Displaced Searches for Axion-Like Particles and Heavy Neutral Leptons at Mu3e

TL;DR

This paper proposes search strategies for the Mu3e experiment to detect light, weakly coupled particles via displaced decay signatures, highlighting Mu3e's potential to explore new parameter space beyond existing constraints.

Contribution

It introduces novel search strategies for displaced decays in Mu3e, demonstrating the experiment's capability to probe unexplored regions of parameter space for axion-like particles and heavy neutral leptons.

Findings

Backgrounds can be fully suppressed with suitable cuts.

Mu3e can probe unexplored parameter space.

Displaced and prompt searches are complementary.

Abstract

We present strategies for the Mu3e experiment to search for light, weakly coupled particles produced in rare muon decays, focusing on displaced $e^+e^-$ decays within the hollow target. In most scenarios the backgrounds can be fully suppressed with a suitable set of cuts. We furthermore quantify the interplay between displaced and prompt searches at Mu3e and existing constraints, showing how Mu3e has a unique opportunity to probe unexplored parameter space.