while the turnaround time for antiproton stores constrains integrated data collection.^[53] Stochastic cooling times scale unfavorably with particle number, demanding innovations like multi-ring setups to achieve luminosities around 10^{34} cm^{-2}s^{-1}.^[52] These colliders have focused on probing symmetries in particle physics, including tests of CP violation through potential production of heavy particles like W' bosons, and early validations of quantum chromodynamics (QCD) via precise measurements of jet production and electroweak parameters.^[53] Their design emphasized high-purity annihilation channels to isolate rare processes, contributing to foundational insights into the Standard Model before higher-luminosity alternatives emerged.^[51] By the early 2010s, proton-antiproton colliders were largely superseded by proton-proton machines due to the inherent inefficiencies in antiproton sourcing and cooling, culminating in the shutdown of the Tevatron in 2011 after it delivered over 10 fb^{-1} of integrated luminosity.^[51] This shift prioritized sustained high-luminosity operations without the need for antimatter production, though conceptual proposals for future upgrades persist in theoretical discussions.^[52]

Heavy-ion colliders

Heavy-ion colliders accelerate beams of heavy atomic nuclei, such as gold or lead ions, each containing multiple nucleons, to relativistic speeds for head-on collisions.^[54] These setups differ from elementary particle collisions by involving macroscopic nuclear matter, where the energy is specified per nucleon pair (√s_NN), enabling the study of collective nuclear effects rather than isolated parton interactions.^[54] The beams are configured to achieve high center-of-mass energies, typically on the order of 100 GeV per nucleon or higher, facilitating the compression and heating of nuclear matter to extreme densities.^[54] The primary physics goals of heavy-ion collisions are to recreate conditions similar to those in the early universe shortly after the Big Bang, probing the transition from hadronic matter to a deconfined state known as quark-gluon plasma (QGP).^[54] This plasma represents a phase of quantum chromodynamics (QCD) where quarks and gluons are no longer confined within hadrons, allowing observation of phase transitions such as deconfinement and chiral symmetry restoration.^[55] Key observables include collective flow phenomena, like elliptic flow (v₂), which measures the azimuthal anisotropy in particle emission and provides insights into the plasma's hydrodynamic behavior and low viscosity.^[54] Beam parameters in heavy-ion colliders are tailored to maximize the production of QGP, with Lorentz factors (γ) often exceeding 100 for ultra-relativistic ions, corresponding to beam energies around 100 GeV per nucleon or more.^[54] Luminosity, a measure of collision rate, scales with the square of the ion charge due to the increased number of nucleons, enabling higher interaction rates compared to lighter beams, though this also amplifies background processes.^[56] These parameters ensure sufficient energy density (ε > 1 GeV/fm³) for QGP formation in the collision overlap region.^[54] In contrast to proton-proton (p-p) collisions, heavy-ion interactions occur over larger volumes due to the extended nuclear size, leading to multiple simultaneous nucleon-nucleon interactions and greater stopping power, where incoming particles deposit more energy into the medium.^[56] Electromagnetic dissociation, arising from the strong fields of highly charged ions, can fragment nuclei before hadronic interactions, a process absent in p-p collisions.^[56] This results in more complex event topologies with higher particle multiplicities, often thousands of charged particles per event, emphasizing macroscopic effects over microscopic ones.^[54] Prominent signatures of QGP formation in heavy-ion collisions include jet quenching, where high-energy partons lose significant energy traversing the dense medium via gluon radiation and collisions, suppressing high-p_T jet yields relative to p-p baselines.^[57] Strangeness enhancement manifests as increased production of particles containing strange quarks, such as kaons and hyperons, attributed to the higher strangeness saturation in the thermalized QGP compared to hadronic matter.^[58] These observables, alongside flow harmonics, collectively indicate the presence of a strongly interacting, nearly perfect fluid-like state.^[54]

