For decades, the leading examples of physics beyond the Standard Model (SM) were particles with TeV-scale masses and large couplings to the SM. While it is of primordial importance to perseverate with the ongoing searches at the large LHC experiments, there is a growing interest in the complementary search for new particles that are much lighter and more weakly-coupled, which would have potentially revolutionary implications for particle physics and cosmology. Indeed, among their many motivations, such particles may yield dark matter with the correct thermal relic density and resolve outstanding discrepancies between theory and low-energy experiments. 

In contrast to TeV-scale particles that are produced more or less isotropically, light particles with masses in the MeV-GeV range are dominantly produced at low pT. In addition, because the new particles are extremely weakly coupled, very large standard model event rates are required to discover the rare new physics events. These rates are not available at high pT, but they are available at low pT: at the 13 TeV LHC, the total inelastic pp scattering cross section is σ_inel(13 TeV) ≈ 75 mb, with most of it in the very forward direction. This implies ≈ 2.3 x 10^16 (2.3 x 10^17) inelastic pp scattering events for an integrated luminosity of 300 fb^-1 at the LHC (3 ab^-1 at the HL-LHC). Due to their weak coupling to the SM, such particles are typically long-lived and travel a macroscopic distance before decaying back into SM particles. Moreover, such particles may be highly collimated. For example, new particles that are produced in pion decays are typically produced within angles of θ ~ Λ_QCD / E of the beam collision axis, where E is the energy of the particle. For E ~ TeV, this implies that even ~500m downstream, such particles have only spread out ~ 10 cm in the transverse plane. A small and inexpensive detector placed in the very forward region may therefore be capable of extremely sensitive searches, provided a suitable location can be found and the signal can be differentiated from the SM background. 

FASER, the ForwArd Search ExpeRiment, is designed to take advantage of this opportunity.

It is a small detector, with volume ~1 m³, that will be placed along the beam collision axis, 480 meters downstream from the ATLAS interaction point.

A detailed description can be found in the FASER web page.

The detector is composed by 1.5 m long, 0.6 T permanent dipole magnets (red) with a R = 10 cm aperture radius. The cylindrical volume enclosed by the first magnet serves as the decay volume for the light, weakly-interacting particles, with the magnet providing a horizontal kick to separate oppositely-charged particles to a detectable distance. Two more magnets are interleaved with the three stations of a silicon-strip tracker (blue), which will observe the characteristic signal of two oppositely charged particles pointing back towards the IP, measure their momenta, and sweep out low-momentum charged particles before they reach the back of the spectrometer.. Scintillator planes (gray) for triggering and precision time measurements are located at the entrance and exit of the spectrometer. Finally, an electromagnetic calorimeter (purple) identifies high energy electrons and photons and measures the total electromagnetic energy.