| Abstract: | We report a measurement of the branching fraction of $B o
K^{ast}(892)ell^+ell^-$ 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 $22pm 6$,
$18 pm 6$, and $38 pm 9$ for the decays $B o K^{ast}(892)mu^+mu^-$, $B
o K^{ast}(892)e^+e^-$, and $B o K^{ast}(892)ell^+ell^-$, 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 $B o
K^{ast}(892)gamma( o e^+e^-)$ background and regions compatible with decays
of charmonium resonances, to be egin{equation} {cal B}(B o
K^{ast}(892)mu^+mu^-) = (1.19 pm 0.31 ^{+0.08}_{-0.07}) imes 10^{-6},
{cal B}(B o K^{ast}(892)e^+e^-) = (1.42 pm 0.48 pm 0.09) imes 10^{-6},
{cal B}(B o K^{ast}(892)ell^+ell^-) = (1.25 pm 0.30 ^{+0.08}_{-0.07})
imes 10^{-6}, end{equation} where the first and second uncertainties are
statistical and systematic, respectively. These results, limited by sample
size, are the first measurements of $B o K^{ast}(892)ell^+ell^-$ branching
fractions from the Belle II experiment. |
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