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    Cortina Gil, E.; Minucci, E.; Padolski, S.; Petrov, P.; Volpe, R.; Numao, T.; Velghe, B.; Bryman, D.; Aliberti, R.; Khoriauli, G.; Kunze, J.; Lomidze, D.; Marchevski, R.; Peruzzo, L.; Vormstein, M.; Norton, A.; Petrucci, F.; Wahl, H.; Iacopini, E.; Lenti, M.; Parenti, A.; Georgiev, G.; Kozhuharov, V.; Moulson, M.; Ambrosino, F.; Corvino, M.; Di Filippo, D.; Massarotti, P.; Piandani, R.; Barbanera, M.; Checcucci, B.; Lubrano, P.; Pepe, M.; Piccini, M.; Costantini, F.; Di Lella, L.; Doble, N.; Giudici, S.; Sozzi, M.; Fantechi, R.; Pontisso, L.; Spinella, F.; Mannelli, I.; Lonardo, A.; Valente, P.; Ammendola, R.; Bonaiuto, V.; Sargeni, F.; Boretto, M.; Soldi, D.; Biino, C.; Filippi, A.; Engelfried, J.; Estrada-Tristan, N.; Ghinescu, S. A.; Hutanu, O. E.; Baeva, A.; Emelyanov, D.; Korotkova, A.; Litov, L.; Madigozhin, D.; Misheva, M.; Movchan, S.; Polenkevich, I.; Potrebenikov, Y.; Fedotov, S.; Khotyantsev, A.; Medvedeva, M.; Mefodev, A.; Kholodenko, S.; Ceccucci, A.; Danielsson, H.; Duval, F.; Federici, L.; Gamberini, E.; Gatignon, L.; Guida, R.; Hahn, F.; Koval, M.; Lichard, P.; Massri, K.; Schuchmann, S.; Bache, T.; Brunetti, M. B.; Duk, V.; Fry, J. R.; Goudzovski, E.; Iacobuzio, L.; Lazzeroni, C.; Newson, F.; Parkinson, C.; Sturgess, A.; Swallow, J.; Heath, H.; Angelucci, B.; Britton, D.; Wrona, B.; Conovaloff, A.; Coward, D.; Rubin, P.

    The journal of high energy physics, 02/2021, Letnik: 2021, Številka: 2
    Journal Article

    A bstract The NA62 experiment at the CERN SPS reports a study of a sample of 4 × 10 9 tagged π 0 mesons from K + → π + π 0 ( γ ), searching for the decay of the π 0 to invisible particles. No signal is observed in excess of the expected background fluctuations. An upper limit of 4 . 4 × 10 − 9 is set on the branching ratio at 90% confidence level, improving on previous results by a factor of 60. This result can also be interpreted as a model- independent upper limit on the branching ratio for the decay K + → π + X , where X is a particle escaping detection with mass in the range 0.110–0.155 GeV /c 2 and rest lifetime greater than 100 ps. Model-dependent upper limits are obtained assuming X to be an axion-like particle with dominant fermion couplings or a dark scalar mixing with the Standard Model Higgs boson.