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Abstract

We report on a Belle II measurement of the branching fraction
($\mathcal{B}$), longitudinal polarization fraction ($f_L$), and CP asymmetry
($\mathcal{A}_{CP}$) of $B^+\to \rho^+\rho^0$ decays. We reconstruct $B^+\to
\rho^+(\to \pi^+\pi^0(\to \gamma\gamma))\rho^0(\to \pi^+\pi^-)$ decays in a
sample of SuperKEKB electron-positron collisions collected by the Belle II
experiment in 2019, 2020, and 2021 at the $\Upsilon$(4S) resonance and
corresponding to 190 fb$^{-1}$ of integrated luminosity. We fit the
distributions of the difference between expected and observed $B$ candidate
energy, continuum-suppression discriminant, dipion masses, and decay angles of
the selected samples, to determine a signal yield of $345 \pm 31$ events. The
signal yields are corrected for efficiencies determined from simulation and
control data samples to obtain $\mathcal{B}(B^+ \to \rho^+\rho^0) = [23.2^{+\
2.2}_{-\ 2.1} (\rm stat) \pm 2.7 (\rm syst)]\times 10^{-6}$, $f_L = 0.943 ^{+\
0.035}_{-\ 0.033} (\rm stat)\pm 0.027(\rm syst)$, and $\mathcal{A}_{CP}=-0.069
\pm 0.068(\rm stat) \pm 0.060 (\rm syst)$. The results agree with previous
measurements. This is the first measurement of $\mathcal{A}_{CP}$ in $B^+\to
\rho^+\rho^0$ decays reported by Belle II.