ForwArd Search ExpeRiment at the LHC

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 m from the IP and just off the beam tunnel, and a near location, just 150 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 ∼10 cm and length of 5-10 m, depending on the location, can discover dark photons in a large and unprobed region of parameter space with dark photon mass m A 0 ∼ 10-500 MeV and kinetic mixing parameter ϵ ∼ 10 −6 − 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.

Heavy neutral fermions at the high-luminosity LHC

“…Perhaps unsurprisingly, a number of new proposals to search for LLLPs have appeared, all based on the idea to exploit LHC's large luminosity: MATHUSLA [6], CODEX-b [7] and FASER [8]. The physics potential of these three experiments has so far not been fully discussed in the literature and it is the aim of the current paper to estimate, and compare to each other and previous experiments, the sensitivity of these proposals for fermionic LLLPs.…”

“…For more details we refer to the original publications for MATHUSLA [6], CODEX-b [7] and FASER [8]. We then describe our numerical simulation of these proposals.…”

“…Being closer to the IP, CODEX-b proposed size is much smaller than that of MATHUSLA. Finally, the authors of FASER [8] propose to construct a very modestly sized detector, situated in the very forward direction, close to either the ATLAS or the CMS IP. Note that FASER is discussed in three different variants in [8,9].…”

“…Finally, the authors of FASER [8] propose to construct a very modestly sized detector, situated in the very forward direction, close to either the ATLAS or the CMS IP. Note that FASER is discussed in three different variants in [8,9]. We will summarize the experimental parameters of the different FASER setups and those of CODEX-b and MATHUSLA in section 3.…”

“…They also considered [7] a light neutral scalar, that mixes with the Higgs, produced in B-mesons decays as a LLLP candidate. The original FASER [8] publication studied dark photons produced in meson decays to estimate the sensitivity of the different FASER setups. Here, we study the reach of these three experimental proposals for the case that the LLLP is a heavy neutral fermion (HNF).…”