Evidence for $CP$ Violation in Time-Integrated $D^0\to h^{-}{h}^{+}$ Decay Rates

A search for time-integrated $CP$ violation in $D^0\to h^{-}{h}^{+}$ ($h=K$, $\pi$) decays is presented using $0.62{\mathrm{fb}}^{-1}$ of data collected by LHCb in 2011. The flavor of the charm meson is determined by the charge of the slow pion in the $D^{*+}\to D^0{\pi }^{+}$ and $D^{*-}\to {\overline{D}}^0{\pi }^{-}$ decay chains. The difference in $CP$ asymmetry between $D^0\to K^{-}{K}^{+}$ and $D^0\to {\pi }^{-}{\pi }^{+}$, $\Delta A_{CP}\equiv A_{CP}(K^{-}{K}^{+})-A_{CP}({\pi }^{-}{\pi }^{+})$, is measured to be $[-0.82\pm 0.21(\mathrm{stat})\pm 0.11(\mathrm{syst})]\%$. This differs from the hypothesis of $CP$ conservation by 3.5 standard deviations.

Figure 1

Fits to the (a) $m(K^{-}{K}^{+})$ and (b) $m({\pi }^{-}{\pi }^{+})$ spectra of $D^{*+}$ candidates passing the selection and satisfying $0<\delta m<15\mathrm{MeV}/c^2$. The dashed line corresponds to the background component in the fit, and the vertical lines indicate the signal window of $1844–1884\mathrm{MeV}/c^2$.

Figure 2

Fits to the $\delta m$ spectra, where the $D^0$ is reconstructed in the final states (a) $K^{-}{K}^{+}$ and (b) ${\pi }^{-}{\pi }^{+}$, with mass lying in the window of $1844–1884\mathrm{MeV}/c^2$. The dashed line corresponds to the background component in the fit.

Figure 3

Time dependence of the measurement. The data are divided into 19 disjoint, contiguous, time-ordered blocks and the value of $\Delta A_{CP}$ measured in each block. The horizontal red dashed line shows the result for the combined sample. The vertical dashed line indicates the technical stop referred to in Table 1.