Angular analysis of $B^+ \to \rho^+\rho^0$ decays reconstructed in 2019-2020 Belle II data

TL;DR

This paper presents the first Belle II measurement of the branching fraction and polarization of B+ to rho+ rho0 decays, demonstrating Belle II's capability and consistency with prior results using 2019-2020 data.

Contribution

First Belle II measurement of B+ to rho+ rho0 decay properties, including branching fraction and polarization, using 2019-2020 data at the Upsilon(4S) resonance.

Findings

Measured branching fraction: (20.6 ± 3.2 ± 4.0)×10^{-6}

Measured longitudinal polarization fraction: 0.936^{+0.049}_{-0.041} ± 0.021

Results are compatible with previous experiments and show Belle II's improved performance.

Abstract

We report on the first Belle II measurement of the branching fraction ($\mathcal{B}$) and longitudinal polarization fraction ($f_L$) 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 and 2020 at the $\Upsilon$(4S) resonance and corresponding to $62.8$ fb$^{-1}$ of integrated luminosity. We fit the distributions of the difference between expected and observed $B$ candidate energy, continuum-suppression variable, dipion masses, and angular distributions of the resulting samples, to determine a signal yield of $104\pm16$ events. The signal yields are corrected for efficiencies determined from simulation and control data samples to obtain $\mathcal{B}(B^+ \to \rho^+\rho^0) = [20.6 \pm 3.2(\rm stat) \pm 4.0(\rm syst)]\times 10^{-6}$, and $f_L(B^+ \to \rho^+\rho^0) = 0.936 ^{+0.049}_{-0.041}(\rm stat)\pm 0.021(\rm syst)$. This first Belle II $B^+ \to \rho^+\rho^0$ angular analysis yields results compatible with previous determinations, and indicates Belle II performance superior to early Belle results.