Observation of $CP$ violation in charm decays at LHCb

A search for charge-parity ($CP$) violation in $D^0 \to K^- K^+$ and $D^0 \to \pi^-\pi^+$ decays is reported, using $pp$ collision data corresponding to an integrated luminosity of 5.9 $\mathrm{fb}^{-1}$ collected at a center-of-mass energy of 13 TeV with the LHCb detector. The flavor of the $D^0$ meson is determined from the charge of the pion in $D^*(2010)^+ \to D^0 \pi^+$ decays or from the charge of the muon in $\kern 0.18em\overline{\kern -0.18em B}{} \to D^0 \mu^- \bar{\nu}_\mu X$ decays. The difference between the $CP$ asymmetries in $D^0 \to K^- K^+$ and $D^0 \to \pi^-\pi^+$ decays is measured to be $\Delta A_{\rm CP} = [ -18.2 \pm 3.2\,(\rm stat.) \pm 0.9\,(\rm syst.) ] \times 10^{-4}$ for $\pi$-tagged and $\Delta A_{\rm CP} = [ -9 \pm 8\,(\rm stat.) \pm 5\,(\rm syst.) ] \times 10^{-4} $ for $\mu$-tagged $D^0$ mesons. The combination with previous LHCb results leads to $$\Delta A_{\rm CP} = ( -15.4 \pm 2.9) \times 10^{-4},$$ where the uncertainty includes both statistical and systematic contributions. The measured value differs from zero by more than five standard deviations. This is the first observation of $CP$ violation in the decay of charm hadrons.