Major facilities

Large Hadron Collider

The Large Hadron Collider (LHC) is a particle accelerator and collider located at CERN near Geneva, Switzerland, designed primarily for proton-proton collisions but also capable of heavy-ion operations. Housed in a 27-kilometre circumference tunnel buried up to 175 metres underground, it circulates two counter-rotating beams of protons or heavy ions at energies up to 6.8 TeV per beam, achieving a centre-of-mass collision energy of 13.6 TeV during its current run.^[59]^[2]^[3] The LHC's engineering relies on advanced superconducting magnet technology to guide and focus the beams. It features 1232 dipole magnets, each 15 metres long and producing a magnetic field of 8.3 tesla, which bend the particle trajectories around the ring; these are complemented by 392 quadrupole magnets for beam focusing. The magnets operate at 1.9 kelvin (-271.3°C), cooled by superfluid liquid helium in a cryogenic system that distributes 120 tonnes of helium through 27 kilometres of piping. The beam pipes maintain an ultrahigh vacuum of approximately 10^{-10} mbar to minimize particle interactions with residual gas.^[60]^[61]^[2] Four main experiments exploit the LHC's collisions: ATLAS and CMS, general-purpose detectors that search for new physics phenomena including the Higgs boson; ALICE, optimized for heavy-ion collisions to study quark-gluon plasma; and LHCb, focused on beauty quark decays to probe matter-antimatter asymmetry. These detectors, each spanning tens of metres and involving thousands of scientists, record and analyze the debris from billions of collisions per second.^[62]^[63]^[64] Operations commenced with first beam circulation on 10 September 2008, followed by Run 1 (2009–2013) at up to 8 TeV, Run 2 (2015–2018) at 13 TeV, and Run 3 (2022–2026) at 13.6 TeV with enhanced luminosity targeting up to 2 × 10^{34} cm^{-2} s^{-1}. The proton physics phase of Run 3 concluded on 4 November 2025, delivering a record-breaking integrated luminosity exceeding 300 fb^{-1} across the experiments since the start of Run 3, enabling high-precision measurements, with heavy-ion runs continuing through the end of 2025. Key milestones include the 2012 discovery of the Higgs boson by ATLAS and CMS, confirming the mechanism for particle mass generation as predicted by the Standard Model.^[2]^[65]^[4]^[66]

Relativistic Heavy Ion Collider

The Relativistic Heavy Ion Collider (RHIC) is a particle accelerator facility located at Brookhaven National Laboratory in Upton, New York, designed primarily for studying heavy-ion collisions to recreate conditions of the early universe. Operational since 2000, RHIC collides beams of heavy atomic nuclei, such as gold ions, at relativistic speeds to probe the properties of quark-gluon plasma (QGP), a state of matter believed to have existed shortly after the Big Bang. Unlike proton colliders focused on elementary particle interactions, RHIC emphasizes nuclear physics, including the behavior of strongly interacting matter under extreme temperatures and densities. The facility consists of two intersecting superconducting rings that enable counter-rotating beams to collide at interaction points where detectors capture the resulting particles and radiation. The collider features two concentric rings with a circumference of 3.8 kilometers, housing approximately 1,740 superconducting magnets to guide and focus the beams. For heavy-ion operations, RHIC accelerates gold (Au) ions to a maximum energy of 100 GeV per nucleon, yielding center-of-mass collision energies up to √s_NN = 200 GeV for Au-Au interactions. In polarized proton mode, it achieves beam energies of up to 255 GeV per beam, corresponding to a center-of-mass energy of 500 GeV, allowing studies of proton spin structure. These capabilities support a versatile program, including variable beam energies down to 3 GeV per nucleon for mapping the quantum chromodynamics (QCD) phase diagram. RHIC hosts major experiments tailored to heavy-ion physics. The Solenoidal Tracker at RHIC (STAR) provides large angular acceptance for tracking charged particles, enabling comprehensive analysis of event topologies and collective flow in collisions. The Pioneering High Energy Nuclear Interaction eXperiment (PHENIX) offered high-precision measurements of rare probes like electrons, muons, and photons from the early collision stages, contributing to insights on energy loss in hot matter. The sPHENIX upgrade, a next-generation detector replacing PHENIX, enhances tracking and calorimetry for heavy-flavor and jet studies; it began commissioning in 2023 and collected its first physics data in 2024, with full operations continuing into 2025 to probe QGP properties with greater precision. Operations at RHIC include annual runs dedicated to heavy-ion collisions, with gold beams injected from the Alternating Gradient Synchrotron and accelerated in the collider rings. Deuteron beams serve as references for asymmetric collisions, such as deuteron-gold, to isolate nuclear effects from cold matter. The facility maintains a dedicated spin physics program, colliding transversely or longitudinally polarized protons to investigate the gluonic contributions to nucleon spin. As of 2025, RHIC conducts its final runs, integrating over 1 inverse femtobarn of polarized proton data at 500 GeV center-of-mass energy across its history. Key achievements include the first experimental evidence for QGP formation in 2005, based on observations of jet quenching and elliptic flow in Au-Au collisions at √s_NN = 200 GeV, as reported by the STAR Collaboration. Subsequent analyses demonstrated the QGP's near-perfect fluidity, with viscous effects matching hydrodynamic models and implying an extremely low shear viscosity-to-entropy ratio, akin to a nearly ideal liquid. These findings established RHIC as a cornerstone for understanding strongly coupled QCD matter. Unique features of RHIC include spin rotators in each ring, which enable control of beam polarization direction—transverse for specific spin-momentum correlations or longitudinal for parity-violating studies—preserving up to 60% polarization during acceleration. Electron cooling, implemented via the Low-Energy RHIC Electron Cooler (LEReC), reduces beam emittance in low-energy heavy-ion modes, improving luminosity for beam energy scans and supporting the final science program through 2025.

