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arXiv · 2511.16564

Differential decay rate of $B^+ \to J/\psi K^+$ with the LHCb Upgrade I experiment

LHCb collaboration·R. Aaij·A. S. W. Abdelmotteleb·C. Abellan Beteta·F. Abudinén·T. Ackernley·A. A. Adefisoye·B. Adeva·M. Adinolfi·P. Adlarson·C. Agapopoulou·C. A. Aidala·Z. Ajaltouni·S. Akar·K. Akiba·M. Akthar·P. Albicocco·J. Albrecht·R. Aleksiejunas·F. Alessio·P. Alvarez Cartelle·R. Amalric·S. Amato·J. L. Amey·Y. Amhis·L. An·L. Anderlini·M. Andersson·P. Andreola·M. Andreotti·S. Andres Estrada·A. Anelli·D. Ao·C. Arata·F. Archilli·Z. Areg·M. Argenton·S. Arguedas Cuendis·L. Arnone·A. Artamonov·M. Artuso·E. Aslanides·R. Ataíde Da Silva·M. Atzeni·B. Audurier·J. A. Authier·D. Bacher·I. Bachiller Perea·S. Bachmann·M. Bachmayer·J. J. Back·P. Baladron Rodriguez·V. Balagura·A. Balboni·W. Baldini·Z. Baldwin·L. Balzani·H. Bao·J. Baptista de Souza Leite·C. Barbero Pretel·M. Barbetti·I. R. Barbosa·R. J. Barlow·M. Barnyakov·S. Barsuk·W. Barter·J. Bartz·S. Bashir·B. Batsukh·P. B. Battista·A. Bay·A. Beck·M. Becker·F. Bedeschi·I. B. Bediaga·N. A. Behling·S. Belin·A. Bellavista·K. Belous·I. Belov·I. Belyaev·G. Benane·G. Bencivenni·E. Ben-Haim·A. Berezhnoy·R. Bernet·S. Bernet Andres·A. Bertolin·F. Betti·J. Bex·O. Bezshyyko·S. Bhattacharya·J. Bhom·M. S. Bieker·N. V. Biesuz·A. Biolchini·M. Birch·F. C. R. Bishop·A. Bitadze·A. Bizzeti
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Abstract

The normalised decay rate of $B^+ \to J/\psi(\to \mu^+\mu^-) K^+$ is measured as a function of the lepton helicity angle using a data sample corresponding to an integrated luminosity of $1.1 \text{fb}^{-1}$ collected during October 2024 with the upgraded (Upgrade I) LHCb detector. This angular distribution can be parameterised by two coefficients, the forward-backward asymmetry, $A_{FB}$, and the flatness parameter, $F_{H}$, whose values are constrained by conservation of angular momentum. These coefficients are measured both integrated and differentially across various kinematic and detector-response variables, and the results are found to be in good agreement with expectations. These measurements show that the detector response of the LHCb Upgrade I experiment is understood to the precision required to reliably extract the angular coefficients associated with rare $b \to s \mu^+\mu^-$ and $b \to d \mu^+\mu^-$ transitions, which are particularly sensitive to physics beyond the Standard Model.

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LHCb collaboration, R. Aaij, A. S. W. Abdelmotteleb, C. Abellan Beteta, F. Abudinén, T. Ackernley, A. A. Adefisoye, B. Adeva, M. Adinolfi, P. Adlarson, C. Agapopoulou, C. A. Aidala, Z. Ajaltouni, S. Akar, K. Akiba, M. Akthar, P. Albicocco, J. Albrecht, R. Aleksiejunas, F. Alessio, P. Alvarez Cartelle, R. Amalric, S. Amato, J. L. Amey, Y. Amhis, L. An, L. Anderlini, M. Andersson, P. Andreola, M. Andreotti, S. Andres Estrada, A. Anelli, D. Ao, C. Arata, F. Archilli, Z. Areg, M. Argenton, S. Arguedas Cuendis, L. Arnone, A. Artamonov, M. Artuso, E. Aslanides, R. Ataíde Da Silva, M. Atzeni, B. Audurier, J. A. Authier, D. Bacher, I. Bachiller Perea, S. Bachmann, M. Bachmayer, J. J. Back, P. Baladron Rodriguez, V. Balagura, A. Balboni, W. Baldini, Z. Baldwin, L. Balzani, H. Bao, J. Baptista de Souza Leite, C. Barbero Pretel, M. Barbetti, I. R. Barbosa, R. J. Barlow, M. Barnyakov, S. Barsuk, W. Barter, J. Bartz, S. Bashir, B. Batsukh, P. B. Battista, A. Bay, A. Beck, M. Becker, F. Bedeschi, I. B. Bediaga, N. A. Behling, S. Belin, A. Bellavista, K. Belous, I. Belov, I. Belyaev, G. Benane, G. Bencivenni, E. Ben-Haim, A. Berezhnoy, R. Bernet, S. Bernet Andres, A. Bertolin, F. Betti, J. Bex, O. Bezshyyko, S. Bhattacharya, J. Bhom, M. S. Bieker, N. V. Biesuz, A. Biolchini, M. Birch, F. C. R. Bishop, A. Bitadze, A. Bizzeti. 2025-11-20. Differential decay rate of $B^+ \to J/\psi K^+$ with the LHCb Upgrade I experiment. https://doi.org/10.1007/jhep04(2026)170

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