Leptophilic ALPs with TWIST data for polarized muon decays

TL;DR

This paper investigates the production of axion-like particles in polarized muon decays, using TWIST data to set bounds on ALP-lepton couplings and exploring the potential for future constraints and collider stability of ALPs.

Contribution

It introduces a novel calculation of muon decay involving ALPs with a $d=7$ operator and derives experimental bounds from TWIST data, expanding the search for ALPs in muon decay processes.

Findings

TWIST data constrains ALP-lepton couplings for $0 < m_{ALP} < m_{mu}/4$

Potential for stronger future constraints with minimal experimental modifications

ALPs could have significant longevity in collider environments within the studied mass range

Abstract

We study the production of axion-like particles (ALPs) in association with electrons and neutrinos in the muon decay process. For this purpose, we compute the decay width of the muon to a four-body channel using a $d=7$ effective operator that couples the ALP to the Standard model fermions, namely leptons and neutrinos. Assuming a dominant coupling of the ALP to the dark sector, we only consider ALP decays to invisible final states. To obtain constraints on our model using the existing measurements, we leverage data from the TRIUMF Weak Interaction Symmetry Test (TWIST) experiment and obtain bounds on the ALP-lepton coupling for masses in the range of $0 < m_{\phi} < m_{\mu}/4$, as allowed by kinematics. Using the precision of current TWIST measurements, we obtain an order of magnitude estimation necessary for future searches to further constrain the parameter space for such a setup. Furthermore, we find that keeping realistic considerations, the new physics contribution can possibly be enhanced even with a minimalistic modification to the fiducial area used in the experiment potentially allowing for stringer constraints. At the end, in an attempt to relax the assumption that ALP decays to invisible only, we also investigate its stability and find potential longevity within collider environments for the mass range considered in this study.