CERN Accélérateur de science

CERN is home to a wide range of experiments. Scientists from institutes all over the world form experimental collaborations to carry out a diverse research programme, ensuring that CERN covers a wealth of topics in physics, from kaons to cosmic rays, and from the Standard Model to supersymmetry.

The largest collaborations run experiments using the Large Hadron Collider (LHC), the most powerful accelerator in the world. In addition, fixed-target experiments, antimatter experiments and experimental facilities make use of the LHC injector chain.

Eight experiments at the Large Hadron Collider (LHC) use detectors to analyse the myriad of particles produced by collisions in the accelerator. These experiments are run by collaborations of scientists from institutes all over the world. Each experiment is distinct, and characterised by its detectors.

The biggest of these experiments, ATLAS and CMS, use general-purpose detectors to investigate the largest range of physics possible. Having two independently designed detectors is vital for cross-confirmation of any new discoveries made. ALICE and LHCb have detectors specialised for focussing on specific phenomena. These four detectors sit underground in huge caverns on the LHC ring.

The smallest experiments on the LHC are TOTEM and LHCf, which focus on "forward particles" – protons or heavy ions that brush past each other rather than meeting head on when the beams collide. TOTEM uses detectors positioned on either side of the CMS interaction point, while LHCf is made up of two detectors which sit along the LHC beamline, at 140 metres either side of the ATLAS collision point. MoEDAL uses detectors deployed near LHCb to search for a hypothetical particle called the magnetic monopole. FASER, the newest LHC experiment, is situated 480 metres from the ATLAS collision point in order to search for light new particles and study neutrinos.

In “fixed-target” experiments, a beam of accelerated particles is directed at a solid, liquid or gas target, which itself can be part of the detection system. 

COMPASS, which looks at the structure of hadrons – particles made of quarks – uses beams from the Super Proton Synchrotron (SPS). The SPS also feeds the North Area (NA), which houses a number of experiments. NA61/SHINE studies a phase transition between hadrons and quark-gluon plasma, and conducts measurements for experiments involving cosmic rays and long-baseline neutrino oscillations. NA62 uses protons from the SPS to study rare decays of kaons. NA63 directs beams of electrons and positrons onto a variety of targets to study radiation processes in strong electromagnetic fields. NA64 is looking for new particles that would mediate an unknown interaction between visible matter and dark matter. UA9 is investigating how crystals could help to steer particle beams in high-energy colliders.

The CLOUD experiment uses beams from the Proton Synchrotron (PS) to investigate a possible link between cosmic rays and cloud formation. DIRAC, which is now analysing data, is investigating the strong force between quarks.

Currently the Antiproton Decelerator and ELENA serve several experiments that are studying antimatter and its properties: AEGIS, ALPHA, ASACUSA, BASE and GBAR. Whereas ATRAP and ACE have now completed their experiments.

Experimental facilities at CERN include the Radioactive Ion Beam facility (ISOLDE), MEDICIS, the neutron time-of-flight facility (n_TOF) and the CERN Neutrino Platform.

Not all experiments rely on CERN’s accelerator complex. AMS, for example, is a CERN-recognised experiment located on the International Space Station, which has its control centre at CERN. The CAST and OSQAR experiments are both looking for hypothetical dark matter particles called axions.

The Gargamelle bubble chamber was designed to detect neutrinos. It operated from 1970 to 1976 with a muon-neutrino beam produced by the CERN Proton Synchrotron, before moving to the Super Proton Synchrotron (SPS) until 1979.

Two moveable detectors UA1 and UA2 ran from 1981 to 1990 to search proton–antiproton collisions for signatures of the W and Z particles, carriers of the electroweak force. In 1983, CERN physicists announced the discovery of the W boson in January and the discovery of the Z boson in June. Two key scientists behind the discoveries – Carlo Rubbia and Simon van der Meer – received the Nobel prize in physics in 1984.

The Large Electron-Positron collider (LEP) ran from 1989 to 2000 and served the ALEPH, DELPHI, L3 and OPAL experiments. LEP then made way for the construction of the Large Hadron Collider (LHC) in the same tunnel.