Pb–Pb collisions at the LHC

Pb–Pb collisions at the LHC
Name	Large Hadron Collider (Pb–Pb programme)
Location	CERN
Status	Active
First beam	2008
Major experiments	ALICE (A Large Ion Collider Experiment), ATLAS experiment, CMS experiment, LHCb
Energy	2.76–5.02 TeV per nucleon pair (typical runs)
Particle	Lead ions (Lead)
Purpose	Study of quark–gluon plasma, high-density Quantum chromodynamics

Pb–Pb collisions at the LHC

Lead–lead heavy-ion collisions at the Large Hadron Collider are high-energy interactions between accelerated lead nuclei designed to recreate conditions similar to the early Big Bang and to study deconfined quark–gluon plasma. Experimental campaigns involve coordinated operation of major detectors such as ALICE (A Large Ion Collider Experiment), ATLAS experiment, and CMS experiment, integrated with accelerator infrastructure at CERN and analyses by collaborations including ALICE Collaboration, ATLAS Collaboration, and CMS Collaboration.

Introduction

Pb–Pb programmes at the Large Hadron Collider follow earlier heavy-ion work at facilities such as Relativistic Heavy Ion Collider, Super Proton Synchrotron, and SPS (accelerator), aiming to probe the phase diagram of Quantum chromodynamics under extreme temperature and density. Run campaigns in 2010, 2011, 2015, 2018 and beyond produced data sets at nucleon-pair center-of-mass energies like 2.76 TeV and 5.02 TeV, enabling comparative studies with results from the PHENIX, STAR detectors and legacy experiments at CERN SPS. The programme involves international institutions such as Institut national de physique nucléaire et de physique des particules, Brookhaven National Laboratory, Lawrence Berkeley National Laboratory, Max Planck Society, and numerous universities.

Experimental setup and detectors

Pb–Pb operation relies on the Large Hadron Collider injector chain including LINAC 3, LEAR, PS (Proton Synchrotron), and Super Proton Synchrotron. Collisions occur at interaction points instrumented by dedicated detectors: ALICE (A Large Ion Collider Experiment) specializes in heavy-ion observables with subsystems like the Time Projection Chamber, Inner Tracking System, V0 detector and Electromagnetic Calorimeter; ATLAS experiment and CMS experiment provide high-resolution tracking and calorimetry with components such as the Transition Radiation Tracker, Muon Spectrometer, Silicon Tracker, and Hadronic Calorimeter. Forward physics and luminosity measurements involve instrumentation like LHCb, Zero Degree Calorimeter, and TOTEM experiment interfaces. Beam conditions and machine parameters are managed by European Organization for Nuclear Research teams at CERN Control Centre with support from accelerator groups and experiments including Accelerator Division (CERN) and Beam Instrumentation Group.

Collision dynamics and observables

Central Pb–Pb collisions produce energy densities exceeding those inferred from Bjorken hydrodynamics estimates and create a short-lived quark–gluon plasma whose evolution is probed via flow harmonics v2, v3, v4, jet quenching, and strangeness enhancement. Observable channels analyzed by ALICE Collaboration, ATLAS Collaboration, and CMS Collaboration include charged-particle multiplicities, transverse momentum spectra, identified hadron yields (e.g., pion, kaon, proton), heavy-flavor production from charm quark and bottom quark decays, quarkonia suppression of states like J/ψ and ϒ (Upsilon), direct photons, dileptons, and reconstructed jets. Correlation measurements such as two-particle angular correlations, ridge phenomena, and femtoscopy (HBT interferometry) provide constraints on source size and lifetime, while nuclear modification factors RAA and azimuthal anisotropy coefficients connect to initial-state geometry and final-state transport properties.

Key results and discoveries

Major LHC heavy-ion findings include robust elliptic flow consistent with near-minimal shear viscosity to entropy density ratio η/s inferred via comparisons to relativistic hydrodynamics models, strong high-pT jet quenching observed by ATLAS experiment and CMS experiment through dijet asymmetry and jet fragmentation modifications, and enhanced production of multi-strange hadrons reported by ALICE (A Large Ion Collider Experiment). Suppression patterns of quarkonia states such as J/ψ and sequential melting of ϒ (Upsilon) families provided experimental confirmation of color screening hypotheses proposed in Matsui and Satz. Measurements of heavy-flavor flow and energy loss connected to theoretical expectations from perturbative QCD and non-perturbative modeling; observation of long-range ridge correlations in small systems prompted cross-disciplinary dialogue with experiments like CMS Collaboration proton-lead results and comparisons with PHENIX and STAR observations at Relativistic Heavy Ion Collider. These discoveries involved coordinated analysis by collaborations from institutions such as University of Oxford, Massachusetts Institute of Technology, CERN, University of Tokyo, and IHEP.

Theoretical interpretation and models

Interpretation of Pb–Pb results synthesizes frameworks including lattice QCD thermodynamics, relativistic viscous hydrodynamics (e.g., MUSIC, VISHNU), parton energy loss formalisms like BDMPS-Z, AMY formalism, and Monte Carlo event generators such as PYTHIA, HIJING, and EPOS. Initial-state geometry and fluctuations are modeled using Glauber model and color-glass condensate approaches developed by communities around McLerran–Venugopalan model and studies at Brookhaven National Laboratory. Extraction of transport coefficients (η/s, heavy-quark diffusion coefficient) draws on global Bayesian analyses and comparisons with computations from AdS/CFT correspondence inspired techniques. Heavy-quark hadronization and recombination are addressed using coalescence models tied to inputs from thermal statistical models and lattice results.

Future directions and upgrades

Planned upgrades include the LHC Run 3 and Run 4 increases in luminosity, the ALICE Upgrade with improved Inner Tracking System and Time Projection Chamber upgrades, and enhanced trigger and readout systems for ATLAS experiment and CMS experiment. Proposed initiatives such as the Future Circular Collider concept, increased heavy-ion luminosity scenarios, and dedicated fixed-target programmes are discussed by international panels including CERN Council and working groups from NuPECC. Future measurements will target precision constraints on η/s(T), heavy-quark transport, jet-medium tomography, electromagnetic probes, and rare probes including heavy quarkonia polarization, multi-charm baryons, and exotic states with support from global theory networks led by institutions like Institute for Nuclear Theory and Perimeter Institute.

Category:Heavy-ion physics