# 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. Fig1a.pdf [26 KiB] HiDef png [207 KiB] Thumbnail [63 KiB] *.C file Fig1b.pdf [26 KiB] HiDef png [197 KiB] Thumbnail [64 KiB] *.C file Fig1c.pdf [26 KiB] HiDef png [213 KiB] Thumbnail [69 KiB] *.C file Fig1d.pdf [26 KiB] HiDef png [232 KiB] Thumbnail [72 KiB] *.C file 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 $\Delta A_{ C\!P }$ 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. Fig2.pdf [21 KiB] HiDef png [1018 KiB] Thumbnail [141 KiB] *.C file Animated gif made out of all figures. PAPER-2016-035.gif Thumbnail

## 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. Table_1.pdf [51 KiB] HiDef png [52 KiB] Thumbnail [9 KiB] tex code Measured asymmetries in $\%$ with their statistical uncertainties. Table_2.pdf [63 KiB] HiDef png [42 KiB] Thumbnail [5 KiB] tex code Systematic uncertainties from the different categories. The quadratic sum is used to compute the total systematic uncertainty. Table_3.pdf [37 KiB] HiDef png [91 KiB] Thumbnail [13 KiB] tex code Summary of the mean decay times of the $D ^0 \rightarrow h^-h^+$ candidates used in the measurements of $\Delta A_{ C\!P }$ and $A_{ C\!P }$ 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. Table_4.pdf [65 KiB] HiDef png [109 KiB] Thumbnail [18 KiB] tex code

## Supplementary Material [file]

 Supplementary material full pdf supple[..].pdf [138 KiB] 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 Fig3.pdf [53 KiB] HiDef png [334 KiB] Thumbnail [91 KiB] *C file Fig4.pdf [14 KiB] HiDef png [82 KiB] Thumbnail [38 KiB] *C file

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