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

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.
Tags Run 1
Working group Time Dependent CP Violation
Principal authors @  Mirco Dorigo, Sebastiano Raiz, Diego Tonelli, Kieran Amos, Radek Zlebcik
Reference submitted to NP
Links arXiv:2606.20797, Inspire,
Bibtex 
@article{Belle-II:2026kzq,
    author = "Abumusabh, Merna and others",
    collaboration = "Belle-II",
    title = "{Measurement of mixing-induced CP violation in the decay $B^0 \to π^0 π^0$}",
    eprint = "2606.20797",
    archivePrefix = "arXiv",
    primaryClass = "hep-ex",
    reportNumber = "Belle II Preprint 2026-013, KEK Preprint 2026-9",
    month = "6",
    year = "2026"
}