Search for Lepton Number and Flavor Violation in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msup><mml:mi>K</mml:mi><mml:mo>+</mml:mo></mml:msup></mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msup><mml:mi>π</mml:mi><mml:mn>0</mml:mn></mml:msup></mml:math> Decays
R. Aliberti, F. Ambrosino, Roberto Ammendola, B. Angelucci, A. Antonelli, G. Anzivino, R. Arcidiacono, T. Bache, A. Baeva, D. Baigarashev, M. Barbanera, J. Bernhard, A. Biagioni, L. Bician, C. Biino, A. Bizzeti, T. Blažek, B. Bloch-Devaux, Vincenzo Bonaiuto, M. Boretto, A. M. Bragadireanu, D. Britton, F. Brizioli, M. B. Brunetti, D. Bryman, F. Bucci, T. Capussela, J. Carmignani, A. Ceccucci, P. Cenci, V. Černý, C. Cerri, B. Checcucci, A. Conovaloff, P. S. Cooper, E. Cortina, M. Corvino, F. Costantini, A. Cotta Ramusino, D. Coward, G. D’Agostini, J.B. Dainton, P. Dalpiaz, H. Danielsson, N. De Simone, D. Di Filippo, L. Di Lella, N. Doble, V. Duk, F. Duval, Babette Döbrich, D. Emelyanov, J. Engelfried, T. Enik, N. Estrada-Tristan, V. Falaleev, R. Fantechi, V. Fascianelli, L. Federici, S. Fedotov, A. Filippi, M. Fiorini, J. R. Fry, Jie Fu, A. Fucci, L. Fulton, E. Gamberini, L. Gatignon, G. Georgiev, S. A. Ghinescu, A. Gianoli, M. Giorgi, S. Giudici, F. Gonnella, E. Goudzovski, C. Graham, R. Guida, E. Gushchin, F. Hahn, H. F. Heath, Jack Henshaw, Eva Barbara Holzer, T. Husek, O. Hutanu, D. Hutchcroft, L. Iacobuzio, E. Iacopini, E. Imbergamo, B. Jenninger, J. Jerhot, R. W. L. Jones, Karol Kampf, V. Kekelidze, S. Kholodenko, G. Khoriauli, A. Khotyantsev, A. Kleimenova, A. Korotkova, M. Koval, V. Kozhuharov
Abstract
Searches for the lepton number violating ${K}^{+}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{\ensuremath{-}}{\ensuremath{\mu}}^{+}{e}^{+}$ decay and the lepton flavor violating ${K}^{+}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}{e}^{+}$ and ${\ensuremath{\pi}}^{0}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{\ensuremath{-}}{e}^{+}$ decays are reported using data collected by the NA62 experiment at CERN in 2017--2018. No evidence for these decays is found and upper limits of the branching ratios are obtained at 90% confidence level: $\mathcal{B}({K}^{+}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{\ensuremath{-}}{\ensuremath{\mu}}^{+}{e}^{+})<4.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}11}$, $\mathcal{B}({K}^{+}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}{e}^{+})<6.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}11}$ and $\mathcal{B}({\ensuremath{\pi}}^{0}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{\ensuremath{-}}{e}^{+})<3.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$. These results improve by 1 order of magnitude over previous results for these decay modes.