Search for lepton flavour violating muon decay mediated by a new light particle in the MEG experiment
A. Baldini, F. Berg, M. Biasotti, G. Boca, P. W. Cattaneo, G. Cavoto, F. Cei, M. Chiappini, G. Chiarello, C. Chiri, A. Corvaglia, A. de Bari, M. De Gerone, M. Francesconi, L. Galli, F. Gatti, F. Grancagnolo, M. Grassi, D. N. Grigoriev, M. Hildebrandt, Z. Hodge, K. Ieki, F.V. Ignatov, R. Iwai, T. Iwamoto, S. Kobayashi, P.-R. Kettle, W. Kyle, N. V. Khomutov, A. Kolesnikov, N. Kravchuk, N. A. Kuchinskiy, T. Libeiro, G. Lim, V. L. Malyshev, Nobuo Matsuzawa, M. Meucci, S. Mihara, W. Molzon, T. Mori, A. Mtchedilishvili, M. Nakao, H. Natori, D. Nicolò, H. Nishiguchi, M. Nishimura, S. Ogawa, Rina Onda, W. Ootani, A. Oya, D. Palo, M. Panareo, A. Papa, V. Pettinacci, G. Pizzigoni, A. Yu. Popov, F. Renga, S. Ritt, A. Rozhdestvensky, M. Rossella, R. Sawada, P. Schwendimann, G. Signorelli, A. Stoykov, G. F. Tassielli, Kazuhiro Toyoda, Y. Uchiyama, M. Usami, C. Voena, K. Yanai, Yu. V. Yudin
Abstract
Abstract We present the first direct search for lepton flavour violating muon decay mediated by a new light particle X, $$\upmu ^+ \rightarrow \mathrm {e}^+\mathrm {X}, \mathrm {X} \rightarrow \upgamma \upgamma \ $$ <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:mrow> <mml:mi>e</mml:mi> </mml:mrow> <mml:mo>+</mml:mo> </mml:msup> <mml:mi>X</mml:mi> <mml:mo>,</mml:mo> <mml:mi>X</mml:mi> <mml:mo>→</mml:mo> <mml:mi>γ</mml:mi> <mml:mi>γ</mml:mi> <mml:mspace/> </mml:mrow> </mml:math> . This search uses a dataset resulting from $$7.5\times 10^{14}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mn>7.5</mml:mn> <mml:mo>×</mml:mo> <mml:msup> <mml:mn>10</mml:mn> <mml:mn>14</mml:mn> </mml:msup> </mml:mrow> </mml:math> 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 $$^2$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mrow/> <mml:mn>2</mml:mn> </mml:msup> </mml:math> for lifetimes below 40 ps, and we set the most stringent branching ratio upper limits in the mass region of 20–40 MeV/c $$^2$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mrow/> <mml:mn>2</mml:mn> </mml:msup> </mml:math> , down to $${\mathcal {O}}(10^{-11})$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>O</mml:mi> <mml:mo>(</mml:mo> <mml:msup> <mml:mn>10</mml:mn> <mml:mrow> <mml:mo>-</mml:mo> <mml:mn>11</mml:mn> </mml:mrow> </mml:msup> <mml:mo>)</mml:mo> </mml:mrow> </mml:math> at 90% confidence level.