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.