Measurement of the branching fraction and $CP$ asymmetries in $B^0 \to J/\psi \pi^0$ decays at Belle II

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.
Tags
Working group Time Dependent CP Violation
Principal authors @  Chunhui Chen, James Cochran, Soeren Prell, Michele Veronesi
Reference Phys.Rev.D 111, 012011 (2025), DOI: 10.1103/PhysRevD.111.012011
Document BELLE2-PUB-PH-2024-049
Links arXiv:2410.08622, Inspire, PRD
Bibtex 
@article{Belle-II:2024hqw,
    author = "Adachi, I. and others",
    collaboration = "Belle-II",
    title = "{Observation of time-dependent CP violation and measurement of the branching fraction of B0{\textrightarrow}J/{\ensuremath{\psi}}{\ensuremath{\pi}}0 decays}",
    eprint = "2410.08622",
    archivePrefix = "arXiv",
    primaryClass = "hep-ex",
    reportNumber = "Belle II preprint: 2024-018, KEK preprint: 2024-14",
    doi = "10.1103/PhysRevD.111.012011",
    journal = "Phys. Rev. D",
    volume = "111",
    number = "1",
    pages = "012011",
    year = "2025"
}