Muon polarization in the MEG experiment: predictions and measurements

Abstract

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 .

Search for the lepton flavour violating decay $$\mu ^+ \rightarrow \mathrm {e}^+ \gamma $$ μ + → e + γ with the full dataset of the MEG experiment

“…The round, Gaussian beam-spot profile has σ x,y ≈10 mm. The muons at the production target are produced fully polarized (P μ + = −1) and they reach the stopping target with a residual polarization P μ + =−0.86 ± 0.02 (stat) +0.05 −0.06 (syst) consistent with the expectations [9].…”

“…The efficiency ratio of the direction match is evaluated from the distribution of accidental background. The effect of muon polarisation [9], which makes the background distribution non-flat (asymmetric) even in case of a fully efficient detector and trigger, is taken into account. Inefficiency due to the AIF-like event cuts and the tail in the time reconstruction are common to signal and RMD, and thus, only tails in the angular responses are relevant.…”

Hunting for ALPs with lepton flavor violation

“…where p a denotes the ALP momentum in the lab frame. By assuming that j decays at rest, as in the case of µ → e experiments such as MEG [36] or Mu3e [17], |p a | reads…”

Constraints on sterile neutrinos in the MeV to GeV mass range

“…For m ν4 = 100 MeV, the factor multiplying |U µ4 U e4 | 2 is 0.52 × 10 −15 . Given the experimental upper limit BR(µ + → e + γ) < 4.2 × 10 −13 [162], it is clear that this does not yield a useful constraint on |U µ4 U e4 | 2 . A massive Dirac neutrino has a magnetic moment [163] (see also [164]; recent reviews include [165,166]).…”