Seema Choudhury

20 September 2022

Abstract: We report a measurement of the branching fraction of $B \to K^{\ast}(892)\ell^+\ell^-$ decays, where $\ell^+\ell^- = \mu^+\mu^-$ or $e^+e^-$, using electron-positron collisions recorded at an energy at or near the $\Upsilon(4S)$ mass and corresponding to an integrated luminosity of $189$ fb$^{-1}$. The data was collected during 2019--2021 by the Belle II experiment at the SuperKEKB $e^{+}e^{-}$ asymmetric-energy collider. We reconstruct $K^{\ast}(892)$ candidates in the $K^+\pi^-$, $K_{S}^{0}\pi^+$, and $K^+\pi^0$ final states. The signal yields with statistical uncertainties are $22\pm 6$, $18 \pm 6$, and $38 \pm 9$ for the decays $B \to K^{\ast}(892)\mu^+\mu^-$, $B \to K^{\ast}(892)e^+e^-$, and $B \to K^{\ast}(892)\ell^+\ell^-$, respectively. We measure the branching fractions of these decays for the entire range of the dilepton mass, excluding the very low mass region to suppress the $B \to K^{\ast}(892)\gamma(\to e^+e^-)$ background and regions compatible with decays of charmonium resonances, to be \begin{align*} {\cal B}(B \to K^{\ast}(892)\mu^+\mu^-) &= (1.19 \pm 0.31 ^{+0.08}_{-0.07}) \times 10^{-6},\\ {\cal B}(B \to K^{\ast}(892)e^+e^-) &= (1.42 \pm 0.48 \pm 0.09)\times 10^{-6}, \\ {\cal B}(B \to K^{\ast}(892)\ell^+\ell^-) &= (1.25 \pm 0.30 ^{+0.08}_{-0.07}) \times 10^{-6}, \end{align*} where the first and second uncertainties are statistical and systematic, respectively. These results, limited by sample size, are the first measurements of $B \to K^{\ast}(892)\ell^+\ell^-$ branching fractions from the Belle II experiment.

Keyword(s): rare decays ; Belle II ; EWP ; b->sll ; FCNC ; BR ; LFU