No abstract

The MEG experiment, designed to search for the µ + → e + γ decay at a 10 −13 sensitivity level, completed datataking in 2013. In order to increase the sensitivity reach of the experiment by an order of magnitude to the level of 6 × 10 −14 for the branching ratio, a total upgrade, involving substantial changes to the experiment, has been undertaken, known as MEG II. We present both the motivation for the upgrade and a detailed overview of the design of the experiment and of the expected detector performance.

The MEG experiment makes use of one of the world's most intense low energy muon beams, in order to search for the lepton flavour violating process μ + → e + γ . We determined the residual beam polarization at the thin stopping target, by measuring the asymmetry of the angular distribution of Michel decay positrons as a function of B. I. Khazin Deceased. a e-mail: fabrizio.cei@pi.infn.it energy. The initial muon beam polarization at the production is predicted to be P μ = −1 by the Standard Model (SM) with massless neutrinos. We estimated our residual muon polarization to be P μ = −0.86 ± 0.02 (stat) +0.05 −0.06 (syst) at the stopping target, which is consistent with the SM predictions when the depolarizing effects occurring during the muon production, propagation and moderation in the target are taken into account. The knowledge of beam polarization is of fundamental importance in order to model the background of our μ + → e + γ search induced by the muon radiative decay: μ + → e +ν μ ν e γ .