We propose a new algorithm for the identification of the production flavor of neutral $D$ mesons in the Belle II experiment. The algorithm exploits the correlation between the flavor of a reconstructed neutral $D$ meson (signal $D$ meson) and the electric charges of particles reconstructed in the rest of the $e^{+}{e}^{-}\to c\overline{c}$ event. These include those originating from the decay of the other charm hadron produced in the event, as well as those possibly produced in association with the signal $D$ meson. We develop the algorithm using simulation and calibrate it in data using decay modes that identify the flavor of the decaying neutral $D$ meson. We use a data sample of $e^{+}{e}^{-}$ collisions, corresponding to $362{\mathrm{fb}}^{-1}$ of integrated luminosity, collected by Belle II at center-of-mass energies near the $\mathrm{\Upsilon }(4S)$ mass. The effective tagging efficiency in data is $(47.91\pm 0.07(\mathrm{stat})\pm 0.51(\mathrm{syst}))\%$, independent of the neutral-$D$-meson decay mode. This charm flavor tagger will approximately double the effective sample size of many $CP$-violation and charm-mixing measurements that so far have exclusively relied on neutral $D$ mesons originating from $D^{*\pm }$ decays. While developed for Belle II, the basic principles underlying the charm flavor tagger can be used in other experiments, including those at hadron colliders.

• Received 6 April 2023
• Accepted 8 May 2023

DOI:https://doi.org/10.1103/PhysRevD.107.112010

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Published by the American Physical Society