https://doi.org/10.1140/epjc/s10052-025-14906-3
Letter
New limit on the 
decay with the MEG II experiment
1
Joint Institute for Nuclear Research, 141980, Dubna, Russia
2
INFN Sezione di Pisa; Dipartimento di Fisica dell’Università, Largo B. Pontecorvo 3, 56127, Pisa, Italy
3
ICEPP, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, 113-0033, Tokyo, Japan
4
INFN Sezione di Pavia; Dipartimento di Fisica dell’Università, Via Bassi 6, 27100, Pavia, Italy
5
INFN Sezione di Roma; Dipartimento di Fisica dell’Università “Sapienza”, Piazzale A. Moro, 00185, Rome, Italy
6
INFN Sezione di Lecce; Dipartimento di Matematica e Fisica dell’Università del Salento, Via per Arnesano, 73100, Lecce, Italy
7
Paul Scherrer Institut PSI, 5232, Villigen, Switzerland
8
Institute for Particle Physics and Astrophysics, ETH Zürich, Otto-Stern-Weg 5, 8093, Zurich, Switzerland
9
INFN Sezione di Genova; Dipartimento di Fisica dell’Università, Via Dodecaneso 33, 16146, Genoa, Italy
10
INFN Sezione di Napoli, Via Cintia, 80126, Naples, Italy
11
Budker Institute of Nuclear Physics of Siberian Branch of Russian Academy of Sciences, 630090, Novosibirsk, Russia
12
Novosibirsk State Technical University, 630092, Novosibirsk, Russia
13
Oliver Lodge Laboratory, University of Liverpool, L69 7ZE, Liverpool, UK
14
KEK, High Energy Accelerator Research Organization, 1-1 Oho, 305-0801, Tsukuba, Ibaraki, Japan
15
University of California, 92697, Irvine, CA, USA
16
Kobe University, 1-1 Rokkodai-cho, Nada-ku, 657-8501, Kobe, Hyogo, Japan
Received:
31
July
2025
Accepted:
6
October
2025
Published online:
21
October
2025
This letter reports the result of the search for the decay undertaken at the Paul Scherrer Institut in Switzerland with the MEG II experiment using the data collected in the 2021–2022 physics runs. The sensitivity of the branching ratio measurement in this search is 
, a factor of 2.4 better than that of the full MEG dataset and obtained in a data taking period of about one fourth that of MEG, thanks to the superior performances of the new detector. The observed data are consistent with the expected background, yielding an upper limit on the branching ratio of 
(90% C.L.). Additional improvements are expected with the data collected during the years 2023–2024. The data-taking will continue in the coming years.
The original online version of this article was revised: In this article the author’s name S. Scarpellini was incorrectly written as S. Scarpelli. Additionally, the author H. Ogawa was mistakenly referred to as S. Ogawa. An additional correction has been made to the Acknowledgment section: the grant number "the Leverhulme Trust LIP-2021-01" should read "the Leverhulme Trust LIP-2021-014".
An erratum to this article is available online at https://doi.org/10.1140/epjc/s10052-025-14986-1.
© The Author(s) 2025
corrected publication 2025
corrected publication 2025
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