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Measurement of the difference of time-integrated CP asymmetries in $D^0 \rightarrow K^{-} K^{+} $ and $D^0 \rightarrow \pi^{-} \pi^{+} $ decays

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Abstract

A search for CP violation in $D^0 \rightarrow K^{-} K^{+} $ and $D^0 \rightarrow \pi^{-} \pi^{+} $ decays is performed using $pp$ collision data, corresponding to an integrated luminosity of $3 fb^{-1}$, collected using the LHCb detector at centre-of-mass energies of 7 and $8 $TeV. The flavour of the charm meson is inferred from the charge of the pion in $D^{*+}\rightarrow D^0\pi^+$ and $D^{*-}\rightarrow \bar{D^0}\pi^{-}$ decays. The difference between the CP asymmetries in $D^0 \rightarrow K^{-} K^{+} $ and $D^0 \rightarrow \pi^{-} \pi^{+} $ decays, $\Delta A_{CP} \equiv A_{CP}(K^{-} K^{+}) - A_{CP}(\pi^{-} \pi^{+})$, is measured to be $\left( -0.10 \pm 0.08(stat) \pm 0.03(syst) \right) \%$. This is the most precise measurement of a time-integrated CP asymmetry in the charm sector from a single experiment.

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Fit to the $\delta m$ spectra, where the $ D ^0$ is reconstructed in the final state (left) $ K ^- K ^+ $ and (right) $\pi ^- \pi ^+ $. The dashed line corresponds to the background component in the fit.

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Contour plot of $\Delta a_{ C P }^{\rm dir}$ versus $a_{ C P }^{\rm ind}$. The point at (0,0) denotes the hypothesis of no $ C P$ violation. The solid bands represent the measurements in Refs. [28,45,46] and the one reported in this Letter. The value of $y_ C P $ is taken from Ref. [47]. The contour lines shows the 68%, 95% and 99% confidence-level intervals from the combination.

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The raw asymmetry, $ A_{\mathrm{raw}}$ , in bins of ($p_z$, $p_x$) of the soft pion with the polarity of the magnet up (left) and down (right). The solid lines show the boundaries corresponding to the baseline selection. The dashed lines represent the edges of the selection used for systematic uncertainty estimation. Candidate $ D ^0 \rightarrow K ^- K ^+ $ decays are shown as an example; however a similar distribution is obtained for $ D ^0 \rightarrow \pi ^- \pi ^+ $ decays\footnotemark.

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The raw asymmetry, $ A_{\mathrm{raw}}$ , in bins of ($p_z$, $p_x$) of the soft pion with the up (left) and down (right) polarity of the magnetic field. The fiducial requirements (non-instrumented beam pipe region) are superimposed as solid black lines. Only candidates close to the beam pipe region are shown. Candidate $ D ^0 \rightarrow K ^- K ^+ $ decays are shown as an example; however a similar distribution is obtained for $ D ^0 \rightarrow \pi ^- \pi ^+ $ decays.

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Animated gif made out of all figures.

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Values of $\Delta A_{ C P }$ measured in the disjoint data subsamples, according to magnet polarity (up, down), centre-of-mass energy of data taking ($\sqrt{s} = 7$ and $8$ TeV ) and trigger category (nTOS, TOS).

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Supplementary Material [file]

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Created on 17 August 2019.Citation count from INSPIRE on 23 August 2019.