Towards a new generation of Charged Lepton Flavour Violation searches at the Paul Scherrer Institut: The MEG upgrade and the Mu3e experiment

Abstract

The MEG experiment has recently set a new upper limit on the branching ratio of the µ+ → e+γ decay, ℬ(µ+ → e+γ) < 4.2 × 10−13 (at 90% confidence level) and un upgrade of the experiment (the MEGII experiment) is ongoing with the aim of improving the single event sensitivity (SES) by one order of magnitude with respect to the previous MEG experiment’s SES. The strong scientific motivation associated with the charged Lepton Flavour Violation (cLFV) searches pushes also towards searching for the complementary muon cLFV µ+ → e+e+e− decay with a completely new apparatus, the Mu3e experiment, aiming at a SES improved by at least three orders of magnitude with respect to the previous SINDRUM experiment’s SES (Mu3e phase I). An ultimate SES of few ×10−16 is foreseen requiring 109 µ/s (Mu3e phase II). Both experiments will be hosted at the Paul Scherrer Institut which currently delivers the most intense continuous low energy muon beam in the world up to few ×108 µ/s. The status of both the MEGII and Mu3e phase I experiments is given.