ALP contributions to $μ\to e$ conversion

Abstract: We study the $\mu\to e$ conversion process in nuclear targets arising in models of axion-like particles (ALPs) with hadronic and charged lepton flavor violating (CLFV) interactions. Contributions to this process generally fall into two categories: spin-independent (SI) and spin-dependent (SD). While the SI contribution can be generated by a dipole operator through purely leptonic ALP interactions, the SD contribution can also be present through ALP-quark interactions at tree-level. It is naively anticipated that the SI contribution would be dominant due to its coherent enhancement. In this $\textit{letter}$, we show that is not generically the case; in particular, for naturally-sized ALP couplings to quarks of order $\sim!m_q/f_a$, the SD interaction induced by ALP-$\pi^0$ mixing turns out to be the leading contribution to $\mu\to e$ conversion. Intuitively, this stems from the suppressed dipole contribution by the QED one-loop factor which counters the effect of SI coherent enhancement. Our study highlights the importance of $\mu\to e$ conversion searches in exploring the parameter space of generic ALP models, and demonstrates the competitiveness of these searches in probing the CLFV ALP parameter space in the heavy mass range of $m_a\gtrsim m_{\mu}$.