# Observation of $CP$ Violation in Charm Decays

29 May 2019

Abstract: 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 $5.9\text{}\text{}{\mathrm{fb}}^{-1}$ 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 ${D}^{*}\left(2010{\right)}^{+}\to {D}^{0}{\pi }^{+}$ 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 $\Delta {A}_{CP}=\left[-18.2±3.2\left(\mathrm{stat}\right)±0.9\left(\mathrm{syst}\right)\right]×{10}^{-4}$ for $\pi$-tagged and $\Delta {A}_{CP}=\left[-9±8\left(\mathrm{stat}\right)±5\left(\mathrm{syst}\right)\right]×{10}^{-4}$ for $\mu$-tagged ${D}^{0}$ mesons. Combining these with previous LHCb results leads to $\Delta {A}_{CP}=\left(-15.4±2.9\right)×{10}^{-4}$, 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.

Published in: Physical Review Letters 122 (2019)