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Measurement of inclusive $B \to X_{u} ℓ ν$ partial branching fractions and $| V_{u b} |$ at Belle II

M. Abumusabh , I. Adachi , K. Adamczyk , L. Aggarwal , H. Ahmed , Y. Ahn , H. Aihara , N. Akopov , S. Alghamdi et al. (Belle II Collaboration)

S. Alghamdi , M. Alhakami , A. Aloisio , N. Althubiti , K. Amos , N. Anh Ky , C. Antonioli , D. M. Asner , H. Atmacan , T. Aushev , R. Ayad , V. Babu , H. Bae , N. K. Baghel , S. Bahinipati , P. Bambade , Sw. Banerjee , M. Barrett , M. Bartl , J. Baudot , A. Beaubien , F. Becherer , J. Becker , J. V. Bennett , F. U. Bernlochner , V. Bertacchi , M. Bertemes , E. Bertholet , M. Bessner , S. Bettarini , V. Bhardwaj , B. Bhuyan , F. Bianchi , T. Bilka , D. Biswas , A. Bobrov , D. Bodrov , G. Bonvicini , J. Borah , A. Boschetti , A. Bozek , M. Bračko , P. Branchini , R. A. Briere , T. E. Browder , A. Budano , S. Bussino , Q. Campagna , M. Campajola , L. Cao , G. Casarosa , C. Cecchi , P. Chang , P. Cheema , L. Chen , B. G. Cheon , C. Cheshta , H. Chetri , K. Chilikin , J. Chin , K. Chirapatpimol , H.-E. Cho , K. Cho , S.-J. Cho , S.-K. Choi , S. Choudhury , S. Chutia , J. Cochran , J. A. Colorado-Caicedo , I. Consigny , L. Corona , J. X. Cui , E. De La Cruz-Burelo , S. A. De La Motte , G. De Nardo , G. De Pietro , R. de Sangro , M. Destefanis , S. Dey , A. Di Canto , J. Dingfelder , Z. Doležal , I. Domínguez Jiménez , T. V. Dong , X. Dong , M. Dorigo , G. Dujany , P. Ecker , J. Eppelt , R. Farkas , P. Feichtinger , T. Ferber , T. Fillinger , C. Finck , G. Finocchiaro , F. Forti , B. G. Fulsom , A. Gabrielli , A. Gale , E. Ganiev , M. Garcia-Hernandez , R. Garg , G. Gaudino , V. Gaur , V. Gautam , A. Gaz , A. Gellrich , G. Ghevondyan , D. Ghosh , H. Ghumaryan , G. Giakoustidis , R. Giordano , A. Giri , P. Gironella Gironell , A. Glazov , B. Gobbo , R. Godang , O. Gogota , P. Goldenzweig , W. Gradl , M. Graf-Schreiber , E. Graziani , D. Greenwald , Y. Guan , K. Gudkova , I. Haide , Y. Han , H. Hayashii , S. Hazra , C. Hearty , M. T. Hedges , A. Heidelbach , G. Heine , I. Heredia de la Cruz , M. Hernández Villanueva , T. Higuchi , M. Hoek , M. Hohmann , R. Hoppe , P. Horak , X. T. Hou , C.-L. Hsu , A. Huang , T. Humair , T. Iijima , K. Inami , N. Ipsita , A. Ishikawa , R. Itoh , M. Iwasaki , P. Jackson , D. Jacobi , W. W. Jacobs , E.-J. Jang , S. Jia , Y. Jin , A. Johnson , M. Kaleta , A. B. Kaliyar , J. Kandra , K. H. Kang , S. Kang , G. Karyan , F. Keil , C. Ketter , M. Khan , C. Kiesling , D. Y. Kim , J.-Y. Kim , K.-H. Kim , H. Kindo , K. Kinoshita , P. Kodyš , T. Koga , S. Kohani , K. Kojima , A. Korobov , S. Korpar , E. Kovalenko , R. Kowalewski , P. Križan , P. Krokovny , T. Kuhr , Y. Kulii , D. Kumar , K. Kumara , T. Kunigo , Y.-J. Kwon , S. Lacaprara , T. Lam , L. Lanceri , J. S. Lange , T. S. Lau , M. Laurenza , R. Leboucher , F. R. Le Diberder , H. Lee , M. J. Lee , C. Lemettais , P. Leo , P. M. Lewis , C. Li , H.-J. Li , L. K. Li , Q. M. Li , W. Z. Li , Y. Li , Y. B. Li , Y. P. Liao , J. Libby , J. Lin , S. Lin , Z. Liptak , M. H. Liu , Q. Y. Liu , Z. Liu , D. Liventsev , S. Longo , A. Lozar , T. Lueck , C. Lyu , J. L. Ma , Y. Ma , M. Maggiora , S. P. Maharana , R. Maiti , G. Mancinelli , R. Manfredi , E. Manoni , M. Mantovano , D. Marcantonio , M. Marfoli , C. Marinas , C. Martellini , A. Martens , T. Martinov , L. Massaccesi , M. Masuda , D. Matvienko , S. K. Maurya , M. Maushart , J. A. McKenna , Z. Mediankin Gruberová , R. Mehta , F. Meier , D. Meleshko , M. Merola , C. Miller , M. Mirra , K. Miyabayashi , H. Miyake , R. Mizuk , G. B. Mohanty , S. Moneta , A. L. Moreira de Carvalho , H.-G. Moser , M. Mrvar , H. Murakami , R. Mussa , I. Nakamura , M. Nakao , Y. Nakazawa , M. Naruki , Z. Natkaniec , A. Natochii , M. Nayak , M. Neu , S. Nishida , R. Nomaru , A. Novosel , S. Ogawa , R. Okubo , H. Ono , F. Otani , G. Pakhlova , A. Panta , S. Pardi , K. Parham , J. Park , K. Park , S.-H. Park , A. Passeri , S. Patra , S. Paul , T. K. Pedlar , R. Pestotnik , M. Piccolo , L. E. Piilonen , P. L. M. Podesta-Lerma , T. Podobnik , C. Praz , S. Prell , E. Prencipe , M. T. Prim , H. Purwar , P. Rados , G. Raeuber , S. Raiz , K. Ravindran , J. U. Rehman , M. Reif , S. Reiter , L. Reuter , D. Ricalde Herrmann , I. Ripp-Baudot , G. Rizzo , S. H. Robertson , J. M. Roney , A. Rostomyan , N. Rout , S. Saha , L. Salutari , D. A. Sanders , S. Sandilya , L. Santelj , V. Savinov , B. Scavino , C. Schmitt , S. Schneider , M. Schnepf , K. Schoenning , C. Schwanda , Y. Seino , A. Selce , K. Senyo , J. Serrano , M. E. Sevior , C. Sfienti , W. Shan , G. Sharma , X. D. Shi , T. Shillington , T. Shimasaki , J.-G. Shiu , D. Shtol , A. Sibidanov , F. Simon , J. B. Singh , J. Skorupa , R. J. Sobie , M. Sobotzik , A. Soffer , A. Sokolov , E. Solovieva , S. Spataro , K. Špenko , B. Spruck , M. Starič , P. Stavroulakis , S. Stefkova , R. Stroili , M. Sumihama , K. Sumisawa , H. Svidras , K. Tackmann , M. Takahashi , M. Takizawa , U. Tamponi , S. Tanaka , S. S. Tang , K. Tanida , F. Tenchini , F. Testa , A. Thaller , T. Tien Manh , O. Tittel , R. Tiwary , E. Torassa , K. Trabelsi , F. F. Trantou , I. Tsaklidis , M. Uchida , I. Ueda , K. Unger , Y. Unno , K. Uno , S. Uno , P. Urquijo , Y. Ushiroda , S. E. Vahsen , R. van Tonder , K. E. Varvell , M. Veronesi , V. S. Vismaya , L. Vitale , V. Vobbilisetti , R. Volpe , M. Wakai , S. Wallner , M.-Z. Wang , A. Warburton , S. Watanuki , C. Wessel , E. Won , X. P. Xu , B. D. Yabsley , W. Yan , W. Yan , J. Yelton , K. Yi , J. H. Yin , K. Yoshihara , C. Z. Yuan , J. Yuan , Y. Yusa , L. Zani , F. Zeng , M. Zeyrek , B. Zhang , V. Zhilich , J. S. Zhou , Q. D. Zhou , L. Zhu , and R. Žlebčík (Belle II Collaboration)

