$\mu \rightarrow$ e conversion and the Mu2e experiment at Fermilab

Abstract

The Mu2e experiment aims to measure the charged-lepton flavour violating (CLFV) neutrino-less conversion of a negative muon into an electron in the field of a nucleus. The conversion process results in a monochromatic electron with an energy slightly below the muon rest mass (104.97 MeV). The goal of the experiment is to improve by four orders of magnitude the previous measurement and reach a single event sensitivity of $3 \times 10^{-17} $on the conversion rate with respect to the muon capture rate. Although the SM is very well tested in many regimes, it is incomplete. In many of Beyond the Standard Model (BSM) scenarios, rates for CLFV processes are within the reach of the next generation of experiments. In particular, if SUSY particles have masses and couplings within the discovery reach of the LHC, CLFV rates may be observable at the next generation experiment. On the contrary, many CLFV searches have a sensitivity to new physics that exceeds the LHC reach bringing the reach of new mass scale up to $10^4$ TeV. In this contest indirected measurements of CLFV will be crucial evidence of new physics. The experiment will use a very intense pulsed negative muon beam directed onto an Aluminium target for a total number of $10^{18}$ stopped muons in three years of running. Production and transport of the muons is accomplished with a complicated and sophisticated magnetic systems composed of a production, a transport and a detector solenoid, hosting a straw-tube tracker and a crystal calorimeter. The entire detector region is surrounded by a Cosmic Ray Veto system. Mu2e is under design and construction at the Muon Campus of Fermilab. In the current schedule, the installation phase will start in 2021, the commisioning in 2022 and the data taking will start in 2023.