Study of $B^+ \to μ^+ ν_μ$ decays at Belle and Belle II

We report a measurement of the branching fraction for the leptonic decay $B^+\toμ^+ν_μ$. This work presents the first $B^+\toμ^+ν_μ$ result using Belle~II data, an updated Belle measurement that supersedes the previous result, and their combination, which yields the most precise search to date. The analysis is based on $1076\,\mathrm{fb}^{-1}$ of $e^+e^-$ collision data collected at a center-of-mass energy of $10.58\,\mathrm{GeV}$ with the Belle and Belle~II detectors at the KEKB and SuperKEKB colliders, respectively. We measure $\mathcal{B}(B^+\toμ^+ν_μ)=(4.4\pm1.9\pm 1.0)\times10^{-7}$, where the first uncertainty is statistical and the second systematic. The observed significance relative to the background-only hypothesis is 2.4 standard deviations. We set a 90% confidence level upper limit of $\mathcal{B}(B^+\toμ^+ν_μ)<6.7\times10^{-7}$ using a frequentist approach and a 90% credibility level upper limit of $\mathcal{B}(B^+\toμ^+ν_μ)<7.2\times 10^{-7}$ using a Bayesian approach. These are the most stringent limits to date. The result is interpreted as an exclusion region in the parameter space of type~II and type~III two-Higgs-doublet models. We search for stable sterile neutrinos with masses $m_N\in[0,1.5]\,\mathrm{GeV}$. No signal is observed, and the resulting exclusion on the squared mixing parameter $|U_{μN}|^2$ provides improvement over previous limits. We report a measurement of the partial branching fraction of semileptonic $B\to X_u\ellν_\ell$ decays with $p_μ^B>2.2\,\mathrm{GeV}$, obtaining $Δ\mathcal{B}(B\to X_u\ellν_\ell)=(2.72\pm0.05\pm0.29)\times10^{-4}$. We present a model-dependent study of weak annihilation decays using the muon momentum spectrum. We observe a signal of 2.4 standard deviations above the background-only hypothesis in regions where the distribution resembles that of $B\to X_u\ellν_\ell$ decays.

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Desy publication database (The Deutsches Elektronen-Synchrotron)
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2026-01-01
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Particle physics theoretical and experimental studies
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article

Study of $B^+ \to μ^+ ν_μ$ decays at Belle and Belle II

M. Alhakami, M. Bartl, C. Schwanda, Y. Han et al.
Desy publication database (The Deutsches Elektronen-Synchrotron)
Particle physics theoretical and experimental studies
article

