Looking for an axion in a haystack of muons

TL;DR

This paper provides a highly precise theoretical calculation of muon decay processes to aid in the search for axion-like particles, enhancing experimental sensitivity by reducing background uncertainties.

Contribution

It introduces a state-of-the-art computation of muon decay with NNLO QED corrections, tailored for polarized muons, to improve axion search strategies in MEG II and Mu3e experiments.

Findings

Enhanced theoretical predictions for muon decay spectra.

Estimated experimental sensitivity to axion-like particles.

Quantified impact of theoretical uncertainties on detection prospects.

Abstract

The search for axion-like particles $X$ in muon decays is an excellent opportunity for the MEG II and Mu3e experiments to extend their horizons beyond $\mu^+ \to e^+ \gamma$ and $\mu^+ \to e^+ e^- e^+$. A suitable process for both experiments is the two-body decay $\mu^+ \to e^+ X$, whose only signature is a monochromatic peak close to the kinematic endpoint of the positron energy spectrum of the $\mu^+ \to e^+ \nu_e \bar\nu_\mu$ background. The hunt for such an elusive signal in a vast amount of irreducible background requires extremely accurate theoretical predictions to be implemented in a Monte Carlo event generator. This work presents a new state-of-the-art computation of $\mu^+ \to e^+ \nu_e \bar\nu_\mu$ for polarised muons, accomplished with the McMule framework. The calculation includes next-to-next-leading order QED corrections and logarithmically enhanced terms at even higher orders. The results are also used to estimate the sensitivity of both experiments on the branching ratio of $\mu^+ \to e^+ X$, in order to evaluate the impact of the theoretical error.