Muon polarization in the MEG experiment: predictions and measurements

TL;DR

This study measures the residual muon polarization in the MEG experiment, confirming it aligns with Standard Model predictions and is crucial for background modeling in the search for lepton flavor violation.

Contribution

The paper provides the first measurement of residual muon polarization at the stopping target in the MEG experiment, including detailed estimation of depolarizing effects.

Findings

Residual muon polarization measured as -0.85 ± 0.03 (stat) +0.04/-0.05 (syst)

Measured polarization consistent with Standard Model predictions

Results improve background modeling for lepton flavor violation searches

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 $\mu^{+} \rightarrow {\rm e}^{+} \gamma$. 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 energy. The initial muon beam polarization at the production is predicted to be $P_{\mu} = -1$ by the Standard Model (SM) with massless neutrinos. We estimated our residual muon polarization to be $P_{\mu} = -0.85 \pm 0.03 ~ {\rm (stat)} ~ { }^{+ 0.04}_{-0.05} ~ {\rm (syst)}$ 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 ${\megsign}$ search induced by the muon radiative decay: $\mu^{+} \rightarrow {\rm e}^{+} \bar{\nu}_{\mu} \nu_{\rm e} \gamma$.