Abstract
The goal of FASER, ForwArd Search ExpeRiment at the LHC, is to discover light, weakly-interacting particles with a small and inexpensive detector placed in the far-forward region of ATLAS or CMS. A promising location in an unused service tunnel 480 m downstream of the ATLAS interaction point (IP) has been identified. Previous studies have found that FASER has significant discovery potential for new particles produced at the IP, including dark photons, dark Higgs bosons, and heavy neutral leptons. In this study, we explore a qualitatively different, `beam dump' capability of FASER, in which the new particles are produced not at the IP, but through collisions in detector elements further downstream. In particular, we consider the discovery prospects for axion-like particles (ALPs) that couple to the standard model through the $a \gamma \gamma$ interaction. TeV-scale photons produced at the IP collide with the TAN neutral particle absorber 130 m downstream, producing ALPs through the Primakoff process, and the ALPs then decay to two photons in FASER. We show that FASER can discover ALPs with masses $m_a \sim 30 - 400~\text{MeV}$ and couplings $g_{a\gamma\gamma} \sim 10^{-6} - 10^{-3}~\text{GeV}^{-1}$, and we discuss the ALP signal characteristics and detector requirements.
Highlights
The Large Hadron Collider (LHC) has already played a remarkable role in pushing back the boundaries of our knowledge about particle physics
Searches for new light, weakly coupled particles could provide the first evidence of physics beyond the standard model, with wide-ranging implications for particle physics and cosmology
This possibility has stimulated a variety of proposals for experiments that could discover these new particles, and it motivates studies to determine the reach and promise of these proposed experiments
Summary
The Large Hadron Collider (LHC) has already played a remarkable role in pushing back the boundaries of our knowledge about particle physics. If new particles are light and weakly interacting, they may be copiously produced in proton-proton collisions, but they preferentially go in the forward direction and escape detection at the ATLAS [8] and CMS [9] experiments This suggests that novel experimental proposals may be able to enhance the LHC’s discovery potential. We studied FASER’s potential to detect dark photons [10], dark Higgs bosons [11], and HNLs [12] In all of these cases, the new physics couples to the SM through dimension-4 interactions (renormalizable portals), and the new particles are produced at the IP. Production in rare meson decays, and the angular acceptance function for FASER are given in Appendices A, B, and C, respectively
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