Observation of $CP$ Violation in Charm Decays

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 charm meson is inferred from the charge of the pion in $D^{*}(2010{)}^{+}\to D^0{\pi }^{+}$ decays or from the charge of the muon in $\overline{B}\to D^0{\mu }^{-}{\overline{\nu }}_{\mu }X$ decays. The difference between the $CP$ asymmetries in $D^0\to K^{-}{K}^{+}$ and $D^0\to {\pi }^{-}{\pi }^{+}$ decays is measured to be $\mathrm{\Delta }{A}_{CP}=[-18.2\pm 3.2(\mathrm{stat})\pm 0.9(\mathrm{syst})]\times {10}^{-4}$ for $\pi$-tagged and $\mathrm{\Delta }{A}_{CP}=[-9\pm 8(\mathrm{stat})\pm 5(\mathrm{syst})]\times {10}^{-4}$ for $\mu$-tagged $D^0$ mesons. Combining these with previous LHCb results leads to $\mathrm{\Delta }{A}_{CP}=(-15.4\pm 2.9)\times {10}^{-4}$, where the uncertainty includes both statistical and systematic contributions. The measured value differs from zero by more than 5 standard deviations. This is the first observation of $CP$ violation in the decay of charm hadrons.

Figure 1

Mass distributions of selected (top) ${\pi }^{\pm }$-tagged and (bottom) ${\mu }^{\pm }$-tagged candidates for (left) $K^{-}{K}^{+}$ and (right) ${\pi }^{-}{\pi }^{+}$ final states of the $D^0$-meson decays, with fit projections overlaid.