FASER, the ForwArd Search ExpeRiment, is designed to search for new, yet undiscovered, light and weakly-interacting particles and study the interactions of high-energy neutrinos.

FASER's Letter of Intent (LOI) and Technical Proposal (TP) were submitted to the LHC Committee (minutes: LOI, TP) in 2018, and FASER was approved by the CERN Research Board in March 2019. FASER is now under construction and will begin taking data in in LHC's Run 3  from 2021-23.

In 2019, the Letter of Intent and Technical Proposal of the FASER neutrino subdetector, FASERν, were submitted to the LHC Committee (minutes: FASERν LOI, FASERν TP). FASERν was approved by the CERN Research Board in December 2019.

New, light particles may be constantly produced in large numbers in proton-proton collisions at the LHC, but still avoid detection. This is because, once produced, such particles go along the proton beam collision axis, where there is no experiment that can detect them in a low-background environment. The background from the interactions of well-known standard model particles needs to be highly suppressed when looking for the rare events associated with new physics particles. On the other hand, new, light, and long-lived particles can easily travel for hundreds of meters without interacting, and then decay to standard model particles, for example, electron-positron pairs, at a distant position well-separated from such background.

Neutrinos are among the least understood particles in the standard model of particle physics, and the experimental study of their interactions, especially at high energies, can have exciting implications for many areas of research. A dedicated subdetector of FASER, dubbed FASERν, which operates in front of the main FASER detector, has been designed to record thousands of such interactions and explore this new high-energy collider neutrino frontier of particle physics.

To explore this far-forward region of the LHC, FASER will be located in the unused LHC service tunnel TI12, which is 480m downstream of the proton-proton interaction point (IP) used by the ATLAS experiment. The fluxes of most high-energy standard model background particles at this position are suppressed to negligible levels, with the only exceptions being the most penetrating known particles, namely muons and neutrinos. The FASER detector will be sensitive to new particles that decay in a cylindrical volume with radius R=10 cm and length L=1.5 m. The electron-positron pairs produced in such decays will be deflected by strong permanent magnets, and their curved tracks will be seen in the FASER spectrometer. They will also deposit their energy in a calorimeter placed downstream of the magnets. A schematic picture of a new-physics decay event in the detector is shown below.

In the accompanying webpages, we give an overview of the most important aspects of the FASER experiment.  A more detailed discussion of the detector location and design, as well as background simulation and measurements, can found in the FASER's Technical Proposal.