Abstract
FASER,the ForwArd Search ExpeRiment,is a proposed experiment dedicated to searching for light, extremely weakly-interacting particles at the LHC. Such particles may be produced in the LHC's high-energy collisions and travel long distances through concrete and rock without interacting. They may then decay to visible particles in FASER, which is placed 480 m downstream of the ATLAS interaction point. In this work we briefly describe the FASER detector layout and the status of potential backgrounds. We then present the sensitivity reach for FASER for a large number of long-lived particle models, updating previous results to a uniform set of detector assumptions, and analyzing new models. In particular, we consider all of the renormalizable portal interactions, leading to dark photons, dark Higgs bosons, and heavy neutral leptons (HNLs); light B-L and $L_i - L_j$ gauge bosons; axion-like particles (ALPs) that are coupled dominantly to photons, fermions, and gluons through non-renormalizable operators; and pseudoscalars with Yukawa-like couplings. We find that FASER and its follow-up, FASER 2, have a full physics program, with discovery sensitivity in all of these models and potentially far-reaching implications for particle physics and cosmology.
Highlights
A focus of energy-frontier particle colliders, such as the LHC, has been searches for new particles with TeV-scale masses and Oð1Þ couplings
For comparison we show the projected sensitivities of other experiments: NA62 assumes 1018 protons on target (POT) while running in a beam dump mode that is being considered for LHC Run 3 [17]; SeaQuest assumes 1.44 × 1018 POT, which could be obtained in two years of parasitic data taking and requires the installation of a calorimeter [19]; the proposed beam dump experiment SHiP assumes ∼2 × 1020 POT collected in 5 years of operation [20]; the proposed electron fixed-target experiment LDMX
IVA, the sensitivity line for NA62 assumes 1018 POT for the experiment running in a beam dump mode that is being considered for LHC Run 3 [17]; SeaQuest assumes 1.44 × 1018 POT, which could be obtained in two years of parasitic data taking and requires the installation of a calorimeter [19]; and the proposed beam dump experiment SHiP assumes ∼2 × 1020 POT collected in 5 years of operation [20]
Summary
A focus of energy-frontier particle colliders, such as the LHC, has been searches for new particles with TeV-scale masses and Oð1Þ couplings. A further, 20-fold increase can be expected in the high luminosity LHC (HL-LHC) era These particles are highly concentrated in the very forward direction; for example, as is discussed in detail below, approximately 0.6% (10%) of all neutral pions are produced within 0.2 mrad (2 mrad) of the beam collision axis, which is the angular acceptance for FASER (FASER 2). The FLUKA results combined with radiation monitor measurements have confirmed low radiation levels in these tunnels These locations, provide extremely low background environments for FASER to search for LLPs that are produced at or close to the IP, propagate in the forward direction close to the beam collision axis, and decay visibly within FASER’s decay volume. We determine the sensitivity reach of both FASER and FASER 2 for a wide variety of proposed particles, updating
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