The Large Hadron Collider (LHC) is a circular particle accelerator at CERN, near Geneva on the border between France and Switzerland. Its approximately 27-kilometre ring accelerates opposing particle beams and brings them into collision at experimental sites. The world’s largest and highest-energy particle accelerator, it principally collides protons, but also operates with heavy atomic nuclei. Its experiments investigate particle physics, test the Standard Model, and search for phenomena beyond that theory. (home.web.cern.ch)
Accelerator design and operation
The LHC occupies an underground tunnel originally built for the Large Electron–Positron Collider. Two beams circulate in opposite directions through separate vacuum pipes, approaching the speed of light. Protons are hadrons: composite particles containing quarks and gluons. In a high-energy proton collision, the fundamental interactions involve these constituents rather than two indivisible objects. Consequently, the energy available to an individual constituent collision varies and is generally below the full proton–proton collision energy. (cds.cern.ch)
The beams are guided by magnets exploiting superconductivity. The machine includes 1,232 main dipole magnets, which bend the beams around the ring, and 392 main quadrupole magnets, which focus them. Its principal magnet system operates at 1.9 kelvin, maintained by cryogenic equipment using superfluid helium. Strong magnetic fields control the particles’ trajectories; they do not provide the accelerating energy. (home.web.cern.ch)
Protons first pass through CERN’s injector accelerators, entering the LHC in bunches at 450 gigaelectronvolts (GeV). Radiofrequency cavities apply oscillating electric fields that increase their energy on successive revolutions. The design energy is 7 teraelectronvolts (TeV) per proton beam, corresponding to 14 TeV for head-on proton–proton collisions. Actual operating energies have increased in stages as the accelerator has been commissioned and upgraded. (home.cern)
Development and operating history
The LHC grew from studies conducted during the 1980s of a high-energy proton collider sharing the existing tunnel. The first beams circulated on September 10, 2008. An electrical fault shortly afterward damaged part of the superconducting magnet system, requiring repairs; beam operation resumed in November 2009. Sustained high-energy physics operation began in 2010 with proton–proton collisions at 7 TeV. (atlas.cern)
Operating periods, called runs, are separated by shutdowns for maintenance and improvements. Run 1 supplied the data used in the Higgs discovery. Run 2 began physics operation on June 3, 2015, at 13 TeV. Run 3 began collision data-taking on July 5, 2022, at 13.6 TeV, equivalent to 6.8 TeV per beam. It ended on June 27, 2026, before the major upgrade programme known as Long Shutdown 3. (home.cern)
Experiments and detection
The collider and its detectors are distinct systems: the accelerator supplies collisions, while experimental collaborations build and operate instruments that measure their products. Four major experiments occupy separate collision points. ATLAS and CMS are general-purpose experiments with overlapping scientific objectives but different technical designs. Their independent measurements permit important results to be cross-checked. (home.cern)
ALICE specializes in heavy-ion physics, particularly matter produced in collisions of lead nuclei. LHCb studies particles containing beauty and charm quarks, including their decays and differences between matter and antimatter. Unlike the approximately enclosing geometries of ATLAS and CMS, LHCb concentrates on particles travelling forward from the collision region. Smaller experiments investigate additional specialized phenomena. (home.cern)
Detectors combine tracking systems, energy-measuring calorimeters, particle-identification instruments, and other subsystems. Charged-particle trajectories curve in magnetic fields, enabling momentum measurements. Short-lived particles are generally reconstructed from their decay products rather than observed directly. Electronic trigger systems select potentially useful events from the collision stream for storage and analysis. (home.cern)
Scientific programme and results
On July 4, 2012, ATLAS and CMS announced a new particle with properties consistent with the Higgs boson, at a mass of approximately 125 GeV. Subsequent measurements established its identification and examined its interactions with other particles. These studies test the Higgs mechanism, associated with the Higgs field and the origin of elementary-particle masses. The discovery did not complete the experimental programme: precision measurements can reveal departures from Standard Model predictions. (home.cern)
Heavy-ion collisions produce quark–gluon plasma, a state in which quarks and gluons are not confined within individual hadrons. Researchers investigate its collective motion and the energy lost by energetic particles traversing it. These measurements explore quantum chromodynamics, the theory of the strong interaction, under extreme conditions resembling aspects of the early universe. (home.cern)
The LHC also searches for new particles and possible dark matter candidates. Invisible particles would be inferred through imbalances in measured momentum, not by direct detector tracks. Such signatures require careful comparison with known processes and detector effects; an unsuccessful search constrains particular models rather than excluding every possible explanation of dark matter. (home.web.cern.ch)
Computing and the high-luminosity upgrade
Collision analysis relies on the Worldwide LHC Computing Grid, a distributed computing infrastructure connecting CERN with research centres internationally. It supplies storage and processing resources for collision records, simulations, and scientific analyses, allowing collaborators to work with data beyond the laboratory itself. (home.cern)
The High-Luminosity LHC upgrade is scheduled to begin operation in mid-2030. Its principal objective is greater integrated luminosity—the accumulated collision exposure—rather than a large increase in collision energy. The programme targets ten times the integrated luminosity of the original design. New accelerator components and extensively upgraded ATLAS and CMS detectors are intended to support more precise Higgs measurements and searches for exceptionally rare processes. (home.cern)