Conversions of Bound Muons: Lepton Flavour Violation from Doubly Charged Scalars

TL;DR

This paper provides the first detailed calculation of muon-to-electron conversion mediated by a doubly charged scalar, highlighting its potential to surpass collider limits in probing new physics and emphasizing the importance of low-energy lepton flavor violation searches.

Contribution

It offers a comprehensive calculation of muon-electron conversion via a doubly charged scalar and compares predictions with experimental data, enhancing understanding of lepton flavor violation mechanisms.

Findings

Conversion rates depend on scalar couplings and future experimental limits.

Potential for muon-electron conversion to set stronger bounds on scalar mass than colliders.

Results support low-energy experiments as vital in new physics searches.

Abstract

We present the first detailed computation of the conversion of a bound muon into an electron mediated by a doubly charged $SU(2)$ singlet scalar. Although such particles are not too exotic, up to now their contribution to $\mu$-$e$ conversion is unknown. We close this gap by presenting a detailed calculation, which will allow the reader not only to fully comprehend the discussion but also to generalise our results to similar cases if needed. We furthermore compare the predictions, for both the general case and for an example model featuring a neutrino mass at 2-loop level, to current experimental data and future sensitivities. We show that, depending on the explicit values of the couplings as well as on the actual future limits on the branching ratio, $\mu$-$e$ conversion may potentially yield a lower limit on the doubly charged singlet scalar mass which is stronger than what could be obtained by colliders. Our results considerably strengthen the case for low-energy lepton flavour violation searches being a very valuable addition to collider experiments.