Measurement of $CP$ asymmetry in $D^0\rightarrow K^-K^+$ decays

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A measurement of the time-integrated $CP$ asymmetry in the Cabibbo-suppressed decay $D^0\rightarrow K^-K^+$ is performed using pp collision data, corresponding to an integrated luminosity of 3fb$^{-1}$, collected with the LHCb detector at centre-of-mass energies of 7 and 8 TeV. The flavour of the charm meson at production is determined from the charge of the pion in $D^{*+}\rightarrow D^0 \pi^+$ and $D^{*-}\rightarrow \bar{D^0} \pi^-$ decays. The time-integrated $CP$ asymmetry $A_{CP}(K^-K^+)$ is obtained assuming negligible $CP$ violation in charm mixing and in Cabibbo-favoured $D^0\rightarrow K^-\pi^+$, $D^+\rightarrow K^-\pi^+\pi^+$ and $D^+\rightarrow \bar{K^0}\pi^+$ decays used as calibration channels. It is found to be $A_{CP}(K^-K^+)=(0.14\pm0.15(stat)\pm0.10(syst))\%$. A combination of this result with previous LHCb measurements yields $A_{CP}(K^-K^+)=(0.04\pm0.12(stat)\pm0.10(syst))\%$, $A_{CP}(\pi^-\pi^+)=(0.07\pm0.14(stat)\pm0.11(syst))\%$. These are the most precise measurements from a single experiment. The result for $A_{CP}(K^-K^+)$ is the most precise determination of a time-integrated $CP$ asymmetry in the charm sector to date, and neither measurement shows evidence of CP asymmetry.

Figures and captions

Fits to the $\delta m $ and to the $m( D ^+ )$ distributions corresponding to the whole data sample and both flavours. Data samples after the kinematic weighting described in the text are used.

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Measurements of $ C\!P$ violation asymmetries in $ D ^0$ $\rightarrow$ $ K ^-$ $ K ^+$ and $ D ^0$ $\rightarrow$ $\pi ^-$ $\pi ^+$ decays. Alongside the two LHCb measurements, presented in this Letter (green ellipse) and in Ref [18] (blue ellipse), and their combination (red ellipse), the latest value of the Heavy Flavour Averaging Group [1] is shown (black ellipse). The latter already includes the measurement of $\Delta A_{\mathcal{C}_{P} }$ with muon(pion)-tagged $ D ^0$ decays, using $3(1)\,$ $\,fb^{-1}$ $pp$ collision data collected with the LHCb detector [18,33]. The $68\%$ confidence level contours are displayed where the statistical and systematic uncertainties are added in quadrature.

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Animated gif made out of all figures.


Tables and captions

Signal yields of the four channels before and after the kinematic weighting. In the case of the weighted samples, effective yields are given.

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Measured asymmetries in $\%$ with their statistical uncertainties.

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Systematic uncertainties from the different categories. The quadratic sum is used to compute the total systematic uncertainty.

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Summary of the mean decay times of the $ D ^0 \rightarrow h^-h^+$ candidates used in the measurements of $\Delta A_{\mathcal{C}_{P} }$ and $ A_{\mathcal{C}_{P} }$ using prompt and semileptonic $ D ^0$ decays, and their combined values. The first uncertainty of the results is statistical, and the second one accounts for the systematics.

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Supplementary Material [file]

Supplementary material full pdf

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This ZIP file contains supplemetary material for the publication LHCb-PAPER-2016-035. The files are: supplementary.pdf : An overview of the extra figure *.pdf, *.png, *.eps, *.C : The figure in variuous formats

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Created on 11 December 2018.Citation count from INSPIRE on 11 December 2018.