Discovering Particles

Experiments at the Large Hadron Collider

Part of the ATLAS detector

Four main experiments are installed at the Large Hadron Collider (LHC). Two are general- purpose experiments, studying many aspects of particle production and decay. These are ATLAS (A Toroidal LHC Apparatus) and CMS (Compact Muon Solenoid), each of which involves more than 3000 scientists and engineers, almost 200 research institutes, and around 40 countries, drawn from all continents except Antarctica. The other experiments, each involving around 1000 scientists and engineers, are more specialised: ALICE, (A Large Ion-Collider Experiment) focuses on heavy-ion collisions, and LHCb (LHC b-hadron experiment) is mainly concerned with the decays of hadrons containing the bottom quark.

The LHC experiments use the largest-volume particle detectors built to date. These detectors have been constructed as a series of layers, or subdetectors, and all follow roughly the same scheme. Starting from the inner layers, which are positioned closest to the interaction region, each detector consists of: subdetectors to measure charged-particle trajectories; subdetectors to stop photons, electrons and hadrons, and measure their energies; subdetectors to record muons, the only charged particles that reach the detector’s outermost layers. The ALICE, ATLAS and CMS detectors are approximately cylindrical in shape, so as to be sensitive to particles emerging in all directions from an interaction. The majority of the particles of interest in LHCb are emitted at small angles, and so the experiment’s detector covers only a narrow cone around the direction of the incoming protons.

ALICE, ATLAS, CMS and LHCb have organisational structures comparable in complexity to those of a large multinational company. Experiment members work in cross-institute groups that take on specific responsibilites. For example, a group might have responsibility for a subdetector, for a piece of electronics, for some computer software, or for a physics measurement.

During operation of the LHC, the experimental areas are sealed off, and the detectors are controlled remotely using computers. Teams of physicists work in shifts to keep the detectors running, and to monitor their performance, twenty-four hours a day.

The four main LHC experiments are complemented by three smaller experiments, each involving fewer than 100 scientists: LHCf (LHC forward experiment), MoEDAL (Monopole and Exotics Detector At the LHC) and TOTEM (TOTal Elastic and diffractive cross-section Measurement).

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