Historical colliders

The development of hadron colliders began in the mid-20th century, with pioneering facilities at CERN establishing the foundational technologies for high-energy particle collisions. These early machines, operating primarily with proton-proton or proton-antiproton beams, enabled breakthroughs in understanding strong interactions and electroweak symmetry breaking, while overcoming challenges like beam cooling and luminosity enhancement.^[67]^[68] The Intersecting Storage Rings (ISR) at CERN marked the debut of hadron collider technology, commencing operations in 1971 and running until 1984. Consisting of two interlaced proton synchrotron rings with a circumference of 942 meters, the ISR accelerated protons to 31 GeV per beam, achieving a center-of-mass energy of 62 GeV for proton-proton collisions—the first such head-on hadron interactions ever produced.^[68]^[67] Key experiments at the ISR, including those detecting hadronic jets, provided early evidence of quark-gluon plasma-like behaviors and validated perturbative quantum chromodynamics predictions.^[69] A pivotal innovation from the ISR was stochastic cooling, invented by Simon van der Meer, which reduced beam emittance by detecting particle deviations and applying corrective feedback, enabling higher luminosity operations.^[68]^[67] Building on ISR advancements, CERN's Super Proton Synchrotron (SPS) transitioned from a fixed-target accelerator to a proton-antiproton collider in 1981, operating in this mode until 1989. The SPS accelerated protons to 450 GeV and produced antiprotons via stochastic cooling in the Antiproton Accumulator, yielding collisions at a center-of-mass energy of approximately 546 GeV.^[70]^[71] The UA1 and UA2 experiments at the SPS delivered landmark discoveries of the W and Z bosons in 1983, confirming the electroweak theory and earning Carlo Rubbia and Simon van der Meer the 1984 Nobel Prize in Physics.^[70]^[71] In the United States, Fermilab's Tevatron operated as a proton-antiproton collider from 1985 to 2011, reaching a center-of-mass energy of 1.96 TeV in a 6.28-kilometer superconducting ring—the highest energy for any accelerator until the LHC.^[72]^[73] The Collider Detector at Fermilab (CDF) and D0 experiments utilized the Tevatron to discover the top quark in 1995, completing the Standard Model's quark sector after extensive searches following the bottom quark's 1977 observation.^[74]^[75] These historical colliders left enduring legacies through technological transfers, such as superconducting magnets and antiproton production techniques from the Tevatron, which informed LHC design and operations.^[73]^[76] ISR and SPS data archives continue to support precision measurements and model validations, while stochastic cooling principles remain integral to modern accelerators.^[77] Shutdowns were driven by resource constraints and the pursuit of higher energies: the ISR was decommissioned in 1984 to repurpose its tunnel for the Large Electron-Positron Collider (LEP); SPS collider operations ceased in 1989 following LEP's startup in its own tunnel; and the Tevatron ended in 2011 amid U.S. funding limitations and the LHC's activation.^[68]^[73]^[78]

