NA62 CollaborationGil, E. CortinaKleimenova, AlinaMinucci, E.Padolski, S.Petrov, P.Shaikhiev, A.Volpe, R.Numao, T.Petrov, Y.Velghe, B.Wong, V. W. S.Bryman, D.Fu, J.Hives, Z.Husek, T.Jerhot, J.Kampf, K.Zamkovsky, M.De Martino, B.Perrin-Terrin, M.Akmete, A. T.Aliberti, R.Khoriauli, G.Kunze, J.Lomidze, D.Peruzzo, L.Vormstein, M.Wanke, R.Dalpiaz, P.Fiorini, M.Mazzolari, A.Neri, I.Norton, A.Petrucci, F.Soldani, M.Wahl, H.Bandiera, L.Ramusino, A. CottaGianoli, A.Romagnoni, M.Sytov, A.Iacopini, E.Latino, G.Lenti, M.Lo Chiatto, P.Panichi, I.Parenti, A.Bizzeti, A.Bucci, F.Antonelli, A.Georgiev, G.Kozhuharov, V.Lanfranchi, G.Martellotti, S.Moulson, M.Spadaro, T.Tinti, G.Ambrosino, F.Capussela, T.Corvino, M.D'Errico, M.Di Filippo, D.Fiorenza, R.Giordano, R.Massarotti, P.Mirra, M.Napolitano, M.Rosa, I.Anzivino, G.Brizioli, F.Imbergamo, E.Lollini, R.Piandani, R.Santoni, C.Barbanera, M.Cenci, P.Checcucci, B.Lubrano, P.Lupi, M.Pepe, M.Piccini, M.Costantini, F.Di Lella, L.Doble, N.Giorgi, M.Giudici, S.Lamanna, G.Lari, E.Pedreschi, E.Sozzi, M.Cerri, C.Fantechi, R.Pontisso, L.Spinella, F.Mannelli, I.D'Agostini, G.Raggi, M.Biagioni, A.Cretaro, P.Frezza, O.Leonardi, E.Lonardo, A.Turisini, M.Valente, P.Vicini, P.Ammendola, R.Bonaiuto, V.Fucci, A.Salamon, A.Sargeni, F.Arcidiacono, R.Bloch-Devaux, B.Boretto, M.Menichetti, E.Migliore, E.Soldi, D.Biino, C.Filippi, A.Marchetto, F.Briano Olvera, A.Engelfried, J.Estrada-Tristan, N.Reyes Santos, M. A.Boboc, P.Bragadireanu, A. M.Ghinescu, S. A.Hutanu, O. E.Bician, L.Blazek, T.Cerny, V.Kucerova, Z.Bernhard, J.Ceccucci, A.Ceoletta, M.Danielsson, H.De Simone, N.Duval, F.Dobrich, B.Federici, L.Gamberini, E.Gatignon, L.Guida, R.Hahn, F.Holzer, E. B.Jenninger, B.Koval, M.Laycock, P.Miotto, G. LehmannLichard, P.Mapelli, A.Marchevski, RadoslavMassri, K.Noy, M.Palladino, V.Pinzino, J.Ryjov, V.Schuchmann, S.Venditti, S.Bache, T.Brunetti, M. B.Duk, V.Fascianelli, V.Fry, J. R.Gonnella, F.Goudzovski, E.Henshaw, J.Iacobuzio, L.Kenworthy, C.Lazzeroni, C.Lurkin, N.Newson, F.Parkinson, C.Romano, A.Sanders, J.Sergi, A.Sturgess, A.Swallow, J.Tomczak, A.Heath, H.Page, R.Trilov, S.Angelucci, B.Britton, D.Graham, C.Protopopescu, D.Carmignani, J.Dainton, J. B.Jones, R. W. L.Ruggiero, G.Fulton, L.Hutchcroft, D.Maurice, E.Wrona, B.Conovaloff, A.Cooper, P.Coward, D.Rubin, P.Baeva, A.Baigarashev, D.Emelyanov, D.Enik, T.Falaleev, V.Fedotov, S.Gorshanov, K.Gushchin, E.Kekelidze, V.Kereibay, D.Kholodenko, S.Khotyantsev, A.Korotkova, A.Kudenko, Y.Kurochka, V.Kurshetsov, V.Litov, L.Madigozhin, D.Medvedeva, M.Mefodev, A.Misheva, M.Molokanova, N.Movchan, S.Obraztsov, V.Okhotnikov, A.Ostankov, A.Polenkevich, I.Potrebenikov, Yu.Sadovskiy, A.Semenov, V.Shkarovskiy, S.Sugonyaev, V.Yushchenko, O.Zinchenko, A.2024-03-182024-03-182024-03-182023-11-1010.1016/j.physletb.2023.138193https://infoscience.epfl.ch/handle/20.500.14299/206351WOS:001158120600001The first search for ultra-rare K+ decays into the pi(+)e(+)e(-) e(+) e(-) final state is reported, using a dataset collected by the NA62 experiment at CERN in 2017-2018. An upper limit of 1.4x10(-8) at 90% CL is obtained for the branching ratio of the K+ -> pi(+) e(+)e(-)e(+)e(-) decay, predicted in the Standard Model to be (7.2 +/- 0.7) x10(-11). Upper limits at 90% CL are obtained at the level of 10(-9) for the branching ratios of two prompt decay chains involving pair-production of hidden-sector mediators: K+. pi(+) aa, a -> e(+)e(-) and K+ -> pi(+) S, S -> A' A', A' -> e(+) e(-). (c) 2023 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP(3).Physical Sciencestext::journal::journal article::research article