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arXiv · 2410.08622

Observation of time-dependent $CP$ violation and measurement of the branching fraction of $B^0 \to J/\psi \pi^0$ decays

Belle II Collaboration·I. Adachi·L. Aggarwal·H. Ahmed·H. Aihara·N. Akopov·A. Aloisio·N. Althubiti·N. Anh Ky·D. M. Asner·H. Atmacan·V. Aushev·M. Aversano·R. Ayad·V. Babu·H. Bae·N. K. Baghel·S. Bahinipati·P. Bambade·Sw. Banerjee·S. Bansal·J. Baudot·A. Baur·A. Beaubien·F. Becherer·J. Becker·J. V. Bennett·F. U. Bernlochner·V. Bertacchi·M. Bertemes·E. Bertholet·M. Bessner·S. Bettarini·V. Bhardwaj·F. Bianchi·T. Bilka·D. Biswas·A. Bobrov·D. Bodrov·A. Bondar·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·J. Cerasoli·M. -C. Chang·P. Chang·R. Cheaib·P. Cheema·C. Chen·B. G. Cheon·K. Chilikin·K. Chirapatpimol·H. -E. Cho·K. Cho·S. -J. Cho·S. -K. Choi·S. Choudhury·J. Cochran·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·R. Dhamija·A. Di Canto·F. Di Capua·J. Dingfelder·Z. Doležal·T. V. Dong·M. Dorigo·S. Dubey·K. Dugic·G. Dujany·P. Ecker·D. Epifanov·P. Feichtinger·T. Ferber·T. Fillinger·C. Finck·G. Finocchiaro·A. Fodor·F. Forti·B. G. Fulsom·A. Gabrielli·E. Ganiev·M. Garcia-Hernandez
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Abstract

We present a measurement of the branching fraction and time-dependent charge-parity ($CP$) decay-rate asymmetries in $B^0 \to J/\psi \pi^0$ decays. The data sample was collected with the Belle~II detector at the SuperKEKB asymmetric $e^+e^-$ collider in 2019-2022 and contains $(387\pm 6)\times 10^6$ $B\overline{B}$ meson pairs from $\Upsilon(4S)$ decays. We reconstruct $392\pm 24$ signal decays and fit the $CP$ parameters from the distribution of the proper-decay-time difference of the two $B$ mesons. We measure the branching fraction to be $B(B^0 \to J/\psi \pi^0)=(2.00 \pm 0.12 \pm 0.09)\times 10^{-5}$ and the direct and mixing-induced $CP$ asymmetries to be $C_{CP}=0.13 \pm 0.12 \pm 0.03$ and $S_{CP}=-0.88 \pm 0.17 \pm 0.03$, respectively, where the first uncertainties are statistical and the second are systematic. We observe mixing-induced $CP$ violation with a significance of $5.0$ standard deviations for the first time in this mode.

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Belle II Collaboration, I. Adachi, L. Aggarwal, H. Ahmed, H. Aihara, N. Akopov, A. Aloisio, N. Althubiti, N. Anh Ky, D. M. Asner, H. Atmacan, V. Aushev, M. Aversano, R. Ayad, V. Babu, H. Bae, N. K. Baghel, S. Bahinipati, P. Bambade, Sw. Banerjee, S. Bansal, J. Baudot, A. Baur, A. Beaubien, F. Becherer, J. Becker, J. V. Bennett, F. U. Bernlochner, V. Bertacchi, M. Bertemes, E. Bertholet, M. Bessner, S. Bettarini, V. Bhardwaj, F. Bianchi, T. Bilka, D. Biswas, A. Bobrov, D. Bodrov, A. Bondar, 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, J. Cerasoli, M. -C. Chang, P. Chang, R. Cheaib, P. Cheema, C. Chen, B. G. Cheon, K. Chilikin, K. Chirapatpimol, H. -E. Cho, K. Cho, S. -J. Cho, S. -K. Choi, S. Choudhury, J. Cochran, 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, R. Dhamija, A. Di Canto, F. Di Capua, J. Dingfelder, Z. Doležal, T. V. Dong, M. Dorigo, S. Dubey, K. Dugic, G. Dujany, P. Ecker, D. Epifanov, P. Feichtinger, T. Ferber, T. Fillinger, C. Finck, G. Finocchiaro, A. Fodor, F. Forti, B. G. Fulsom, A. Gabrielli, E. Ganiev, M. Garcia-Hernandez. 2024-10-11. Observation of time-dependent $CP$ violation and measurement of the branching fraction of $B^0 \to J/\psi \pi^0$ decays. https://doi.org/10.1103/physrevd.111.012011

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