Search for K+ decays to a muon and invisible particles
E. Cortina, A. Kleimenova, E. Minucci, S. Padolski, P. Petrov, A. Shaikhiev, R. Volpe, T. Numao, Y. Petrov, B. Velghe, D. Bryman, Jie Fu, T. Husek, J. Jerhot, Karol Kampf, M. Zamkovsky, R. Aliberti, G. Khoriauli, J. Kunze, D. Lomidze, L. Peruzzo, M. Vormstein, R. Wanke, P. Dalpiaz, M. Fiorini, I. Neri, A. Norton, F. Petrucci, H. D. Wahl, A. Cotta Ramusino, A. Gianoli, E. Iacopini, G. Latino, M. Lenti, A. Parenti, A. Bizzeti, F. Bucci, A. Antonelli, G. Georgiev, V. Kozhuharov, G. Lanfranchi, S. Martellotti, M. Moulson, T. Spadaro, F. Ambrosino, T. Capussela, M. Corvino, D. Di Filippo, P. Massarotti, M. Mirra, M. Napolitano, G. Saracino, G. Anzivino, F. Brizioli, E. Imbergamo, R. Lollini, R. Piandani, C. Santoni, M. Barbanera, P. Cenci, B. Checcucci, P. Lubrano, M. Lupi, M. Pepé, M. Piccini, F. Costantini, L. Di Lella, N. Doble, M. Giorgi, S. Giudici, G. Lamanna, E. Lari, E. Pedreschi, M. Sozzi, C. Cerri, R. Fantechi, L. Pontisso, F. Spinella, I. Mannelli, G. D’Agostini, M. Raggi, A. Biagioni, E. Leonardi, A. Lonardo, P. Valente, P. Vicini, Roberto Ammendola, Vincenzo Bonaiuto, A. Fucci, Á. Salamon, F. Sargeni, R. Arcidiacono, B. Bloch-Devaux, M. Boretto, E. Menichetti, E. Migliore, D. Soldi, C. Biino, A. Filippi, F. Marchetto
Abstract
The NA62 experiment at CERN reports searches for K + → μ + N and K + → μ + ν X decays, where N and X are massive invisible particles, using the 2016–2018 data set. The N particle is assumed to be a heavy neutral lepton , and the results are expressed as upper limits of O ( 10 − 8 ) of the neutrino mixing parameter | U μ 4 | 2 for N masses in the range 200–384 MeV/ c 2 and lifetime exceeding 50 ns. The X particle is considered a scalar or vector hidden sector mediator decaying to an invisible final state, and upper limits of the decay branching fraction for X masses in the range 10–370 MeV/ c 2 are reported for the first time, ranging from O ( 10 − 5 ) to O ( 10 − 7 ) . An improved upper limit of 1.0 × 10 − 6 is established at 90% CL on the K + → μ + ν ν ν ¯ branching fraction.