Measurement of the CP violation parameter $A_{\Gamma}$ in $D^0 \to K^+K^-$ and $D^0 \to \pi^+\pi^-$ decays

Asymmetries in the time-dependent rates of $D^0 \to K^+K^−$ and $D^0 \to \pi^+\pi^-$ decays are measured in a pp collision data sample collected with the LHCb detector during LHC Run 1, corresponding to an integrated luminosity of 3 $fb^{−1}$. The asymmetries in effective decay widths between $D^0$ and $\overline{D}^0$ decays, sensitive to indirect CP violation, are measured to be $A_{\Gamma}(K^+K^−) = (−0.30 \pm 0.32 \pm 0.10) \times 10^{−3}$ and $A_{\Gamma}(\pi^+\pi−) = (0.46 \pm 0.58 \pm 0.12) \times 10^{−3}$, where the first uncertainty is statistical and the second systematic. These measurements show no evidence for CP violation and improve on the precision of the previous best measurements by nearly a factor of two