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arXiv · 2604.08396

Search for the lepton-flavour violating decays $B^+ \to \pi^+ \mu^\pm e^\mp$

LHCb collaboration·R. Aaij·M. Abdelfatah·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·S. Akar·K. Akiba·P. Albicocco·J. Albrecht·R. Aleksiejunas·F. Alessio·P. Alvarez Cartelle·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·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·Z. B. Bai·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. Baron·S. Barsuk·W. Barter·J. Bartz·S. Bashir·B. Batsukh·P. B. Battista·A. Bavarchee·A. Bay·A. Beck·M. Becker·F. Bedeschi·I. B. Bediaga·N. A. Behling·S. Belin·A. Bellavista·I. Belov·I. Belyaev·G. Bencivenni·E. Ben-Haim·J. L. M. Berkey·R. Bernet·A. Bertolin·F. Betti·J. Bex·O. Bezshyyko·S. Bhattacharya·M. S. Bieker·N. V. Biesuz·A. Biolchini·M. Birch·F. C. R. Bishop·A. Bitadze·A. Bizzeti·T. Blake·F. Blanc·J. E. Blank·S. Blusk·J. A. Boelhauve
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Abstract

The first search for the lepton-flavour violating decays $B^+ \to \pi^+ \mu^{\pm} e^{\mp}$ in proton-proton collisions is presented, using data collected by the LHCb experiment between 2011 and 2018, corresponding to an integrated luminosity of 9 fb$^{-1}$. No significant signal is observed and an upper limit on the branching fraction is set at $\mathcal{B}(B^+ \to \pi^+ \mu^{\pm} e^{\mp}) < 1.8 \times 10^{-9}$ at the $90\%$ confidence level, two orders of magnitude more restrictive than the current world average. This is the first constraint on lepton-flavour violating $b \to d$ quark transitions at the LHC and also sets the most stringent upper limits to date on $b \to d \mu^{\pm} e^{\mp}$ transitions. Limits on left-handed and scalar scenarios beyond the Standard Model are also reported.

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LHCb collaboration, R. Aaij, M. Abdelfatah, 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, S. Akar, K. Akiba, P. Albicocco, J. Albrecht, R. Aleksiejunas, F. Alessio, P. Alvarez Cartelle, 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, 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, Z. B. Bai, 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. Baron, S. Barsuk, W. Barter, J. Bartz, S. Bashir, B. Batsukh, P. B. Battista, A. Bavarchee, A. Bay, A. Beck, M. Becker, F. Bedeschi, I. B. Bediaga, N. A. Behling, S. Belin, A. Bellavista, I. Belov, I. Belyaev, G. Bencivenni, E. Ben-Haim, J. L. M. Berkey, R. Bernet, A. Bertolin, F. Betti, J. Bex, O. Bezshyyko, S. Bhattacharya, M. S. Bieker, N. V. Biesuz, A. Biolchini, M. Birch, F. C. R. Bishop, A. Bitadze, A. Bizzeti, T. Blake, F. Blanc, J. E. Blank, S. Blusk, J. A. Boelhauve. 2026-04-09. Search for the lepton-flavour violating decays $B^+ \to \pi^+ \mu^\pm e^\mp$. https://arxiv.org/abs/2604.08396

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