Scientific contributions

Discoveries in particle physics

Hadron colliders have been instrumental in confirming key predictions of the Standard Model of particle physics through the discovery of fundamental particles and precise measurements of their properties. These facilities, such as the Super Proton Synchrotron (SPS) at CERN and the Tevatron at Fermilab, have enabled high-energy proton-antiproton and proton-proton collisions that probe the electroweak and strong interactions at scales unattainable by other means.^[79] Major breakthroughs include the identification of the W and Z bosons, the top quark, and the Higgs boson, each validating aspects of the electroweak symmetry breaking mechanism and quark sector. The W and Z bosons, carriers of the weak force, were discovered in 1983 at CERN's SPS collider using the UA1 and UA2 experiments in proton-antiproton collisions at a center-of-mass energy of 540 GeV. These neutral (Z) and charged (W±) particles confirmed the electroweak theory proposed by Glashow, Weinberg, and Salam, with measured masses of approximately 91 GeV/c² for the Z boson and 80 GeV/c² for the W boson, aligning closely with theoretical predictions. Their detection through leptonic decay channels, such as Z → e⁺e⁻ and W → eν, provided direct evidence for the unification of electromagnetic and weak forces. In 1995, the top quark was discovered at Fermilab's Tevatron collider by the CDF and DØ collaborations in proton-antiproton collisions at 1.8 TeV.^[80] As the heaviest known elementary particle with a mass of about 173 GeV/c², the top quark is produced primarily in pairs via the strong interaction through gluon fusion or quark-antiquark annihilation. Its identification relied on decay signatures like top → Wb, followed by W → ℓν and b-jet reconstruction, completing the set of six quarks predicted by the Standard Model and enabling studies of flavor-changing processes. The Higgs boson, responsible for imparting mass to other particles via the Higgs mechanism, was observed in 2012 at CERN's Large Hadron Collider (LHC) by the ATLAS and CMS experiments in proton-proton collisions at 8 TeV.^[66] With a mass of approximately 125 GeV/c², it was detected through multiple decay channels, including the diphoton channel H → γγ, which offers a clean signature due to the boson's scalar nature and low background. This discovery, announced with a significance exceeding 5σ, provided empirical support for the Brout-Englert-Higgs mechanism and electroweak symmetry breaking. Beyond direct discoveries, hadron colliders have yielded precision measurements essential for validating the Standard Model. At the LHC and Tevatron, analyses of jet production and event shapes have refined the strong coupling constant α_s, demonstrating its evolution with energy scale as predicted by quantum chromodynamics (QCD), with current world-average values around 0.118 at the Z mass scale. Measurements of B-meson decays, particularly from b-quark flavor-changing processes at LHCb, CDF, and DØ, have constrained Cabibbo-Kobayashi-Maskawa (CKM) matrix elements like |V_cb| and |V_ub|, testing unitarity and CP violation with uncertainties below 5%.^[81] These results align with global fits and help calibrate theoretical models for rare decays. Searches for physics beyond the Standard Model at hadron colliders have set stringent limits without confirming new particles. LHC experiments have excluded supersymmetric partners of Standard Model particles up to masses of several TeV in various models, based on null results in multi-jet plus missing energy signatures from Run 2 data. Similarly, constraints on large extra dimensions have set lower limits on the fundamental scale M_D ≳ 6-11 TeV for 2-6 extra dimensions in the ADD model from graviton production searches in dilepton and diphoton channels (as of 2023), with no evidence observed at 13 TeV; further constraints from Run 3 data as of 2025 continue to refine these bounds.^[82] These bounds refine theoretical parameter spaces while motivating refined models for future probes.

