Run 2 (LHC)

Run 2 (LHC)
Name	Run 2 (LHC)
Location	CERN
Status	Completed
Start	2015
End	2018
Energy	13 TeV (centre-of-mass)
Type	Particle accelerator
Beam	protons, lead ions
Operators	European Organization for Nuclear Research (CERN), ATLAS, CMS, ALICE, LHCb

Run 2 (LHC) was the second operational data-taking period of the Large Hadron Collider at CERN from 2015 to 2018, delivering high-energy proton collisions at 13 teraelectronvolts and a program of heavy-ion operation. The run involved upgrades to the superconducting magnets, RF systems, injector complex, and data acquisition system to increase luminosity and enable precision measurements across the Standard Model and searches for BSM phenomena. Major collaborations including ATLAS, CMS, ALICE, and LHCb exploited the dataset to refine parameters such as the Higgs boson couplings, top-quark properties, and rare decay rates while constraining models like Supersymmetry, Extra dimensions, and Dark matter candidates.

Overview

Run 2 was initiated after the long shutdown LS1 and followed Run 1 (2010–2012), with goals set by CERN Council and the European Strategy for Particle Physics to probe higher centre-of-mass energy and integrated luminosity. The period saw coordinated operation of injector machines including the Proton Synchrotron and Super Proton Synchrotron feeding the LHC ring, relying on maintenance and refurbishment overseen by the LHC Machine Committee and Accelerator Physics Group. The run supported a broad program spanning precision tests in experiments such as ATLAS, CMS and LHCb alongside heavy-ion physics at ALICE and performance studies by TOTEM and LHCf. Run 2's dataset underpinned publications in journals from the Physical Review Letters cohort to the Journal of High Energy Physics.

Machine configuration and upgrades

Key hardware improvements included consolidated superconducting cable repairs following the 2008 incident, installation of upgraded beam collimation systems, crab cavity research, and replacement of quench protection system components to permit reliable operation at 6.5 TeV per beam. Injector upgrades incorporated enhancements to the LINAC4 and PS Booster improving bunch brightness for experiments such as LHCb and ALICE. The beam instrumentation suite benefited from new beam position monitor electronics and transverse feedback systems used in coordination with the Machine Protection System and Beam Dumping System. Work on High-Luminosity LHC preparatory systems began, interoperating with collaborations including CERN IT Department, Engineering Department, and external partners like Fermilab, DESY, and KEK.

Physics program and major results

The Run 2 physics agenda emphasized precision Higgs studies, top-quark sector measurements, electroweak processes, heavy-flavor physics, QCD studies, and searches for BSM signatures including SUSY, vector-like quarks, leptoquarks, and axion-like particles. ATLAS and CMS produced combined measurements of the Higgs boson mass, production cross sections, and decay modes to ZZ, WW, ττ, bb, and γγ channels, constraining couplings predicted by the Standard Model. LHCb achieved world-leading results on CP violation and rare decays like B_s → μ+ μ− and angular analyses of B meson decays, informing flavor models and global fits used by groups such as CKMfitter and UTfit. ALICE advanced understanding of the quark–gluon plasma via measurements of flow harmonics, jet quenching, and strange hadron production in heavy-ion collisions. TOTEM measured total cross sections and elastic scattering at 13 TeV, informing phenomenology used by PDG and HERAPDF fits.

Data collection and performance

Across 2015–2018 the LHC delivered an integrated luminosity of about 150 inverse femtobarns to ATLAS and CMS, with LHCb and ALICE receiving tailored datasets optimized for forward physics and heavy-ion runs. Peak instantaneous luminosities reached unprecedented levels for hadron collider operation, with pile-up mitigation strategies implemented via upgraded trigger systems, silicon pixel detector replacements, and advanced particle-flow algorithms developed by collaborations including CERN Openlab partners. Data quality monitoring and calibration were coordinated with computing grids including the Worldwide LHC Computing Grid, CERN Data Centre, and Tiered sites like Fermilab Tier-1, GridPP, and INFN CNAF. Statistical analyses relied on frameworks such as ROOT, GEANT4, and fits using RooFit and HistFactory.

Notable analyses and discoveries

Run 2 confirmed and refined the properties of the Higgs boson discovered in Run 1, providing measurements of signal strengths and coupling modifiers consistent with Standard Model expectations within uncertainties reported by ATLAS and CMS. Precision top-quark mass and production asymmetry studies by CMS and ATLAS reduced systematic errors impacting electroweak fits involving the W boson and Z boson. LHCb reported anomalies in lepton-flavor universality tests in B → K(*) ℓ+ℓ− transitions that stimulated theoretical work from groups at Cambridge University, University of Chicago, CERN Theory Department, and Institute for Advanced Study. ALICE presented high-statistics evidence for collective behavior in small systems, influencing models from Relativistic Heavy Ion Collider communities and groups at Brookhaven National Laboratory. Searches constrained large regions of parameter space for models proposed by Pierre Ramond-inspired SUSY scenarios, Arkani-Hamed–Dimopoulos–Dvali large extra-dimensions, and simplified dark-matter mediators discussed at conferences like ICHEP and EPS-HEP.

Challenges and legacy

Run 2 faced technical challenges including managing high pile-up, radiation damage to inner trackers, cryogenics reliability for the superconducting magnets, and resiliency against single-event upsets in electronics, prompting upgrades coordinated with industrial partners and national laboratories such as CERN, Fermilab, KEK, DESY, INFN, and CEA. The dataset and techniques developed during Run 2 laid groundwork for the High-Luminosity LHC era, informing detector upgrade projects like the ATLAS Inner Tracker replacement, CMS High Granularity Calorimeter, LHCb Upgrade I, and ALICE ITS Upgrade, and contributing to training of a generation of physicists from institutions including University of Oxford, Massachusetts Institute of Technology, University of California, Berkeley, École Polytechnique, University of Tokyo, and Tata Institute of Fundamental Research. Run 2's comprehensive publications, public datasets, and software releases continue to support phenomenologists, experimentalists, and educators across the global high-energy physics community.

Category:Large Hadron Collider