Diverse experiments at CERN
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 the Standard Model to supersymmetry and from exotic isotopes to cosmic rays.
Several 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.
Nine 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-MAPP uses detectors deployed near LHCb to search for a hypothetical particle called the magnetic monopole. FASER and SND@LHC, the two newest LHC experiments, are situated close to the ATLAS collision point in order to search for light new particles and to 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.
CERN’s experimental programme has consisted of hundreds of experiments spanning decades.
Among these were pioneering experiments for electroweak physics, a branch of physics that unifies the electromagnetic and weak fundamental forces. In 1958, an experiment at the Synchrocyclotron discovered a rare pion decay that spread CERN’s name around the world. Then in 1973, the Gargamelle bubble chamber presented first direct evidence of the weak neutral current. Ten years later, CERN physicists working on the UA1 and UA2 detectors announced the discovery of the W boson in January and Z boson in June – the two carriers of the electroweak force. Two key scientists behind the discoveries – Carlo Rubbia and Simon van der Meer – received the Nobel prize in physics in 1984.
From 1989, the Large Electron-Positron collider (LEP) enabled the ALEPH, DELPHI, L3 and OPAL experiments to put the Standard Model of particle physics on a strong experimental basis. In 2000, LEP made way for the construction of the Large Hadron Collider (LHC) in the same tunnel.
CERN’s huge contributions to electroweak physics are just some of the highlights of the experiments over the years.