https://doi.org/10.1140/epjc/s10052-020-8364-1
Regular Article - Experimental Physics
Search for lepton flavour violating muon decay mediated by a new light particle in the MEG experiment
1
Dipartimento di Fisica, INFN Sezione di Pisa dell’Università, Largo B. Pontecorvo 3, 56127, Pisa, Italy
2
Paul Scherrer Institut PSI, 5232, Villigen, Switzerland
3
Swiss Federal Institute of Technology ETH, 8093, Zurich, Switzerland
4
Dipartimento di Fisica, INFN Sezione di Genova dell’Università, Via Dodecaneso 33, 16146, Genoa, Italy
5
Dipartimento di Fisica, INFN Sezione di Pavia dell’Università, Via Bassi 6, 27100, Pavia, Italy
6
Dipartimento di Fisica, INFN Sezione di Roma dell’Università “Sapienza”, Piazzale A. Moro, 00185, Rome, Italy
7
Dipartimento di Matematica e Fisica, INFN Sezione di Lecce dell’Università del Salento, Via per Arnesano, 73100, Lecce, Italy
8
Budker Institute of Nuclear Physics of Siberian Branch of Russian Academy of Sciences, 630090, Novosibirsk, Russia
9
Novosibirsk State Technical University, 630092, Novosibirsk, Russia
10
Novosibirsk State University, 630090, Novosibirsk, Russia
11
ICEPP, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, 113-0033, Tokyo, Japan
12
University of California, 92697, Irvine, CA, USA
13
Joint Institute for Nuclear Research, 141980, Dubna, Russia
14
KEK, High Energy Accelerator Research Organization, 1-1 Oho, 305-0801, Tsukuba, Ibaraki, Japan
av
mitsutaka.r.nakao@gmail.com
Received:
5
May
2020
Accepted:
15
August
2020
Published online:
17
September
2020
We present the first direct search for lepton flavour violating muon decay mediated by a new light particle X, 
. This search uses a dataset resulting from 
stopped muons collected by the MEG experiment at the Paul Scherrer Institut in the period 2009–2013. No significant excess is found in the mass region 20–45 MeV/c
for lifetimes below 40 ps, and we set the most stringent branching ratio upper limits in the mass region of 20–40 MeV/c, down to 
at 90% confidence level.
© The Author(s) 2020
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Funded by SCOAP3
