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

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Abstract

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.

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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]

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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.