Improved constraint on the MINERνA medium energy neutrino flux using ν¯e−→ν¯e− data

Zazueta, L.; Akhter, S.; Ahmad Dar, Z.; Akbar, F.; Ansari, V.; Ascencio, M. V.; Athar, M. Sajjad; Bashyal, A.; Bercellie, A.; Betancourt, M.; Bonilla, J. L.; Bravar, A.; Cai, T.; Díaz, G. A.; da Motta, H.; Felix, J.; Fields, L.; Filkins, A.; Fine, R.; Gago, A. M.; Gallagher, H.; Ghosh, A.; Gran, R.; Granados, E.; Harris, D. A.; Henry, S.; Jena, D.; Jena, S.; Kleykamp, J.; Klustová, A.; Kordosky, M.; Last, D.; Lozano, A.; Lu, X.-G.; Maher, E.; Manly, S.; Mann, W. A.; McFarland, K. S.; Messerly, B.; Miller, J.; Moreno, O.; Morfín, J. G.; Nelson, J. K.; Nguyen, C.; Olivier, A.; Paolone, V.; Perdue, G. N.; Plows, K.-J.; Ramírez, M. A.; Ruterbories, D.; Schellman, H.; Solano Salinas, C. J.; Su, H.; Sultana, M.; Valencia, E.; Vaughan, N. H.; Waldron, A. V.; Yaeggy, B.

doi:10.1103/PhysRevD.107.012001

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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