Projected sensitivity of the \anubis detector to photophobic and fermion-coupled axion-like particles

Abstract

Axion-like particles (ALPs) are a common benchmark for experiments at the lifetime frontier, as their suppressed couplings to Standard Model particles can render them long-lived on detector scales. This article presents the projected sensitivity of the ANUBIS detector at the High-Luminosity LHC (HL-LHC) to two scenarios: a photophobic ALP model and a modified version of the flavour-diagonal, fermion-coupled ALP benchmark of the Physics Beyond Colliders initiative that introduces an additional $h\to Za$ decay. For the photophobic case, the ANUBIS detector will reach a peak sensitivity of $σ_{Wa}\cdot Br(a\rightarrow gg)=6.2\times10^{-4}$~pb at $cτ=1.95$ m for an ALP mass of $10$ GeV. This will improve on the current exclusion limits from the ATLAS experiment by two orders of magnitude and extend the $cτ$ coverage by three orders of magnitude. For the fermion-coupled case with a dimension-seven Higgs--ALP interaction, the peak sensitivity to the fermion coupling is $C_{a^Φ}=4.4\times10^{-8}$, reached for ALP masses between $16$ and $23$ GeV. Overall, the ANUBIS detector is shown to provide unique coverage of the parameter space of both models, in regions that can be probed neither by the main LHC experiments nor by forward detectors at the HL-LHC.