Study of $B^+ \to μ^+ ν_μ$ decays at Belle and Belle II

M. Alhakami, M. Bartl, C. Schwanda, Y. Han, J. Kandra, Robert Kowalewski, K. Chirapatpimol, S. Raiz, H. Chetri, S. A. De La Motte, R. Okubo, P. Cheema, A. Beaubien, A. Natochii, L. Aggarwal, Jake S. Bennett, P. Chang, Varsha Gautam, J. Becker, Andrej Lozar, J. Park, J. Borah, S. Kohani, A. Di Canto, K. Park, J. Baudot, A. Thaller, Jitendra Kumar, D. Greenwald, C. Wessel, A. Martens, A. Rostomyan, T. Shimasaki, T. Fillinger, Marcel Hohmann, P. Jackson, G. Inguglia, M. Bertemes, Suxian Li, Lorenzo G. Vitale, P. Feichtinger, I. Ripp-Baudot, Kristina Gudkova, I. Tsaklidis, Anil Panta, E. Kovalenko, Seema Choudhury, T. Shillington, C. Miller, F. Otani, K. Adamczyk, K. Schoenning, C. Mariñas, A. Giri, T. Podobnik, Anh-Ky Nguyen, Philipp Horak, Shun Watanuki, A. Gabrielli, P. Urquijo, Q. Campagna, Daniel Enrique Ricalde Herrmann, G. Bonvicini, N. Althubiti, R. Mizuk, C. Lyu, R. Giordano, A. Passeri, Agrim Aggarwal, Yongheon Ahn, Martin Edmund Sevior, Rajeev Kumar, B. Scavino, R. 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HOPPE, Hiroaki Aihara, Jianshe Zhou, Pedro Luis Manuel Podesta Lerma, William Wescott Jacobs, A. Boschetti, Paolo Leo, Dmitri Liventsev, G. Schnell, Cheshta Cheshta, Foteini Faidra Trantou, Sourabh Chutia, Rajesh Kumar Maiti, Saurabh Sandilya, Raynette van Tonder, Emilie Bertholet, Jonas Eppelt, Rahul Tiwary, Clotilde Lemettais, Peter Križan, Pere Gironella Gironell, Pavel Pakhlov, Ezio Torassa, Norayr Akopov, Junxi Cui, Roberta Volpe, Jing-Ge Shiu, Mario Merola, Giuliana Rizzo, Mehmet Tevfik Zeyrek, Mirco Dorigo, Claudia Cecchi, Dmitry Shtol, Minori Watanabe, Eldar Ganiev, Petar Rados, Peter Kodys, Florian Urs Bernlochner, Gevorg Karyan, Giuseppe Finocchiaro, Vidya Sagar Vobbilisetti, Michael Thomas Hedges, Antonio Budano, Alexander Bobrov, Wei Shan, Sookyung Choi, Pavel Krokovny, Isamu Nakamura, Tommy Martinov, Matthew Barrett, Karim Trabelsi, Qingyuan Liu, Maiko Takahashi, Boris Shwartz, Anja Novosel, Vitalii Lisovskyi, Stefano Spataro, Patrick Ecker, Lea Reuter, Abner Soffer, Shu-Ping Lin, Wolfgang Gradl, Jens Soeren Lange, Martin Sobotzik, Soeren Prell, Alexander Kuzmin, Hobin Lee, Eduard De La Cruz Burelo, Andreas Warburton, Romulus Godang, Michele Mantovano, Junaid Ur Rehman, Martin Bessner, Ludovico Massaccesi, Zhiqing Liu, Ryosuke Itoh, Toru Iijima, Megumi Naruki, Marko Staric, Giacomo De Pietro, Ravinder Dhayal, Bryan Gregory Fulsom, Laura Zani, Dmitry Matvienko, Masako Iwasaki, Swarna Prabha Maharana, Frank Meier, Wenzhe Li, Tak Shun Lau, Sen Jia, Roberto Mussa, Cecilia Antonioli, Alexander Glazov, Kevin Ernest Varvell, Mattia Marfoli, Swagato Banerjee, Johan Andres Colorado, Stefano Moneta, Riccardo de Sangro, Sahil Saha, Mikihiko Nakao, Meihong Liu, Pablo Goldenzweig, Hiroaki Ono, Roberto Stroili, Alberto Aloisio, Frank Simon, Kay Kinoshita, Fabrizio Bianchi, Luigi Corona, Hisaki Hayashii, Doris Yangsoo Kim, Thibaud Humair, Rok Pestotnik, Renu Garg, Sungjin Cho, Seokhee Park, Justine Serrano, Munira Khan, BELLE II Collaboration, Lorenz Ennio Gaertner, ByungGu Cheon, Benigno Gobbo, Hideki Miyake, Naveen Kumar Baghel, Xu Dong, Stefan Wallner, Andreas Gellrich, Diptaparna Biswas, Jing Yuan, Galina Pakhlova, Rishabh Mehta, Makoto Takizawa, Kai Yi, Xiaodong Shi, Steven H. 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article en

Abstract

We report a measurement of the branching fraction for the leptonic decay $B^+\toμ^+ν_μ$. This work presents the first $B^+\toμ^+ν_μ$ result using Belle~II data, an updated Belle measurement that supersedes the previous result, and their combination, which yields the most precise search to date. The analysis is based on $1076\,\mathrm{fb}^{-1}$ of $e^+e^-$ collision data collected at a center-of-mass energy of $10.58\,\mathrm{GeV}$ with the Belle and Belle~II detectors at the KEKB and SuperKEKB colliders, respectively. We measure $\mathcal{B}(B^+\toμ^+ν_μ)=(4.4\pm1.9\pm 1.0)\times10^{-7}$, where the first uncertainty is statistical and the second systematic. The observed significance relative to the background-only hypothesis is 2.4 standard deviations. We set a 90% confidence level upper limit of $\mathcal{B}(B^+\toμ^+ν_μ)<6.7\times10^{-7}$ using a frequentist approach and a 90% credibility level upper limit of $\mathcal{B}(B^+\toμ^+ν_μ)<7.2\times 10^{-7}$ using a Bayesian approach. These are the most stringent limits to date. The result is interpreted as an exclusion region in the parameter space of type~II and type~III two-Higgs-doublet models. We search for stable sterile neutrinos with masses $m_N\in[0,1.5]\,\mathrm{GeV}$. No signal is observed, and the resulting exclusion on the squared mixing parameter $|U_{μN}|^2$ provides improvement over previous limits. We report a measurement of the partial branching fraction of semileptonic $B\to X_u\ellν_\ell$ decays with $p_μ^B>2.2\,\mathrm{GeV}$, obtaining $Δ\mathcal{B}(B\to X_u\ellν_\ell)=(2.72\pm0.05\pm0.29)\times10^{-4}$. We present a model-dependent study of weak annihilation decays using the muon momentum spectrum. We observe a signal of 2.4 standard deviations above the background-only hypothesis in regions where the distribution resembles that of $B\to X_u\ellν_\ell$ decays.

Desy publication database (The Deutsches Elektronen-Synchrotron)
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Particle physics theoretical and experimental studies
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