Measurement of the Difference of Time-Integrated $CP$ Asymmetries in $D^0\to K^{-}{K}^{+}$ and $D^0\to {\pi }^{-}{\pi }^{+}$ Decays

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

Figure 1

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.

Figure 2

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