FASER, or the Forward Search Experiment, is designed to search for light and extremely weakly interacting particles. The existence of such new particles is predicted by many models beyond the Standard Model that attempt to solve some of the biggest puzzles in physics, such as the nature of dark matter, the origin of neutrino masses, and the imbalance between matter and antimatter in the present-day universe.
The four main LHC detectors, ALICE, ATLAS, CMS and LHCb, are not suited to detect signals from light and weakly interacting particles produced parallel to the proton beamline, because they have holes along the beamline to let the proton beams through. Located along the beam trajectory, 480 metres downstream of the ATLAS detector, FASER is ideally positioned to detect the particles into which such light and weakly interacting particles will decay.
FASER also has a subdetector called FASERν, which is specifically designed to detect neutrinos. No neutrino produced at a particle collider has ever been detected, despite colliders producing them in huge numbers and at high energies. As a result, neutrino interactions at these high energies have not yet been studied in detail. FASER’s main detector cannot detect the neutrinos produced at the LHC, because it lacks a target with enough material to successfully detect the very weak interactions the particles have with matter. So FASERν is made up of emulsion films and tungsten plates to act as both the target and the detector to see the neutrino interactions.
FASER is supported by the Heising-Simons and Simons Foundations and was installed during the second long shutdown of CERN’s accelerator complex (2019-2021). It started taking data at the beginning of Run 3 of the LHC in July 2022.