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

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Abstract

The isospin asymmetries of $B \to K^{(*)}\mu^+\mu^-$ decays and the partial branching fractions of $B^0 \to K^0\mu^+\mu^-$ and $B^+ \to K^{*+}\mu^+\mu^-$ 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 $B \to K^{*}\mu^+\mu^-$ decay measurements are consistent with the Standard Model prediction of negligible isospin asymmetry. The observation of the decay $B^0 \to K^0_{\rm\scriptscriptstyle S}\mu^+\mu^-$ is reported with 5.7 $\sigma$ significance. Assuming that the branching fraction of $B^0 \to K^0\mu^+\mu^-$ is twice that of $B^0 \to K^0_{\rm\scriptscriptstyle S}\mu^+\mu^-$, the branching fractions of $B^0 \to K^0\mu^+\mu^-$ and $B \to K^{*+}\mu^+\mu^-$ are found to be ($0.31^{+0.07}_{-0.06}) \times 10^{-6}$ and ($1.16\pm0.19) \times 10^{-6}$, respectively.

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Mass of the di-muon versus the mass of the $ B ^+ \!\rightarrow K ^+ \mu ^+ \mu ^- $ candidates. Only the di-muon mass region close to the $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ and $\psi {(2S)}$ masses is shown. The lines show the boundaries of the regions which are removed. Regions (a)--(c) are explained in the text.

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Mass distributions and fits of the signal channels integrated over the full $ q^{2}$ region. For the $ K ^0_{\rm\scriptscriptstyle S}$ channels, the plots are shown separately for the L and D $ K ^0_{\rm\scriptscriptstyle S}$ reconstruction categories, (a,b) and (c,d) respectively. The signal component is shown by the dashed line, the partially reconstructed component in 2(a) and 2(c) is shown by the dotted line while the solid line shows the entire fit model.

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Efficiency of the $ K ^0_{\rm\scriptscriptstyle S}$ channels with respect to the $ K ^+$ channels for (left) $ B \!\rightarrow K \mu ^+ \mu ^- $ and (right) $ B \!\rightarrow K^{*} \mu ^+ \mu ^- $ , calculated using the simulation. The efficiencies are shown for both L and D $ K ^0_{\rm\scriptscriptstyle S}$ reconstruction categories and include the visible branching fraction of $ K ^0 \rightarrow K ^0_{\rm\scriptscriptstyle S} \rightarrow \pi ^+ \pi ^- $. The error bars are not visible as they are smaller than the marker size.

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Differential branching fractions of (left) $ B ^0 \!\rightarrow K ^0 \mu ^+ \mu ^- $ and (right) $ B ^+ \!\rightarrow K^{*+} \mu ^+ \mu ^- $ . The theoretical SM predictions are taken from Refs. [23,24].

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Isospin asymmetry of (left) $ B \!\rightarrow K \mu ^+ \mu ^- $ and (right) $ B \!\rightarrow K^{*} \mu ^+ \mu ^- $ . For $ B \!\rightarrow K^{*} \mu ^+ \mu ^- $ the theoretical SM prediction, which is very close to zero, is shown for $ q^{2}$ below 8.68 $ {\mathrm{\,Ge V\!/}c^2}$ , from Ref. [25].

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Signal yields of the $ B \!\rightarrow K^{(*)} \mu ^+ \mu ^- $ decays. The upper bound of the highest $ q^{2}$ bin, $ q^{2} _{\mathrm{max}}$, is 19.3 $ {\mathrm{\,Ge V^2\!/}c^4}$ and 23.0 $ {\mathrm{\,Ge V^2\!/}c^4}$ for $ B \!\rightarrow K^{*} \mu ^+ \mu ^- $ and $ B \!\rightarrow K \mu ^+ \mu ^- $ , respectively.

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Partial branching fractions of $ B ^0 \!\rightarrow K ^0 \mu ^+ \mu ^- $ and isospin asymmetries of $ B \!\rightarrow K \mu ^+ \mu ^- $ decays. The significance of the deviation of $ A_{\rm I}$ from zero is shown in the last column. The errors include the statistical and systematic uncertainties.

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Partial branching fractions of $ B ^+ \!\rightarrow K^{*+} \mu ^+ \mu ^- $ and isospin asymmetries of $ B \!\rightarrow K^{*} \mu ^+ \mu ^- $ decays. The significance of the deviation of $ A_{\rm I}$ from zero is shown in the last column. The errors include the statistical and systematic uncertainties.

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Supplementary Material [file]

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Created on 20 April 2019.Citation count from INSPIRE on 25 April 2019.