Exclusive $B\to X_u\ell \hat{I}{½}_{\ell }$ Decays with Hadronic Full-event-interpretation Tagging in 62.8 fb${}^{-1}$ of Belle II Data

Abstract

We present a reconstruction in early data of the semileptonic decay $B^{+}\to \ddot{I}{€}^0{\ell }^{+}\hat{I}{½}_{\ell }$, and first results of a reconstruction of the decays $B^{+}\to \ddot{I}{}^0{\ell }^{+}\hat{I}{½}_{\ell }$ and $B^0\to \ddot{I}{}^{-}{\ell }^{+}\hat{I}{½}_{\ell }$ in a sample corresponding to 62.8 fb${}^{-1}$ of Belle II data using hadronic $B$-tagging via the full-event-interpretation algorithm. We determine the total branching fractions via fits to the distribution of the square of the missing mass, and find $\mathcal{B}(B^{+}\to \ddot{I}{€}^0{\ell }^{+}\hat{I}{½}_{\ell })$ = (8.29 $\pm$ 1.99(stat) $\pm$ 0.46(syst)) $\times 10^{-5}$. We obtain $95\%$ CL upper limits on the branching fractions with $\mathcal{B}(B^0\to \ddot{I}{}^{-}{\ell }^{+}\hat{I}{½}_{\ell })<3.37\times 10^{-4}$ and $\mathcal{B}(B^{+}\to \ddot{I}{}^0{\ell }^{+}\hat{I}{½}_{\ell })<19.7\times 10^{-5}$. We also obtain an updated branching fraction for the $B^0\to \ddot{I}{€}^{-}{\ell }^{+}\hat{I}{½}_{\ell }$ decay, $\mathcal{B}(B^0\to \ddot{I}{€}^{-}{\ell }^{+}\hat{I}{½}_{\ell })$ = (1.47 $\pm$ 0.29(stat) $\pm$ 0.05(syst)) $\times 10^{-4}$, based on the sum of the partial branching fractions in three bins of the squared momentum transfer to the leptonic system.