$B$-flavor tagging at Belle II

Abstract

We report on new flavor tagging algorithms developed to determine the quark-flavor content of bottom ($B$) mesons at Belle II. The algorithms provide essential inputs for measurements of quark-flavor mixing and charge-parity violation. We validate and evaluate the performance of the algorithms using hadronic $B$ decays with flavor-specific final states reconstructed in a data set corresponding to an integrated luminosity of $62.8$ fb${}^{-1}$, collected at the $\hat{I}\yen$(4$S$) resonance with the Belle II detector at the SuperKEKB collider. We measure the total effective tagging efficiency to be ${\epsilon }_{\mathrm{eff}}=(30.0\pm 1.2(\text{stat})\pm 0.4(\text{syst}))\%$ for a category-based algorithm and ${\epsilon }_{\mathrm{eff}}=(28.8\pm 1.2(\text{stat})\pm 0.4(\text{syst}))\%$ for a deep-learning-based algorithm.