LHC Run 2

LHC Run 2 LHC Run 2 was the 2015–2018 operational period of the Large Hadron Collider at CERN near Geneva that delivered proton–proton collisions at a center-of-mass energy of 13 TeV and substantially increased integrated luminosity to extend searches beyond the Standard Model. It combined upgraded accelerator hardware with enhanced performance of the major experiments ATLAS, CMS, LHCb, and ALICE, enabling precision measurements of the Higgs boson properties, searches for supersymmetry, and studies of heavy-flavor and quark–gluon plasma phenomena. The run’s outputs informed planning for the High-Luminosity Large Hadron Collider and influenced theoretical work across collaborations involving institutions such as Fermilab, DESY, SLAC National Accelerator Laboratory, KEK, and university groups worldwide.

Background and Objectives

Run 2 followed the first high-energy phase after the 2010–2012 Run 1 and the Long Shutdown 1 upgrades at CERN. Objectives included achieving 13 TeV collisions to probe mass scales beyond reach in Run 1, increasing integrated luminosity to test rare processes like Higgs pair production and electroweak vector-boson scattering, and providing differential measurements to constrain parton distribution functions used by groups at Brookhaven National Laboratory, Lawrence Berkeley National Laboratory, Imperial College London, University of Oxford, and University of Cambridge. Scientific aims tied into searches for phenomena predicted by theories explored at conferences such as Lepton-Photon Conference, Rencontres de Moriond, and ICHEP, and to provide data for global fits by collaborations including Particle Data Group, ATLAS Collaboration, CMS Collaboration, and LHCb Collaboration.

Machine Upgrades and Operation Parameters

During Long Shutdown 1 and commissioning prior to Run 2, the Superconducting Magnet circuits, RF cavities, and cryogenic systems at CERN Accelerator School-linked teams were upgraded alongside the Injector Complex improvements from PS Booster and SPS. Run 2 running parameters featured 25 ns bunch spacing, higher beam energy from upgraded superconducting magnets developed by industrial partners and institutes like European Organization for Nuclear Research teams, and progressive increases in peak luminosity validated by beam diagnostics groups from University of Manchester, ETH Zurich, and University of Tokyo. Operation involved complex coordination among CERN Control Centre operators, beam dynamics specialists familiar with synchrotron radiation management, and machine-protection systems including monitoring by groups from Paul Scherrer Institute and National Technical University of Athens.

Major Experiments and Detector Upgrades

The ATLAS and CMS experiments implemented significant upgrades to tracking, trigger, and readout systems with contributions from institutions such as CERN, INFN, CNRS, Max Planck Society, University of California, Berkeley, and Peking University. LHCb revamped its vertex locator and implemented a real-time analysis strategy influenced by High Energy Physics community software frameworks, while ALICE upgraded its inner tracking system and readout to study heavy-ion collisions, supported by partners including GSI Helmholtz Centre, Universidad de Zaragoza, and National Centre for Nuclear Research. Forward detectors like TOTEM and LHCf extended diffraction and soft-QCD measurements, with instrumentation and alignment work involving Instituto Nazionale di Fisica Nucleare and Kyoto University groups. Detector improvements enabled analyses carried out by multinational teams from University of Chicago, Yale University, Princeton University, University of Melbourne, and McGill University.

Key Physics Results and Discoveries

Run 2 yielded high-statistics measurements of the Higgs boson production modes, differential cross sections, and couplings with inputs compared across theoretical predictions from groups such as CERN Theory Department, Institute for Advanced Study, Perimeter Institute, and CERN Yellow Reports. Searches placed stringent limits on supersymmetry models explored by SUSY Working Groups and on heavy resonances like Z' boson and W' boson scenarios motivated by extensions studied at KITP, SLAC, and Harvard University. Heavy-flavor physics by LHCb produced precise determinations of CP violation parameters and rare decay limits related to the CKM matrix; results engaged experts from Belle II, BaBar, and CLEO. ALICE advanced measurements of quark–gluon plasma signatures, jet quenching, and strangeness enhancement compared with results from RHIC at Brookhaven National Laboratory. Combined electroweak measurements, top-quark properties, and precision QCD tests constrained parton-distribution fits by groups at NNPDF, CTEQ, and MMHT.

Data Analysis, Challenges, and Systematics

Analysis efforts required large-scale computing coordination across the Worldwide LHC Computing Grid with Tiered sites at CERN and national centers like Fermilab, TRIUMF, GridPP, Riken, and INFN-CNAF. Systematic uncertainties from detector calibration, luminosity determination, and theoretical modeling engaged collaborations with experts from Jet Energy Calibration Working Group, PDF fitting teams, and Monte Carlo generator developers at PYTHIA, HERWIG, and MadGraph projects. Pileup mitigation, trigger bandwidth constraints, and real-time alignment prompted algorithmic advances from groups at Google Research-adjacent HPC centers, Oak Ridge National Laboratory, and university computer-science departments. Statistical combinations used methodologies favored by Cowan-style likelihood frameworks, with collaborative review processes involving editorial boards from Physical Review Letters, Journal of High Energy Physics, and Physics Letters B.

Legacy and Impact on Future Runs

Run 2 set the stage for the High-Luminosity Large Hadron Collider upgrade, influencing designs for new inner trackers, higher-granularity calorimetry, and radiation-hard electronics developed with partners such as CERN, ASML, Nokia Bell Labs, and national laboratories. Results shaped theoretical directions at institutes including Perimeter Institute and DAMTP and informed upgrade priorities for ATLAS Phase-II and CMS Phase-II upgrades coordinated through consortia involving INFN, CEA Saclay, and Max Planck Society. The data and software legacy—open datasets, analysis preservation, and machine-learning tools—benefit education and outreach at universities such as University of Oxford, MIT, Stanford University, and University of Tokyo and have been incorporated into proposals to agencies like European Research Council, U.S. Department of Energy, and Japan Society for the Promotion of Science. Category:Large Hadron Collider