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.
Highlights
The search for new physics at the LHC has primarily focused on high-pT physics at the ATLAS [1] and CMS [2] experiments
We study two representative locations for FASER, both of which are on the beam collision axis, but are just off the beam line: a “far” location 400 m downstream from the interaction point (IP), where the beam is curved, and a “near” location 150 m downstream from the IP, just behind the TAN neutral particle absorber, where the beam lines are split into two beam pipes
In the high-momentum, forward regions, the expected event yield is comparable and can even exceed that from meson decays, even though the proton bremsstrahlung cross section is far below that of meson production. This is due to the different characteristics of the two processes: the A0 spectrum from meson decays is centered around pT ∼ ΛQCD and decreases at high pT, the dark photon bremsstrahlung spectrum follows the characteristics of photon bremsstrahlung and peaks around the collinear cutoff, with pT ≈ mA0 and the high-pT tail of the distribution suppressed by ∼1=p2T [see Eq (B3)]
Summary
The search for new physics at the LHC has primarily focused on high-pT physics at the ATLAS [1] and CMS [2] experiments. Even extremely weakly coupled new particles may be produced in sufficient numbers in the very forward region Such particles may be highly collimated, as they are typically produced within θ ∼ ΛQCD=E ∼ mrad of the beam line, where ΛQCD ≃ 250 MeV and E ∼ 100 GeV − 1 TeV is the energy of the particle. A small, inexpensive detector placed in the very forward region may be capable of extremely sensitive searches, provided a suitable location can be found and the signal can be differentiated from the SM background. Given this potential, we propose a new experiment, Forward search experiment, or FASER.. In Appendixes A and B, we present results for the representative off-axis detector location and details of our proton bremsstrahlung rate calculation, respectively
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