Measurement of the $CP$ violation parameter $A_\Gamma$ in $D^0\rightarrow K^+K^-$ and $D^0\rightarrow \pi^+\pi^-$ decays with LHCb Run 2 data

Time-dependent $CP$ asymmetries in the decay rates of the $D^0$ and $\overline{D}^0$ mesons into the $CP$-symmetric, singly Cabibbo-suppressed final states $K^+K^-$ and $\pi^+\pi^-$ are measured in $pp$ collisions collected by the LHCb experiment in 2015 and in 2016 at the centre-of-mass energy of $13\,\text{TeV}$, corresponding to an integrated luminosity of $1.7\,\text{fb}^{-1}$. The strong-interaction decay $D^{*+} \rightarrow D^0 \pi^+$ is used to infer the flavour of the $D^0$ meson at production. The asymmetry between the effective decay widths of the $D^0$ and $\overline{D}^0$ decays, sensitive to indirect $CP$ violation, is measured to be \begin{align*} A_\Gamma(D^0\rightarrow K^+K^-) &= (-60.9 \pm 3.8 \pm 0.6 )\cdot 10^{-4},\\ A_\Gamma(D^0\rightarrow \pi^+\pi^-) &= (-62.5 \pm 6.7 \pm 0.5 )\cdot 10^{-4}, \end{align*} where the first uncertainty is statistical, the second one systematic and the central values are blinded pending the receipt of the approval of the analysis procedure by the LHCb collaboration. The obtained statistical precision is comparable to the previous, world-leading measurement of $A_\Gamma$ by the LHCb collaboration, whereas the systematic uncertainty is reduced by a factor of $\approx2$.