Eur. Phys. J. C (2016) 76: 223
https://doi.org/10.1140/epjc/s10052-016-4047-3

Regular Article - Experimental Physics

Muon polarization in the MEG experiment: predictions and measurements

The MEG Collaboration

¹ Paul Scherrer Institut PSI, 5232, Villigen, Switzerland
² Swiss Federal Institute of Technology ETH, 8093, Zurich, Switzerland
³ ICEPP, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-0033, Japan
⁴ INFN Sezione di Pisa, dell’Università, Largo B. Pontecorvo 3, 56127, Pisa, Italy
⁵ Dipartimento di Fisica, dell’Università, Largo B. Pontecorvo 3, 56127, Pisa, Italy
⁶ Scuola Normale Superiore, Piazza dei Cavalieri, 56127, Pisa, Italy
⁷ University of California, Irvine, CA, 92697, USA
⁸ INFN Sezione di Pavia, dell’Università, Via Bassi 6, 27100, Pavia, Italy
⁹ Dipartimento di Fisica, dell’Università, Via Bassi 6, 27100, Pavia, Italy
¹⁰ INFN Sezione di Roma, dell’Università “Sapienza”, Piazzale A. Moro, 00185, Rome, Italy
¹¹ Dipartimento di Fisica, dell’Università “Sapienza”, Piazzale A. Moro, 00185, Rome, Italy
¹² INFN Sezione di Genova, dell’Università, Via Dodecaneso 33, 16146, Genova, Italy
¹³ Dipartimento di Fisica, dell’Università, Via Dodecaneso 33, 16146, Genova, Italy
¹⁴ Budker Institute of Nuclear Physics of Siberian Branch of Russian Academy of Sciences, 630090, Novosibirsk, Russia
¹⁵ KEK, High Energy Accelerator Research Organization, 1-1 Oho, Tsukuba, Ibaraki, 305-0801, Japan
¹⁶ Joint Institute for Nuclear Research, 141980, Dubna, Russia
¹⁷ INFN Sezione di Lecce, dell’Università del Salento, Via per Arnesano, 73100, Lecce, Italy
¹⁸ Dipartimento di Matematica e Fisica, dell’Università del Salento, Via per Arnesano, 73100, Lecce, Italy
¹⁹ Novosibirsk State Technical University, 630092, Novosibirsk, Russia
²⁰ Novosibirsk State University, 630090, Novosibirsk, Russia
²¹ INFN, Laboratori Nazionali di Frascati, via E.Fermi, 40 Frascati, 00044, Rome, Italy

^* e-mail: fabrizio.cei@pi.infn.it

Received: 21 October 2015
Accepted: 31 March 2016
Published online: 22 April 2016

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 . 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 by the Standard Model (SM) with massless neutrinos. We estimated our residual muon polarization to be 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 search induced by the muon radiative decay: .