Test of lepton flavor universality and search for lepton flavor violation in $B\to K\ell \ell$ decays

Abstract

We present measurements of the branching fractions for the decays $B\to K{\mu }^{+}{\mu }^{-}$ and $B\to K{e}^{+}{e}^{-}$, and their ratio ($R_K$), using a data sample of 711 $f{b}^{-1}$ that contains $772\times 10^6$ $B\bar{B}$ events. The data were collected at the ${\rm Y}(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^{+}{e}^{-}$ collider. The ratio $R_K$ is measured in five bins of dilepton invariant-mass-squared ($q^2$): $q^2\in (0.1,4.0),(4.0,8.12),(1.0,6.0)$, $(10.2,12.8)$ and ($>14.18)Ge{V}^2/c^4$, along with the whole $q^2$ region. The $R_K$ value for $q^2\in (1.0,6.0)Ge{V}^2/c^4$ is $1.03_{-0.24}^{+0.28}\pm 0.01$. The first and second uncertainties listed are statistical and systematic, respectively. All results for $R_K$ are consistent with Standard Model predictions. We also measure $CP$-averaged isospin asymmetries in the same $q^2$ bins. The results are consistent with a null asymmetry, with the largest difference of 2.6 standard deviations occurring for the $q^2\in (1.0,6.0)Ge{V}^2/c^4$ bin in the mode with muon final states. The measured differential branching fractions, $d\mathcal{B}/d{q}^2$, are consistent with theoretical predictions for charged $B$ decays, while the corresponding values are below the expectations for neutral $B$ decays. We have also searched for lepton-flavor-violating $B\to K{\mu }^{\pm }{e}^{\mp }$ decays and set $90\%$ confidence-level upper limits on the branching fraction in the range of $10^{-8}$ for $B^{+}\to K^{+}{\mu }^{\pm }{e}^{\mp }$, and $B^0\to K^0{\mu }^{\pm }{e}^{\mp }$ modes.

arXiv comment18 pages, 7 figures

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