Beam-Target Helicity Asymmetry for γ→n→→π−p in the N* Resonance Region

Ho, D.; Peng, P.; Bass, C.; Collins, P.; D’Angelo, A.; Deur, A.; Fleming, J.; Hanretty, C.; Kageya, T.; Khandaker, M.; Klein, F. J.; Klempt, E.; Laine, V.; Lowry, M. M.; Lu, H.; Nepali, C.; Nikonov, V. A.; O’Connell, T.; Sandorfi, A. M.; Sarantsev, A. V.; Schumacher, R. A.; Strakovsky, I. I.; Švarc, A.; Walford, N. K.; Wei, X.; Whisnant, C. S.; Workman, R. L.; Zonta, I.; Adhikari, K. P.; Adikaram, D.; Akbar, Z.; Amaryan, M. J.; Anefalos Pereira, S.; Avakian, H.; Ball, J.; Bashkanov, M.; Battaglieri, M.; Batourine, V.; Bedlinskiy, I.; Biselli, A.; Briscoe, W. J.; Burkert, V. D.; Carman, D. S.; Celentano, A.; Charles, G.; Chetry, T.; Ciullo, G.; Clark, L.; Colaneri, L.; Cole, P. L.; Contalbrigo, M.; Crede, V.; Dashyan, N.; De Sanctis, E.; De Vita, R.; Djalali, C.; Dupre, R.; El Alaoui, A.; El Fassi, L.; Elouadrhiri, L.; Eugenio, P.; Fedotov, G.; Fegan, S.; Fersch, R.; Filippi, A.; Fradi, A.; Ghandilyan, Y.; Gilfoyle, G. P.; Girod, F. X.; Glazier, D. I.; Gleason, C.; Gohn, W.; Golovatch, E.; Gothe, R. W.; Griffioen, K. A.; Guidal, M.; Guo, L.; Hakobyan, H.; Harrison, N.; Hattawy, M.; Hicks, K.; Holtrop, M.; Hughes, S. M.; Ilieva, Y.; Ireland, D. G.; Ishkhanov, B. S.; Isupov, E. L.; Jenkins, D.; Jiang, H.; Jo, H. S.; Joo, K.; Joosten, S.; Keller, D.; Khachatryan, G.; Kim, A.; Kim, W.; Klein, A.; Kubarovsky, V.; Kuleshov, S. V.; Lanza, L.; Lenisa, P.; Livingston, K.; MacGregor, I. J.; Markov, N.; McKinnon, B.; Mineeva, T.; Mokeev, V.; Montgomery, R. A.; Movsisyan, A.; Munoz Camacho, C.; Murdoch, G.; Niccolai, S.; Niculescu, G.; Osipenko, M.; Paolone, M.; Paremuzyan, R.; Park, K.; Pasyuk, E.; Phelps, W.; Pogorelko, O.; Price, J. W.; Procureur, S.; Protopopescu, D.; Ripani, M.; Riser, D.; Ritchie, B. G.; Rizzo, A.; Rosner, G.; Sabatié, F.; Salgado, C.; Sharabian, Y. G.; Skorodumina, Iu.; Smith, G. D.; Sober, D. I.; Sokhan, D.; Sparveris, N.; Strauch, S.; Tian, Ye; Torayev, B.; Ungaro, M.; Voskanyan, H.; Voutier, E.; Watts, D. P.; Wood, M. H.; Zachariou, N.; Zhang, J.; Zhao, Z. W.; CLAS Collaboration

doi:10.1103/PhysRevLett.118.242002

We search for the rare decay $B^{+} \to K^{+} ν \bar{ν}$ in a $362 {fb}^{- 1}$ sample of electron-positron collisions at the $ϒ (4 S)$ resonance collected with the Belle II detector at the SuperKEKB collider. We use the inclusive properties of the accompanying $B$ meson in $ϒ (4 S) \to B \bar{B}$ events to suppress background from other decays of the signal $B$ candidate and light-quark pair production. We validate the measurement with an auxiliary analysis based on a conventional hadronic reconstruction of the accompanying $B$ meson. For background suppression, we exploit distinct signal features using machine learning methods tuned with simulated data. The signal-reconstruction efficiency and background suppression are validated through various control channels. The branching fraction is extracted in a maximum likelihood fit. Our inclusive and hadronic analyses yield consistent results for the $B^{+} \to K^{+} ν \bar{ν}$ branching fraction of $[2.7 \pm 0.5 (stat) \pm 0.5 (syst)] \times 10^{- 5}$ and $[{1.1}_{- 0.8}^{+ 0.9} (stat)_{- 0.5}^{+ 0.8} (syst)] \times 10^{- 5}$ , respectively. Combining the results, we determine the branching fraction of the decay $B^{+} \to K^{+} ν \bar{ν}$ to be $[2.3 \pm 0.5 (stat)_{- 0.4}^{+ 0.5} (syst)] \times 10^{- 5}$ , providing the first evidence for this decay at 3.5 standard deviations. The combined result is 2.7 standard deviations above the standard model expectation.

Published by the American Physical Society

2024

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