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arXiv · 2606.20797

Measurement of mixing-induced CP violation in the decay $B^0 \to \pi^0 \pi^0$

Belle II Collaboration·M. Abumusabh·I. Adachi·K. Adamczyk·A. Aggarwal·Y. Ahn·H. Aihara·M. Akdag·N. Akopov·S. Alghamdi·M. Alhakami·N. Althubiti·K. Amos·M. Angelsmark·N. Anh Ky·C. Antonioli·K. Arai·H. Atmacan·T. Aushev·V. Aushev·R. Ayad·V. Babu·H. Bae·N. K. Baghel·S. Bahinipati·P. Bambade·Sw. Banerjee·S. Bansal·M. Barrett·M. Bartl·J. Baudot·A. Beaubien·F. Becherer·J. Becker·G. F. Benfratello·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·A. Bondar·G. Bonvicini·J. Borah·A. Boschetti·A. Bozek·M. Bračko·P. Branchini·N. Brenny·R. A. Briere·T. E. Browder·A. Budano·S. Bussino·F. Callet·Q. Campagna·M. Campajola·L. Cao·M. Carminati·G. Casarosa·C. Cecchi·M. -C. Chang·P. Cheema·C. Chen·L. Chen·B. G. Cheon·C. Cheshta·H. Chetri·K. Chilikin·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·H. Crotte Ledesma·S. Cuccuini·J. X. Cui·S. Das·E. De La Cruz-Burelo·S. A. De La Motte·G. de Marino·G. De Nardo·G. De Pietro·R. de Sangro·M. Destefanis·S. Dey·R. Dhayal
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Abstract

Measurements of charge-parity (CP) violation in quark transitions provide stringent tests of the current theory and sensitive probes of particles or interactions beyond it. We report the first measurement of mixing-induced CP violation in $B^0\to\pi^0\pi^0$ decays. The measurement uses 190 million $\Upsilon(4S)\to B^{0}\overline{B}{}^0$ events collected with the Belle II experiment at the SuperKEKB asymmetric-energy electron-positron collider. A new approach that exploits the quantum entanglement of the $B^{0}\overline{B}{}^0$ pair enables a decay-time-dependent analysis without reconstructing the $B^0 \to \pi^0\pi^0$ decay position. We measure the mixing-induced CP-violating coefficient to be $S_{\pi^{0}\pi^{0}} = 0.61^{+0.75}_{-0.79}\,(\mathrm{stat}) \pm 0.11\,(\mathrm{syst})$, attaining a precision that would require a data set twenty times as large with a conventional analysis. We also measure the direct CP-violating coefficient to be $C_{\pi^{0}\pi^{0}} = 0.05 \pm 0.28\,(\mathrm{stat}) \pm 0.07\,(\mathrm{syst})$. These results substantially improve the constraints from $B \to \pi\pi$ decays on the CP-violating phase $\phi_2$ of the quark-mixing matrix using far less data than had previously been assumed necessary.

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Belle II Collaboration, M. Abumusabh, I. Adachi, K. Adamczyk, A. Aggarwal, Y. Ahn, H. Aihara, M. Akdag, N. Akopov, S. Alghamdi, M. Alhakami, N. Althubiti, K. Amos, M. Angelsmark, N. Anh Ky, C. Antonioli, K. Arai, H. Atmacan, T. Aushev, V. Aushev, R. Ayad, V. Babu, H. Bae, N. K. Baghel, S. Bahinipati, P. Bambade, Sw. Banerjee, S. Bansal, M. Barrett, M. Bartl, J. Baudot, A. Beaubien, F. Becherer, J. Becker, G. F. Benfratello, 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, A. Bondar, G. Bonvicini, J. Borah, A. Boschetti, A. Bozek, M. Bračko, P. Branchini, N. Brenny, R. A. Briere, T. E. Browder, A. Budano, S. Bussino, F. Callet, Q. Campagna, M. Campajola, L. Cao, M. Carminati, G. Casarosa, C. Cecchi, M. -C. Chang, P. Cheema, C. Chen, L. Chen, B. G. Cheon, C. Cheshta, H. Chetri, K. Chilikin, 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, H. Crotte Ledesma, S. Cuccuini, J. X. Cui, S. Das, E. De La Cruz-Burelo, S. A. De La Motte, G. de Marino, G. De Nardo, G. De Pietro, R. de Sangro, M. Destefanis, S. Dey, R. Dhayal. 2026-06-18. Measurement of mixing-induced CP violation in the decay $B^0 \to \pi^0 \pi^0$. https://arxiv.org/abs/2606.20797

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