New limit on the $${\upmu ^+ \rightarrow e^+ \upgamma }$$ decay with the MEG II experiment
Abstract This letter reports the result of the search for the decay $${\upmu ^+ \rightarrow e^+ \upgamma }$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msup> <mml:mi>μ</mml:mi> <mml:mo>+</mml:mo> </mml:msup> <mml:mo>→</mml:mo> <mml:msup> <mml:mi>e</mml:mi> <mml:mo>+</mml:mo> </mml:msup> <mml:mi>γ</mml:mi> </mml:mrow> </mml:math> 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 $${2.2 \times 10^{-13}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mn>2.2</mml:mn> <mml:mo>×</mml:mo> <mml:msup> <mml:mn>10</mml:mn> <mml:mrow> <mml:mo>-</mml:mo> <mml:mn>13</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:math> , 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 $$\mathcal{B} ({\upmu ^+ \rightarrow \textrm{e}^+ \upgamma }) < {1.5 \times 10^{-13}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>B</mml:mi> <mml:mrow> <mml:mo>(</mml:mo> <mml:mrow> <mml:msup> <mml:mi>μ</mml:mi> <mml:mo>+</mml:mo> </mml:msup> <mml:mo>→</mml:mo> <mml:msup> <mml:mtext>e</mml:mtext> <mml:mo>+</mml:mo> </mml:msup> <mml:mi>γ</mml:mi> </mml:mrow> <mml:mo>)</mml:mo> </mml:mrow> <mml:mo><</mml:mo> <mml:mrow> <mml:mn>1.5</mml:mn> <mml:mo>×</mml:mo> <mml:msup> <mml:mn>10</mml:mn> <mml:mrow> <mml:mo>-</mml:mo> <mml:mn>13</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:mrow> </mml:math> (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.
Authors
- A. Oya (ORCID: https://orcid.org/0009-0004-3810-6350)
- G. Dal Maso (ORCID: https://orcid.org/0000-0002-6493-0317)
- M. Rossella
- S. Ritt (ORCID: https://orcid.org/0000-0001-6338-6447)
- M. De Gerone (ORCID: https://orcid.org/0000-0002-5489-6581)
- S. Ban (ORCID: https://orcid.org/0000-0003-0869-2243)
- P.-R. Kettle
- N. Kuchinskiy
- B. Vitali (ORCID: https://orcid.org/0000-0003-2490-8593)
- F.V. Ignatov (ORCID: https://orcid.org/0000-0001-7061-6060)
- P. W. Cattaneo (ORCID: https://orcid.org/0000-0001-6877-6882)
- Y. Uchiyama (ORCID: https://orcid.org/0000-0003-0545-3888)
- A. Venturini (ORCID: https://orcid.org/0000-0001-5838-4738)
- H. Benmansour (ORCID: https://orcid.org/0000-0003-1125-4367)
- Kensuke Yamamoto (ORCID: https://orcid.org/0000-0003-4031-5835)
- K. Afanaciev
- Alexander Kolesnikov (ORCID: https://orcid.org/0000-0001-6828-8993)
- F. Grancagnolo
- A. Papa
- S. V. Karpov (ORCID: https://orcid.org/0000-0003-0035-651X)
- Weiyuan Li (ORCID: https://orcid.org/0009-0003-6658-909X)
- H. Nishiguchi
- T. Iwamoto
- D. Nicolò (ORCID: https://orcid.org/0000-0001-6414-946X)
- D. Pasciuto
- G. Gallucci (ORCID: https://orcid.org/0000-0003-3554-9733)
- L. Ferrari Barusso (ORCID: https://orcid.org/0000-0002-9740-7346)
- M. Grassi (ORCID: https://orcid.org/0000-0003-2422-6736)
- C. Voena (ORCID: https://orcid.org/0000-0001-8938-5055)
- M. Hildebrandt (ORCID: https://orcid.org/0000-0002-9425-5543)
- F. Gatti
- N. V. Khomutov (ORCID: https://orcid.org/0009-0003-2641-2492)
- A. Matsushita
- F. Cei (ORCID: https://orcid.org/0000-0002-8313-3540)
- W. Ootani (ORCID: https://orcid.org/0000-0003-4939-1850)
- María Cecilia Chiappini (ORCID: https://orcid.org/0000-0001-5296-4299)
- A. Corvaglia (ORCID: https://orcid.org/0000-0002-2386-3592)
- W. Molzon (ORCID: https://orcid.org/0000-0001-9232-6064)
- D. Palo (ORCID: https://orcid.org/0000-0001-9256-5348)
- A. M. Baldini
- G. Signorelli
- S. Mihara
- T. Yonemoto
- В. А. Крылов (ORCID: https://orcid.org/0000-0003-1726-956X)
- A. de Bari (ORCID: https://orcid.org/0000-0001-6693-0284)
- M. Panareo (ORCID: https://orcid.org/0000-0002-7757-5553)
- L. Gerritzen (ORCID: https://orcid.org/0000-0002-3580-1073)
- T. Mori (ORCID: https://orcid.org/0000-0002-5907-7091)
- Ryoji Yokota
- Hiroaki Suzuki (ORCID: https://orcid.org/0000-0002-8899-0955)
- F. Ikeda
- E. G. Grandoni
- G. Cavoto (ORCID: https://orcid.org/0000-0003-2161-918X)
- Masato Takahashi (ORCID: https://orcid.org/0009-0001-3214-3888)
- D. N. Grigoriev
- V. Malyshev
- A. Ochi
- G. Boca
- A.S. Popov
- L. Galli
- S. Ogawa
- A. Rozhdestvensky
- F. Leonetti
- N. Kravchuk
- M. Francesconi
- F. Renga (ORCID: https://orcid.org/0000-0001-8129-8504)
- S. Scarpelli
- R. Umakoshi
Institutions
- High Energy Accelerator Research Organization (JP)
- University of Liverpool (GB)
- University of California, Irvine (US)
- Paul Scherrer Institute (CH)
- University of California System (US)
- Joint Institute for Nuclear Research (RU)
- ETH Zurich (CH)
- Istituto Nazionale di Fisica Nucleare, Sezione di Lecce (IT)
- Institute of Particle Physics (CA)
- Istituto Nazionale di Fisica Nucleare, Sezione di Napoli (IT)
- Istituto Nazionale di Fisica Nucleare, Sezione di Pavia (IT)
- Istituto Nazionale di Fisica Nucleare, Sezione di Genova (IT)
- Istituto Nazionale di Fisica Nucleare, Sezione di Roma I (IT)
- Istituto Nazionale di Fisica Nucleare, Sezione di Pisa (IT)
- Budker Institute of Nuclear Physics (RU)
- Kobe University (JP)
- The University of Tokyo (JP)
- Novosibirsk State Technical University (RU)
Publication Details
- Journal
- The European Physical Journal C
- Published
- 2025-10-21
- DOI
- https://doi.org/10.1140/epjc/s10052-025-14906-3
- Citations
- 9
- Primary Topic
- Particle physics theoretical and experimental studies
- Type
- article
- Field-Weighted Citation Impact
- 16.83
Funders
- U.S. Department of Energy
- Leverhulme Trust
- Schweizerischer Nationalfonds zur Förderung der Wissenschaftlichen Forschung
- Horizon 2020 Framework Programme
- Japan Society for the Promotion of Science