Angular analysis of $B^{+}\to \ddot{I}{}^{+}\ddot{I}{}^0$ decays reconstructed in 2019-2020 Belle II data

Abstract

We report on the first Belle II measurement of the branching fraction ($\mathcal{B}$) and longitudinal polarization fraction ($f_L$) of $B^{+}\to \ddot{I}{}^{+}\ddot{I}{}^0$ decays. We reconstruct $B^{+}\to \ddot{I}{}^{+}(\to \ddot{I}{€}^{+}\ddot{I}{€}^0(\to {\hat{I}}^3{\hat{I}}^3))\ddot{I}{}^0(\to \ddot{I}{€}^{+}\ddot{I}{€}^{-})$ decays in a sample of SuperKEKB electron-positron collisions collected by the Belle II experiment in 2019 and 2020 at the $\hat{I}\yen$(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 $104\pm 16$ events. The signal yields are corrected for efficiencies determined from simulation and control data samples to obtain $\mathcal{B}(B^{+}\to \ddot{I}{}^{+}\ddot{I}{}^0)=[20.6\pm 3.2(\mathrm{stat})\pm 4.0(\mathrm{syst})]\times 10^{-6}$, and $f_L(B^{+}\to \ddot{I}{}^{+}\ddot{I}{}^0)=0.936_{-0.041}^{+0.049}(\mathrm{stat})\pm 0.021(\mathrm{syst})$. This first Belle II $B^{+}\to \ddot{I}{}^{+}\ddot{I}{}^0$ angular analysis yields results compatible with previous determinations, and indicates Belle II performance superior to early Belle results.