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Phys. Rev. D 113, 032004 – Published 9 February, 2026

DOI: https://doi.org/10.1103/59ws-zxbt

Abstract

A sample of $365 {fb}^{- 1}$ of $e^{+} e^{-} \to ϒ (4 S) \to B \bar{B}$ data collected by the Belle II experiment is used to measure the partial branching fractions of charmless semileptonic $B$ meson decays and determine the magnitude of the Cabibbo-Kobayashi-Maskawa (CKM) matrix element $V_{u b}$ . Events containing a signal electron or muon $ℓ$ and a fully reconstructed hadronic $B$ decay that constrains the signal kinematics are selected, while the rest of the event defines the hadronic system $X_{u}$ associated with the signal. To discriminate the signal from the 50-times larger background originating from CKM-favored semileptonic $B$ decays, a template fit is performed in both signal and control regions after applying an optimized selection. The partial branching fraction measured for lepton energies greater than 1 GeV in the signal $B$ meson rest frame is $Δ B (B \to X_{u} ℓ ν) = (1.54 \pm 0.08 (stat) \pm 0.12 (syst)) \times 10^{- 3}$ . From this measurement, using the Gambino, Giordano, Ossola, Uraltsev theoretical framework, $| V_{u b} | = (4.01 \pm 0.1 9_{- 0.08}^{+ 0.07}) \times 10^{- 3}$ is determined, where the uncertainties are experimental and theoretical, respectively. This value is consistent with the world average obtained from previous inclusive measurements. Different theoretical predictions and partial branching fractions measured in other phase-space regions, defined by additional selections on the $X_{u}$ and leptonic system masses, are also used to determine $| V_{u b} |$ . This allows for a comparison of the resulting values across theoretical frameworks and phase-space regions.

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Phys. Rev. D 113, 032004– Published 9 February, 2026

Vol. 113, Iss. 3 — 1 February 2026

Received 6 December 2025
Accepted 13 January 2026

DOI: https://doi.org/10.1103/59ws-zxbt

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP³.

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Measurement of inclusive $B \to X_{u} ℓ ν$ partial branching fractions and $| V_{u b} |$ at Belle II

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Measurement of inclusive B→Xuℓν partial branching fractions and |Vub| at Belle II

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Measurement of inclusive $B \to X_{u} ℓ ν$ partial branching fractions and $| V_{u b} |$ at Belle II