Looking for an axion in a haystack of muons

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.