Measurement of the $C P$ asymmetry in $B^+ \rightarrow K^+ \mu^+ \mu^-$ decays

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A measurement of the $C\P$ asymmetry in $B^+ \rightarrow K^+ \mu^+ \mu^-$ decays is presented using $pp$ collision data, corresponding to an integrated luminosity of 1.0${ fb}^{-1}$, recorded by the LHCb experiment during 2011 at a centre-of-mass energy of 7 TeV. The measurement is performed in seven bins of $\mu^+ \mu^-$ invariant mass squared in the range ${0.05<q^{2}<22.00{\mathrm{GeV^2/}c^4}}$, excluding the ${J/\psi}$ and $\psi{(2S)}$ resonance regions. Production and detection asymmetries are corrected for using the $B^+ \rightarrow J/\psi K^+$ decay as a control mode. Averaged over all the bins, the $C\P$ asymmetry is found to be ${{\cal A}_{C\P} = 0.000\pm 0.033{(stat.)} \pm0.005 {(syst.)} \pm 0.007{}(J/\psi K^+)}$, where the third uncertainty is due to the $C\P$ asymmetry of the control mode. This is consistent with the Standard Model prediction.

Figures and captions

Invariant mass distributions of $ { B ^+ \rightarrow K ^+ \mu ^+\mu ^- }$ candidates for the full $ q^2$ range. The results of the unbinned maximum likelihood fits are shown with blue, solid lines. Also shown are the signal component (red, short-dashed), the combinatorial background (grey, long-dashed), and the partially-reconstructed background (magenta, dot-dashed). The peaking backgrounds $ { B ^+ \rightarrow K ^+ \pi ^+ \pi ^- }$ (green, double-dot-dashed) and $ { B ^+ \rightarrow \pi ^+ \mu ^+\mu ^- }$ (teal, dotted) are also shown under the signal peak. The four datasets are (a) $ B ^+ $ and (b) $ B ^- $ for one magnet polarity, and (c) $ B ^+ $ and (d) $ B ^- $ for the other.

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Measured value of $ {\cal A}_{ C P }$ in $ { B ^+ \rightarrow K ^+ \mu ^+\mu ^- }$ decays in bins of the $\mu ^+\mu ^-$ invariant mass squared ( $ q^2$ ). The points are displayed at the mean value of $ q^2$ in each bin. The uncertainties on each $ {\cal A}_{ C P }$ value are the statistical and systematic uncertainties added in quadrature. The excluded charmonium regions are represented by the vertical red lines, the dashed line is the weighted average, and the grey band indicates the 1$\sigma$ uncertainty on the weighted average.

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Animated gif made out of all figures.


Tables and captions

Systematic uncertainties on $ {\cal A}_{ C P }$ from non-cancelling asymmetries arising from kinematic differences between $ { B ^+ \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} K ^+ }$ and $ { B ^+ \rightarrow K ^+ \mu ^+\mu ^- }$ decays, and fit uncertainties arising from the choice of signal shape, mass fit range and combinatorial background shape, and from the treatment of the asymmetries in the $ { B ^+ \rightarrow \pi ^+ \mu ^+\mu ^- }$ and partially-reconstructed (PR) backgrounds. The total is the sum in quadrature of each component.

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Values of $ {\cal A}_{ C P }$ and the signal yields in the seven $ q^2$ bins, the weighted average, and their associated uncertainties.

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Supplementary Material [file]

Supplementary material full pdf

Supple[..].pdf [132 KiB]

This ZIP file contains supplemetary material for the publication LHCb-PAPER-2013-043. The files are: Supplementary.pdf : An overview of the extra figures *.pdf, *.png, *.eps : The figures in various formats

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Created on 20 February 2021.