Kavli Affiliate: T. Higuchi
| First 5 Authors: Belle II Collaboration, I. Adachi, L. Aggarwal, H. Ahmed, N. Akopov
| Summary:
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 $CP$ violation
parameters for decays into longitudinally polarized $rho^+ rho^-$ pairs. This
analysis is based on a data sample containing $(387pm6) times 10^6$ BBbar
pairs collected with the Belle~II detector at the SuperKEKB asymmetric-energy
$e^+e^-$ collider in 2019-2022. We obtain ${B}(B^0torho^+rho^-) = (2.88
^{+0.23}_{-0.22} {}^{+0.29}_{-0.27}) times 10^{-5}, f_{L} = 0.921
^{+0.024}_{-0.025} {}^{+0.017}_{-0.015}$, $S = -0.26pm0.19pm0.08$, and $C =
-0.02pm0.12^{+0.06}_{-0.05}$, 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 $phi_2 =
(92.6^{+4.5}_{-4.8})^circ$.
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