Search for the lepton flavor violating decay μ + → e + e - e + with the Mu3e experiment at PSI

Abstract

Abstract Mu3e is an experiment under construction at the Paul Scherrer Institute (Switzerland) aiming to find or exclude the charged lepton flavor violating decay μ + → e + e - e + at branching fractions above 10-16. A first running phase using an existing beamline targets a single event sensitivity of 2 · 10-15, while the ultimate sensitivity is reached in a second phase with a new high intensity muon beamline. Achieving such sensitivity requires a high muon rate along with a detector of large kinematic acceptance able to reconstruct the low momenta of the decay positrons and electrons with minimal material budget. The Mu3e experiment is mounted with ultra thin tracking detectors based on high-voltage monolithic active pixel sensors, combined with scintillating fibers and tiles for precise timing. This paper presents an overview of the experimental requirements, the technical design, and the construction readiness of Mu3e.