# ALP contributions to $μ\to e$ conversion

**Authors:** Kaori Fuyuto, Emanuele Mereghetti  
**arXiv:** [2307.13076](https://arxiv.org/abs/2307.13076)  
**Date:** 2023-07-24  
**Summary page:** https://www.summarizepaper.com/en/arxiv-id/2307.13076v1/

## Key points

- Authors investigate $\mu\to e$ conversion in nuclear targets within the framework of axion-like particles (ALPs) with hadronic and charged lepton flavor violating (CLFV) interactions
- Contributions to the process can be categorized into spin-independent (SI) and spin-dependent (SD)
- Traditionally, SI contribution expected to dominate due to coherent enhancement
- However, authors demonstrate that SD interaction induced by ALP-$\pi^0$ mixing is actually the leading contribution
- Suppressed dipole contribution caused by QED one-loop factor counteracts effect of SI coherent enhancement
- For ALP couplings to quarks on order of $m_q/f_a$, SD interaction becomes dominant
- Study shows how searches for $\mu\to e$ conversion can probe CLFV ALP parameters in heavy mass ranges ($m_a \gtrsim m_{\mu}$)
- Also explores generic ALP models' parameter space
- Provides valuable insights for future research in this field

## Summary

In their study, authors Kaori Fuyuto and Emanuele Mereghetti investigate the process of $\mu\to e$ conversion in nuclear targets within the framework of axion-like particles (ALPs) with hadronic and charged lepton flavor violating (CLFV) interactions. They analyze the contributions to this process, which can be categorized into two types: spin-independent (SI) and spin-dependent (SD). Traditionally, it has been expected that the SI contribution would dominate due to its coherent enhancement. This is because the SI contribution can be generated by a dipole operator through purely leptonic ALP interactions. However, Fuyuto and Mereghetti challenge this assumption and demonstrate that the leading contribution to $\mu\to e$ conversion is actually the SD interaction induced by ALP-$\pi^0$ mixing. The authors explain that this unexpected result arises from the suppressed dipole contribution caused by the QED one-loop factor, which counteracts the effect of SI coherent enhancement. Additionally, they find that for ALP couplings to quarks on the order of $m_q/f_a$, where $m_q$ represents quark masses and $f_a$ denotes an ALP decay constant, the SD interaction becomes dominant. This study emphasizes how searches for $\mu\to e$ conversion can effectively probe CLFV ALP parameters in heavy mass ranges ($m_a \gtrsim m_{\mu}$), as well as exploring generic ALP models' parameter space. It also highlights a previously overlooked aspect of ALP physics that provides valuable insights for future research in this field.