Measurement of $BF$, $f_L$ and $CP$ asymmetries in $B^0 \to \rho^+\rho^-$ decays and constraints on $\phi_2$ at Belle II

Abstract We present a measurement of the branching fraction and fraction of longitudinal polarization of $B^0 \to \rho^+ \rho^-$ decays, which have two $\pi^0$'s in the final state. We also measure time-dependent {\it CP} violation parameters for decays into longitudinally polarized $\rho^+ \rho^-$ pairs. This analysis is based on a data sample containing $(387\pm6) \times 10^6$$\Upsilon(4S)$ mesons collected with the Belle~II detector at the SuperKEKB asymmetric-energy $e^+e^-$ collider in 2019-2022. We obtain $\mathcal{B}(B^0\to\rho^+\rho^-) = (2.89 ^{+0.23}_{-0.22} {}^{+0.29}_{-0.27}) \times 10^{-5}, f_{L} = 0.921 ^{+0.024}_{-0.025} {}^{+0.017}_{-0.015}$, \mbox{$S = -0.26\pm0.19\pm0.08$}, and $C = -0.02\pm0.12^{+0.06}_{-0.05}$, where the first uncertainties are statistical and the second are systematic. We use these results to perform an isospin analysis to constrain the CKM angle $\phi_2$ and obtain two solutions; the result consistent with other Standard Model constraints is $\phi_2 = (92.6^{+4.5}_{-4.7})^\circ$.
Tags Run 1
Working group Time Dependent CP Violation
Principal authors @  Yu Nakazawa, Ryogo Okubo
Reference Phys.Rev.D 111, 092001 (2025), DOI: 10.1103/PhysRevD.111.092001
Document BELLE2-PUB-PH-2025-015
Links arXiv:2412.19624, Inspire, PRD
Bibtex 
@article{Belle-II:2024frs,
    author = "Adachi, I. and others",
    collaboration = "Belle-II",
    title = "{Measurement of the branching fraction, polarization, and time-dependent CP asymmetry in B0{\textrightarrow}{\ensuremath{\rho}}+{\ensuremath{\rho}}- decays and constraint on the CKM angle {\ensuremath{\phi}}2}",
    eprint = "2412.19624",
    archivePrefix = "arXiv",
    primaryClass = "hep-ex",
    reportNumber = "Belle II preprint 2024-024, KEK preprint 2024-23",
    doi = "10.1103/PhysRevD.111.092001",
    journal = "Phys. Rev. D",
    volume = "111",
    number = "9",
    pages = "092001",
    year = "2025"
}