Search for lepton flavour violating muon decay mediated by a new light particle in the MEG experiment
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Regular Article - Experimental Physics
Search for lepton flavour violating muon decay mediated by a new light particle in the MEG experiment The MEG Collaboration A. M. Baldini1a , F. Berg2,3 , M. Biasotti4a,4b , G. Boca5a,5b , P. W. Cattaneo5a , G. Cavoto6a,6b , F. Cei1a,1b , M. Chiappini1a,1b , G. Chiarello6a,6b , C. Chiri7a,7b , A. Corvaglia7a,7b , A. de Bari5a,5b , M. De Gerone4a , M. Francesconi1a , L. Galli1a , F. Gatti4a,4b , F. Grancagnolo7a , M. Grassi1a , D. N. Grigoriev8,9,10 , M. Hildebrandt2 , Z. Hodge2,3 , K. Ieki11 , F. Ignatov8,10 , R. Iwai11 , T. Iwamoto11 , S. Kobayashi11 , P.-R. Kettle2 , W. Kyle12 , N. Khomutov13 , A. Kolesnikov13 , N. Kravchuk13 , N. Kuchinskiy13 , T. Libeiro12 , G. M. A. Lim12 , V. Malyshev13 , N. Matsuzawa11 , M. Meucci6a,6b , S. Mihara14 , W. Molzon12 , Toshinori Mori11 , A. Mtchedilishvili2 , M. Nakao11,a , H. Natori11 , D. Nicolò1a,1b , H. Nishiguchi14 , M. Nishimura11 , S. Ogawa11 , R. Onda11 , W. Ootani11 , A. Oya11 , D. Palo12 , M. Panareo7a,7b , A. Papa1a,1b,2 , V. Pettinacci6a , G. Pizzigoni4a,4b , A. Popov8,10 , F. Renga6a , S. Ritt2 , A. Rozhdestvensky13 , M. Rossella5a , R. Sawada11 , P. Schwendimann2 , G. Signorelli1a , A. Stoykov2 , G. F. Tassielli7a , K. Toyoda11 , Y. Uchiyama11 , M. Usami11 , C. Voena6a , K. Yanai11 , Yu. V. Yudin8,10 1
Dipartimento di Fisicab , INFN Sezione di Pisaa dell’Università, Largo B. Pontecorvo 3, 56127 Pisa, Italy Paul Scherrer Institut PSI, 5232 Villigen, Switzerland 3 Swiss Federal Institute of Technology ETH, 8093 Zurich, Switzerland 4 Dipartimento di Fisicab , INFN Sezione di Genovaa dell’Università, Via Dodecaneso 33, 16146 Genoa, Italy 5 Dipartimento di Fisicab , INFN Sezione di Paviaa dell’Università, Via Bassi 6, 27100 Pavia, Italy 6 Dipartimento di Fisicab , INFN Sezione di Romaa dell’Università “Sapienza”, Piazzale A. Moro, 00185 Rome, Italy 7 Dipartimento di Matematica e Fisicab , INFN Sezione di Leccea 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, Tokyo 113-0033, Japan 12 University of California, Irvine, CA 92697, USA 13 Joint Institute for Nuclear Research, 141980 Dubna, Russia 14 KEK, High Energy Accelerator Research Organization, 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan 2
Received: 5 May 2020 / Accepted: 15 August 2020 © The Author(s) 2020
Abstract We present the first direct search for lepton flavour violating muon decay mediated by a new light particle X, μ+ → e+ X, X → γγ . This search uses a dataset resulting from 7.5 × 1014 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/c2 for lifetimes below 40 ps, and we set the most stringent branching ratio upper limits in the mass region of
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