Applications in nuclear physics

Hadron colliders, particularly through heavy-ion collision programs at facilities like the Relativistic Heavy Ion Collider (RHIC) and the Super Proton Synchrotron (SPS), enable the study of nuclear matter under extreme temperatures and densities, recreating conditions akin to those in the early universe. These experiments probe the transition from hadronic matter to a deconfined state known as the quark-gluon plasma (QGP), providing insights into quantum chromodynamics (QCD) at high energy densities. Key observables from the 2000s onward, such as collective flow and particle suppression patterns, have established the formation of a strongly interacting QGP with near-perfect fluid properties.^[83]^[84] Evidence for QGP formation emerged prominently from RHIC experiments in the early 2000s, building on earlier SPS indications, through measurements of elliptic flow and other hydrodynamic signatures indicating a low-viscosity medium. Hydrodynamic modeling of these data reveals a shear viscosity-to-entropy density ratio \eta/s approaching the quantum lower bound of $1/(4\pi), consistent with a nearly ideal fluid behavior in the QGP phase. This low viscosity, extracted from viscous hydrodynamics fits to particle spectra and flow harmonics at RHIC energies, underscores the strongly coupled nature of the plasma, differing markedly from weakly interacting gas expectations.^[83] Jet quenching, observed as the energy loss of high-momentum partons traversing the QGP, manifests in the suppression of high-transverse momentum (p_T) hadrons in heavy-ion collisions relative to proton-proton baselines. The nuclear modification factor R_{AA}, defined as the yield ratio normalized by the number of binary collisions, drops below unity (R_{AA} < 1) for p_T > 4 GeV/c in central Au+Au collisions at RHIC, signaling medium-induced gluon radiation and parton energy degradation. This phenomenon, first quantified in PHENIX data from 2002, provides a tomographic probe of the QGP's density profile and transport coefficients.^[85] The suppression of J/ψ mesons in hot, dense media offers evidence for chiral symmetry restoration within the QGP, as color screening and Debye mass effects dissolve the quarkonium binding in the deconfined phase. At SPS energies, NA50 measurements in Pb-Pb collisions showed anomalous J/ψ yield reductions beyond cold nuclear absorption expectations, interpreted as QGP onset signatures where elevated temperatures restore approximate chiral invariance, altering meson spectral functions. This suppression pattern, scaling with collision centrality, aligns with lattice QCD predictions for the deconfinement transition.^[86] Polarized proton collisions at RHIC have illuminated the spin structure of the nucleon, particularly the gluon helicity contribution \Delta g, which quantifies the net polarized gluon momentum fraction carrying the proton's spin. STAR and PHENIX analyses of double-longitudinal spin asymmetries in jet and pion production yield \Delta g / g \approx 0.1-0.2 in the probed x range (0.01-0.3), indicating gluons contribute modestly to the proton spin puzzle while revealing transversity and sea quark polarizations. These results constrain polarized parton distribution functions, essential for understanding nuclear spin dynamics. Insights from hadron collider QGP studies extend to astrophysics, linking laboratory recreations to neutron star mergers and early universe cosmology. In binary neutron star mergers, post-merger ejecta may transiently form hot QGP, with gravitational wave signatures like tidal deformability echoing heavy-ion flow patterns and viscosity bounds. Similarly, QGP properties inform Big Bang nucleosynthesis models, where the plasma's equation of state influences baryon-to-photon ratios and light element abundances microseconds after the epoch of recombination.^[87]

Future developments

Upgrades to existing facilities

The High-Luminosity Large Hadron Collider (HL-LHC) represents a major upgrade to the existing LHC at CERN, scheduled to begin operations in 2029 following the completion of Long Shutdown 3 (LS3) in 2026.^[88] This enhancement aims to increase the instantaneous luminosity by a factor of 5 to 7.5 compared to the current LHC, reaching up to 7.5 × 10^{34} cm^{-2} s^{-1}, enabling the collection of approximately 3000 fb^{-1} of integrated luminosity by around 2040.^[89] Key components include new inner triplet quadrupoles using niobium-tin (Nb3Sn) superconducting magnets capable of generating 12 T fields to achieve tighter beam focusing.^[90] Civil engineering work for the HL-LHC, including underground modifications, commenced in 2024 during preparations for LS3.^[91] At Brookhaven National Laboratory, the Relativistic Heavy Ion Collider (RHIC) has undergone significant upgrades, notably the sPHENIX detector, which began receiving beams in 2023 and features advanced calorimetry for high-precision measurements of heavy-ion collisions.^[92] This upgrade replaces much of the original PHENIX detector with state-of-the-art tracking and electromagnetic calorimetry to study quark-gluon plasma properties in greater detail.^[93] As of 2025, RHIC is in its 25th and final run with sPHENIX, after which operations will transition to preparations for the Electron-Ion Collider (EIC), with construction slated to start post-2025 and first beams expected around 2035.^[94]^[95] Technological advancements integral to these upgrades include crab cavities, which will be installed in the HL-LHC to rotate proton bunches electromagnetically, compensating for the crossing angle and boosting luminosity by up to 50% without increasing beam intensity.^[96] Additionally, high-temperature superconductors, such as magnesium diboride cables operating at around 20 K, are being integrated into HL-LHC electrical transfer lines to enhance energy efficiency and reduce cryogenic demands.^[97] These upgrades are driven by the need to probe rare processes in particle physics, such as the Higgs boson self-coupling, and to achieve higher precision in quantum chromodynamics (QCD) calculations, allowing for more accurate predictions of strong interaction phenomena.^[98] As of November 2025, LHC Run 3 continues with proton collisions at 13.6 TeV, targeting over 500 fb^{-1} of data by mid-2026 before the transition to HL-LHC preparations.^[3]^[99]

