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arXiv · 2510.01331

Measurement of time-dependent $CP$ asymmetries in $B^0 \to K_{\rm S}^0 \: \pi^{+} \pi^{-} \gamma$ decays at Belle and Belle II

Belle·Belle II Collaborations·:·M. Abumusabh·I. Adachi·L. Aggarwal·H. Ahmed·Y. Ahn·H. Aihara·N. Akopov·S. Alghamdi·M. Alhakami·K. Amos·N. Anh Ky·D. M. Asner·H. Atmacan·T. Aushev·R. Ayad·V. Babu·S. Bahinipati·P. Bambade·Sw. Banerjee·M. Barrett·M. Bartl·J. Baudot·A. Beaubien·F. Becherer·J. Becker·J. V. Bennett·V. Bertacchi·E. Bertholet·M. Bessner·S. Bettarini·V. Bhardwaj·B. Bhuyan·F. Bianchi·T. Bilka·D. Biswas·A. Bobrov·D. Bodrov·J. Borah·A. Boschetti·A. Bozek·M. Bračko·P. Branchini·R. A. Briere·T. E. Browder·A. Budano·S. Bussino·Q. Campagna·M. Campajola·G. Casarosa·C. Cecchi·P. Cheema·C. Chen·L. Chen·C. Cheshta·H. Chetri·J. Chin·K. Chirapatpimol·H. -E. Cho·K. Cho·S. -J. Cho·S. -K. Choi·S. Choudhury·J. A. Colorado-Caicedo·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·A. Di Canto·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. Gale·M. Garcia-Hernandez·R. Garg·G. Gaudino·V. Gaur·V. Gautam·A. Gaz·A. Gellrich·G. Ghevondyan
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Abstract

We present a measurement of the time-dependent $CP$ asymmetry in $B^0 \to K_{\rm S}^0 \: \pi^{+} \pi^{-} \gamma$ decays using a data set of 365 fb$^{-1}$ recorded by the Belle II experiment and the final data set of 711 fb$^{-1}$ recorded by the Belle experiment at the ${\rm \Upsilon(4S)}$ resonance. The direct and mixing-induced time-dependent $CP$ violation parameters $C$ and $S$ are determined along with two additional quantities, $S^{+}$ and $S^{-}$, defined in the two halves of the $m^2(K_{\rm S}^0 \pi^{+})-m^2(K_{\rm S}^0 \pi^{-})$ plane. The measured values are $C = -0.17 \pm 0.09 \pm 0.04$, $S = -0.29 \pm 0.11 \pm 0.05$, $S^{+} = -0.57 \pm 0.23 \pm 0.10$ and $S^{-} = 0.31 \pm 0.24 \pm 0.05$, where the first uncertainty is statistical and the second systematic.

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Belle, Belle II Collaborations, :, M. Abumusabh, I. Adachi, L. Aggarwal, H. Ahmed, Y. Ahn, H. Aihara, N. Akopov, S. Alghamdi, M. Alhakami, K. Amos, N. Anh Ky, D. M. Asner, H. Atmacan, T. Aushev, R. Ayad, V. Babu, S. Bahinipati, P. Bambade, Sw. Banerjee, M. Barrett, M. Bartl, J. Baudot, A. Beaubien, F. Becherer, J. Becker, J. V. Bennett, V. Bertacchi, E. Bertholet, M. Bessner, S. Bettarini, V. Bhardwaj, B. Bhuyan, F. Bianchi, T. Bilka, D. Biswas, A. Bobrov, D. Bodrov, J. Borah, A. Boschetti, A. Bozek, M. Bračko, P. Branchini, R. A. Briere, T. E. Browder, A. Budano, S. Bussino, Q. Campagna, M. Campajola, G. Casarosa, C. Cecchi, P. Cheema, C. Chen, L. Chen, C. Cheshta, H. Chetri, J. Chin, K. Chirapatpimol, H. -E. Cho, K. Cho, S. -J. Cho, S. -K. Choi, S. Choudhury, J. A. Colorado-Caicedo, 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, A. Di Canto, 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. Gale, M. Garcia-Hernandez, R. Garg, G. Gaudino, V. Gaur, V. Gautam, A. Gaz, A. Gellrich, G. Ghevondyan. 2025-10-01. Measurement of time-dependent $CP$ asymmetries in $B^0 \to K_{\rm S}^0 \: \pi^{+} \pi^{-} \gamma$ decays at Belle and Belle II. https://doi.org/10.1007/jhep01(2026)134

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