Seven 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.
While the main focus of research at CERN has moved in recent years towards the LHC, experiments at other accelerators and facilities both on-site and off remain an important part of the laboratory’s activities.
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). NA61/SHINE studies the properties of hadrons in collisions of beam particles with fixed targets. NA62 uses protons from the SPS to study rare decays of kaons. DIRAC is investigating the strong force between quarks at the Proton Synchrotron (PS). The CLOUD experiment is investigating a possible link between cosmic rays and cloud formation. ACE, AEGIS, ALPHA, ASACUSA, and ATRAP all use antiprotons from the Antiproton Decelerator, while the CAST experiment is looking for hypothetical particles coming not from collisions at the accelerators but from the Sun.
Experimental facilities at CERN include the Radioactive Ion Beam facility (ISOLDE), the neutron time-of-flight facility (nTOF) and the CERN Neutrino Platform. AMS, which is a CERN-recognised experiment located on the International Space Station, has its control centre at CERN.