Proposed next-generation projects

The Future Circular Collider (FCC) is a proposed post-Large Hadron Collider (LHC) facility at CERN, featuring a 91 km circumference tunnel to enable higher-energy collisions. The hadron component, FCC-hh, aims for proton-proton collisions at a center-of-mass energy of 100 TeV, representing a significant leap from the LHC's 14 TeV, while the initial electron-positron stage, FCC-ee, would operate at up to 365 GeV for precision studies.^[100] Construction could begin after a CERN Council decision expected around 2028, with operations targeted for the late 2040s following a 15-year research program in the ee mode. Key challenges include an estimated construction cost exceeding 20 billion euros for the full project, geological constraints in the Geneva region, and the need for advanced superconducting magnets to achieve the required magnetic fields of 16 tesla. The Electron-Ion Collider (EIC) at Brookhaven National Laboratory represents another major proposal, focusing on colliding polarized electrons with ions to probe quantum chromodynamics (QCD).^[101] It will achieve center-of-mass energies up to 140 GeV for electron-ion collisions, using upgraded infrastructure from the Relativistic Heavy Ion Collider to enable high-luminosity operations with polarized beams.^[102] Selected by the U.S. Department of Energy in 2020, construction is underway with operations planned for the 2030s, aiming to provide detailed "tomography" of quark-gluon plasma (QGP) formation and the internal structure of nucleons.^[103] Challenges encompass integrating new electron accelerators into the existing 3.9 km ring and managing beam polarization at high intensities, with a total project cost estimated at around 1 billion dollars.^[104] In China, the Circular Electron Positron Collider (CEPC) project includes provisions for a subsequent Super Proton-Proton Collider (SPPC) in a shared 100 km tunnel, emphasizing a staged approach to high-energy physics.^[105] The CEPC phase would serve as a Higgs factory at 240 GeV center-of-mass energy for electron-positron collisions, producing millions of Higgs bosons for precision measurements, while the SPPC would enable proton-proton collisions up to 125 TeV.^[106] Proposed in 2012, the initiative awaits final approval, with potential construction starting as early as 2027 and operations in the 2040s, at an estimated cost of about 5 billion euros.^[107] Site selection in geologically stable regions like Qinhuangdao poses logistical hurdles, alongside demands for energy-efficient designs to minimize operational power consumption exceeding 100 megawatts.^[108] These projects are driven by the need to surpass LHC limitations in energy reach and precision, targeting deeper insights into the Higgs mechanism, potential extensions of the Standard Model, and elusive phenomena like dark matter candidates through enhanced sensitivity to new particles and interactions.^[109] For instance, higher energies would facilitate rare Higgs decay studies and electroweak symmetry breaking probes, while electron-ion capabilities at the EIC could illuminate neutrino mass origins via QCD connections.^[110] Overall, they address fundamental questions in particle and nuclear physics, including the nature of mass generation and matter-antimatter asymmetry, amid global collaboration to mitigate financial and technical risks.^[111]

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CMS | CERN
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[hep-ex/0101052] J/psi suppression in Pb-Pb collisions at CERN SPS
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None
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Electron-Ion Collider (EIC)
### Summary of Electron-Ion Collider (EIC) from https://www.bnl.gov/eic/
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a 3.9 km-circumference collider ...
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The Circular Electron Positron Collider ... - CEPC
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Mar 19, 2025 · The European physics laboratory CERN is planning to build a mega collider by 2070. Critics say the plan could lead to its ruin.
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[2203.09451] Snowmass2021 White Paper AF3-CEPC - arXiv
Mar 15, 2022 · The 100 km tunnel of CEPC could also host a Super proton proton Collider (SppC) to reach energies well beyond the LHC. CEPC Conceptual Design ...