Crossing into the $m_a > f_a$ Region for Leptophilic ALPs

TL;DR

This paper explores leptophilic axion-like particles (ALPs) in the parameter space where their mass exceeds their decay constant, analyzing their potential to explain electron magnetic moment anomalies and proposing future experimental tests.

Contribution

It investigates the less-explored region where ALP mass exceeds decay constant, focusing on leptophilic ALPs and their phenomenological implications.

Findings

Leptophilic ALPs can explain the electron magnetic moment tension.

The $m_a > f_a$ region is viable and uninvestigated in previous studies.

Future $ o e$ conversion experiments can test this parameter space.

Abstract

Axion-like particles (ALPs) are typically identified as pseudoscalars whose couplings are shift-symmetry invariant with the exception of their couplings to gauge bosons and their mass term. Additionally, the ALP mass $m_a$ is usually assumed to be (much) smaller than the ALP decay constant $f_a$. The latter condition is conservative, at best, and excludes part of the ALP parameter space that is presently viable. We revisit the interpretation of the $m_a\ll f_a$ and perform an analysis focussing on leptophilic ALPs. In particular, we explore regions of the parameter space still uninvestigated, where $m_a>f_a$, thus providing a phenomenological study of the ALP-lepton couplings complementary to the existing literature. We point out that a leptophilic ALP may explain the $-3.8\sigma$ tension in the anomalous magnetic dipole moment of the electron for the Caesium determination in a large region of the $m_a\times f_a$ parameter space, testable in the near future through studies on $\mu\to e$ conversion in nuclei.