Evidence for CP violation in time-integrated $D^0 \rightarrow h^-h^+$ decay rates

A search for time-integrated $CP$ violation in $D^0 \rightarrow h^-h^+$ ($h=K$, $\pi$) decays is presented using 0.62~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 $D^{*+} \rightarrow D^0 \pi^+$ and $D^{*-} \rightarrow \overline{D}^0 \pi^-$ decay chains. The difference in $CP$ asymmetry between $D^0 \rightarrow K^- K^+$ and $D^0 \rightarrow \pi^- \pi^+$, $\Delta A_{CP} \equiv A_{CP}(K^-K^+) \, - \, A_{CP}(\pi^-\pi^+)$, is measured to be $\left[ -0.82 \pm 0.21 (\mathrm{stat.}) \pm 0.11 (\mathrm{syst.}) \right]\%$. This differs from the hypothesis of $CP$ conservation by $3.5$ standard deviations.