Expériences
A range of experiments at CERN investigate physics from cosmic rays to supersymmetry
Diverses expériences au 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.
Accelerators use electromagnetic fields to accelerate and steer particles. Radiofrequency cavities boost the particle beams, while magnets focus the beams and bend their trajectory.
In a circular accelerator, the particles repeat the same circuit for as long as necessary, getting an energy boost at each turn. In theory, the energy could be increased over and over again. However, the more energy the particles have, the more powerful the magnetic fields have to be to keep them in their circular orbit.
A linear accelerator, on the contrary, is exclusively formed of accelerating structures since the particles do not need to be deflected, but they only benefit from a single acceleration pass. In this case, increasing the energy means increasing the length of the accelerator.
As physicists have explored higher and higher energies, accelerators have become larger and larger: the size of an accelerator is a compromise between energy, the radius of curvature (if it’s circular), the feasibility and the cost.
Colliders are accelerators that generate head-on collisions between particles. Thanks to this technique, the collision energy is higher because the energy of the two particles is added together.
The Large Hadron Collider is the largest and most powerful collider in the world. It boosts the particles in a loop 27 kilometres in circumference at an energy of 6.5 TeV (teraelectronvolts), generating collisions at an energy of 13 TeV.
The type of particles, the energy of the collisions and the luminosity are among the important characteristics of an accelerator.
An accelerator can circulate a lot of different particles, provided that they have an electric charge so that they can be accelerated with an electromagnetic field. The CERN accelerator complex accelerates protons, but also nuclei of ionized atoms (ions), such as the nuclei of lead, argon or xenon atoms. Some LHC runs are thus dedicated to lead-ion collisions. The ISOLDE facility accelerates beams of exotic nuclei for nuclear physics studies.
The energy of a particle is measured in electronvolts. One electronvolt is the energy gained by an electron that accelerates through a one-volt electrical field. As they race around the LHC, the protons acquire an energy of 6.5 million million electronvolts, known as 6.5 tera-electronvolts or TeV. It is the highest energy reached by an accelerator, but in everyday terms, this is a ridiculously tiny energy; roughly the energy of a safety pin dropped from a height of just two centimetres. But an accelerator concentrates that energy at the infinitesimal scale to obtain very high concentrations of energy close to those that existed just after the Big Bang.
Luminosity is a key indicator of an accelerator’s performance: it indicates the number of potential collisions per surface unit over a given period of time. The instantaneous luminosity is expressed in cm-2s-1 and the integrated luminosity, corresponding to the number of collisions that can occur over a given period, is measured in inverse femtobarn. One inverse femtobarn corresponds to 100 million millions (potential) collisions.
CERN operates a complex of nine accelerators and two decelerators. These accelerators supply experiments or are used as injectors, accelerating particles for larger accelerators. Some, such as the Proton Synchrotron (PS) or Super Proton Synchrotron (SPS) do both at once, preparing particles for experiments that they supply directly and injecting into larger accelerators.
The Large Hadron Collider is supplied with protons by a chain of four accelerators that boost the particles and divide them into bunches.
The accelerators are controlled by operators 24 hours a day from the CERN Control Centre.
Les expériences LHC
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.
Les expériences avec cibles fixes
Dans les expériences avec cibles fixes, un faisceau de particules accélérées est dirigé sur une cible solide, liquide ou gazeuse, qui peut faire partie intégrante du système de détection.
Le SPS alimente également la zone Nord, qui abrite un certain nombre d’expériences. NA61/SHINE étudie une transition de phase entre les hadrons et le plasma de quarks et de gluons, et effectue des mesures pour des expériences étudiant les rayons cosmiques et les oscillations neutrinos longue distance. NA62 utilise des protons du SPS pour analyser des modes rares de désintégration des kaons. NA63 dirige des faisceaux d’électrons et de positons sur diverses cibles afin d’étudier les processus de rayonnement dans les champs électromagnétiques forts. NA64 recherche de nouvelles particules qui serviraient de médiateurs pour une interaction inconnue entre la matière visible et la matière noire. NA65 étudie la production de neutrinos tau. UA9 étudie comment les cristaux pourraient aider à diriger les faisceaux de particules dans les collisions à haute énergie.
L’expérience CLOUD utilise les faisceaux du Synchrotron à protons (PS) pour étudier un lien possible entre les rayons cosmiques et la formation des nuages.
DIRAC, qui analyse actuellement les données, explore les interactions fortes entre les quarks. L’expérience COMPASS, qui s’est achevée en 2022, étudiait la structure des hadrons (particules constituées de quarks).
Physics Beyond Colliders
Physics Beyond Colliders (PBC) is an exploratory study aimed at exploiting the full scientific potential of CERN’s complex of accelerators and experiment areas.
PBC
Physics Beyond Colliders
Les expériences sur l’antimatière
Currently the Antiproton Decelerator and ELENA serve several experiments that are studying antimatter and its properties: AEGIS, ALPHA, ASACUSA, BASE and GBAR. PUMA is designed to carry antiprotons to ISOLDE. Earlier experiments (ATHENA, ATRAP and ACE) are now completed.
Les installations pour les expériences
Experimental facilities at CERN include ISOLDE, MEDICIS, the neutron time-of-flight facility (n_TOF) and the CERN Neutrino Platform.
Les expériences hors accélérateur
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 looking for hypothetical dark matter particles called axions, are now completed and are currently analysing their data.
AMS
Alpha Magnetic Spectrometer
Les expériences passées
Des centaines d’expériences menées sur plusieurs décennies ont fait partie du programme d’expérimentation du CERN.
Parmi elles figuraient des expériences pionnières de la physique électrofaible, une branche de la physique qui unifie deux des quatre forces fondamentales, à savoir la force faible et la force électromagnétique. En 1958, une expérience auprès du Synchrocyclotron découvre une désintégration rare du pion, qui permettra de faire connaître le CERN dans le monde entier. Par la suite, en 1973, la chambre à bulles Gargamelle présente le premier signe direct de l’existence du courant neutre faible. Dix ans plus tard, les scientifiques du CERN qui travaillent alors sur les détecteurs UA1 et UA2 annoncent la découverte du boson W (en janvier 1983) puis du boson Z (en juin 1983) – les deux porteurs de la force électrofaible. Les deux scientifiques qui ont joué un rôle clé dans ces travaux, Carlo Rubbia et Simon van der Meer, recevront le prix Nobel de physiqueen 1984.
À partir de 1989, le Grand collisionneur électron-positon (LEP) permettra aux expériences ALEPH, DELPHI, L3 et OPAL d’établir sur des bases expérimentales solides le Modèle standard de la physique des particules. Le LEP cessera d’être exploité en 2000 pour permettre la construction dans le même tunnel du Grand collisionneur de hadrons (LHC).
Les contributions majeures du CERN à la physique électrofaible ne sont que quelques-uns des résultats marquants obtenus par les expériences au fil des années.