FASER: ForwArd Search ExpeRiment at the LHC

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.

What's Out There ?

New Experiment
Today's Focus: the forward-region of pp-collisions.

Radiative Muon background
Expected proton loss rate as a function of the LHC cell in the dispersion su ASER is located in cell 12.

Process
Event Rate 7.4K µ + EM shower 22K µ + hadronic shower 21K . Event rates for muons and muon-induced processes that enter FASER from the with energy ≥ 100 GeV in Run 3 with integrated luminosity 150 fb −1 . The b the subset of all µ + γ brem events in which the photon pair converts in FASE the calorimeter.
I. Bremsstrahlung is the dominant radiative process, but most photons co before reaching FASER. An estimated 41,000 photons with energies above 1 Muon's from IP in processes with E > 100 GeV in Run-3 conditions with 150 fb −1 Upstream tracking section: 10 emulsion films interleaved with 10-mm-thick Styrofoam, designed to detect two almost-parallel tracks.
Emulsion film: two 65 µm thick emulsion layers, poured onto both sides of a 200 µm-thick plastic base.
Downstream sampling calorimeter section -Emulsion Cloud Chamber (ECC): a repeated structure of emulsion films interleaved with 1 mm or 5 mm thick lead plates for the EM-shower energy measurement. 12 X 0 in length .
The emulsion films are vacuum-packed with a light-tight bag and enclosed in an acrylic box.
Two removable emulsion detectors have been placed on the front and back faces of the acrylic box to provide the possibility of a prompt check of the track density shortly after TS1.