Searching for a new light gauge boson with axial couplings in muon beam dump experiments

TL;DR

This paper develops a formalism for axial gauge bosons with hypercharge, baryon, and lepton number interactions, emphasizing their detection prospects in muon beam dump experiments and their distinctive axion-like behavior.

Contribution

It introduces a comprehensive framework for axial $U(1)$ gauge bosons, analyzing their interactions, mixing effects, and experimental signatures, especially in muon beam dump setups.

Findings

Axially coupled $U$ bosons behave like axion-like particles at high energies.

Enhanced couplings to muons increase detection prospects in muon beam dump experiments.

Exclusion and discovery regions depend on $U$ boson decay modes and experiment geometry.

Abstract

We present a formalism for new $U(1)$ interactions involving weak hypercharge, baryon, and lepton numbers, and a possible axial symmetry generator $F_A$ in the presence of a second Brout-Englert-Higgs doublet. The resulting $U$ boson, after mixing with the $Z$, interpolates between a generalised dark photon, a dark $Z$, and an axially coupled gauge boson. We especially focus on the axial couplings originating from $F_A$ or from mixing with the $Z$, determined by the scalar sector via parameters like $\tan\beta$ and the v.e.v. of an extra dark singlet. We explore the distinctive features of axially coupled interactions, especially in the ultrarelativistic limit, where the $U$ boson behaves much as an axion-like particle, with enhanced interactions to quarks and leptons. This enhancement is particularly relevant for future muon beam dump experiments, since the muon mass considerably increases the effective coupling, proportional to $2m_\mu/m_U$, compared to analogous experiments with electrons. We also analyse the shape of the expected beam dump exclusion or discovery regions, influenced by $U$ boson interactions and the experiment geometry. Different situations are considered, limited in particular by cases for which the $U$ decays before reaching the detector, or has too small couplings to produce detectable events. We also compare to vectorially coupled bosons and axion-like pseudoscalars, highlighting the importance of understanding the parameter space for future experiment design and optimisation.