FASER: ForwArd Search ExpeRiment at the LHC

TL;DR

FASER is a new, small detector placed near the LHC beam line to search for light, weakly-coupled particles like dark photons, offering a cost-effective way to explore unprobed regions of new physics parameter space.

Contribution

This paper proposes the FASER experiment, a novel small detector located downstream of the LHC interaction point to detect light, weakly-coupled particles, expanding the search for new physics.

Findings

FASER can discover dark photons with masses 10 MeV to 1 GeV and kinetic mixing parameters 10^{-7} to 10^{-3}.

A small cylindrical detector is sufficient for significant sensitivity.

FASER can also detect other new physics phenomena.

Abstract

New physics has traditionally been expected in the high-$p_T$ region at high-energy collider experiments. If new particles are light and weakly-coupled, however, this focus may be completely misguided: light particles are typically highly concentrated within a few mrad of the beam line, allowing sensitive searches with small detectors, and even extremely weakly-coupled particles may be produced in large numbers there. We propose a new experiment, ForwArd Search ExpeRiment, or FASER, which would be placed downstream of the ATLAS or CMS interaction point (IP) in the very forward region and operated concurrently there. Two representative on-axis locations are studied: a far location, $400~\text{m}$ from the IP and just off the beam tunnel, and a near location, just $150~\text{m}$ from the IP and right behind the TAN neutral particle absorber. For each location, we examine leading neutrino- and beam-induced backgrounds. As a concrete example of light, weakly-coupled particles, we consider dark photons produced through light meson decay and proton bremsstrahlung. We find that even a relatively small and inexpensive cylindrical detector, with a radius of $\sim 10~\text{cm}$ and length of $5-10~\text{m}$, depending on the location, can discover dark photons in a large and unprobed region of parameter space with dark photon mass $m_{A'} \sim 10~\text{MeV} - 1~\text{GeV}$ and kinetic mixing parameter $\epsilon \sim 10^{-7} - 10^{-3}$. FASER will clearly also be sensitive to many other forms of new physics. We conclude with a discussion of topics for further study that will be essential for understanding FASER's feasibility, optimizing its design, and realizing its discovery potential.