Measurement of the isospin asymmetry in $B\to K^{(\ast )}{\mu }^{+}{\mu }^{-}$ decays

Abstract

The isospin asymmetries of $B\to K^{(\ast )}{\mu }^{+}{\mu }^{-}$ decays and the partial branching fractions of $B^0\to K^0{\mu }^{+}{\mu }^{-}$ and $B^{+}\to K^{\ast +}{\mu }^{+}{\mu }^{-}$ are measured as a function of the di-muon mass squared $q^2$ using an integrated luminosity of 1.0 fb${}^{-1}$ collected with the LHCb detector. The $B\to K{\mu }^{+}{\mu }^{-}$ isospin asymmetry integrated over $q^2$ is negative, deviating from zero with over 4 $\sigma$ significance. The $B\to K^{\ast }{\mu }^{+}{\mu }^{-}$ decay measurements are consistent with the Standard Model prediction of negligible isospin asymmetry. The observation of the decay $B^0\to K_{\mathrm{S}}^0{\mu }^{+}{\mu }^{-}$ is reported with 5.7 $\sigma$ significance. Assuming that the branching fraction of $B^0\to K^0{\mu }^{+}{\mu }^{-}$ is twice that of $B^0\to K_{\mathrm{S}}^0{\mu }^{+}{\mu }^{-}$, the branching fractions of $B^0\to K^0{\mu }^{+}{\mu }^{-}$ and $B\to K^{\ast +}{\mu }^{+}{\mu }^{-}$ are found to be ($0.31_{-0.06}^{+0.07})\times 10^{-6}$ and ($1.16\pm 0.19)\times 10^{-6}$, respectively.

arXiv comment21 pages. Updated to correspond to proof from JHEP

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