Searching for new physics at $\mu\rightarrow e$ facilities with $\mu^+$ and $\pi^+$ decays at rest

TL;DR

This paper explores how muon and pion decay experiments can detect new light particles through monoenergetic positron signals, potentially surpassing current constraints with rapid data collection.

Contribution

It proposes using muon and pion decay at rest experiments to search for new physics particles, highlighting their potential to probe unexplored parameter space efficiently.

Findings

Mu2e can explore new parameter space for axion-like particles and Z' bosons.

Detection of monoenergetic positrons can indicate new physics beyond current constraints.

Projected sensitivities can be achieved within days of data collection.

Abstract

We investigate the ability of $\mu\rightarrow e$ facilities, Mu2e and COMET, to probe, or discover, new physics with their detector validation datasets. The validation of the detector response may be performed using a dedicated run with $\mu^+$, collecting data below the Michel edge, $E_e\lesssim 52$ MeV; an alternative strategy using $\pi^+\rightarrow e^+ \nu_e$ may also be considered. We focus primarily on a search for a monoenergetic $e^+$ produced via two-body decays $\mu^+ \rightarrow e^+ X$ or $\pi^+\rightarrow e^+X$, with $X$ a light new physics particle. Mu2e can potentially explore new parameter space beyond present astrophysical and laboratory constraints for a set of well motivated models including: axion like particles with flavor violating couplings ($\mu^+ \rightarrow e^+ a$), massive $Z'$ bosons ($\mu^+ \rightarrow Z' e^+$), and heavy neutral leptons ($\pi^+\rightarrow e^+N$). The projected sensitivities presented herein can be achieved in a matter of days.