Observation of $CP$ violation in charm decays

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 6 ${\mathrm{fb}}^{-1}$ 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 $D^{\ast }(2010{)}^{+}\to D^0{\pi }^{+}$ decays or from the charge of the muon in $\overline{B}\to D^0{\mu }^{-}{\bar{\nu }}_{\mu }X$ decays. The difference between the $CP$ asymmetries in $D^0\to K^{-}{K}^{+}$ and $D^0\to {\pi }^{-}{\pi }^{+}$ decays is measured to be $\Delta A_{CP}=[-18.2\pm 3.2(stat.)\pm 0.9(syst.)]\times 10^{-4}$ for $\pi$-tagged and $\Delta A_{CP}=[-9\pm 8(stat.)\pm 5(syst.)]\times 10^{-4}$ for $\mu$-tagged $D^0$ mesons. Combining these with previous LHCb results leads to $$\Delta A_{CP}=(-15.4\pm 2.9)\times 10^{-4},$$ where the uncertainty includes both statistical and systematic contributions. The measured value differs from zero by more than five standard deviations. This is the first observation of $CP$ violation in the decay of charm hadrons.

All figures and tables, along with any supplementary material and additional information, are available at https://lhcbproject.web.cern.ch/lhcbproject/Publications/p/LHCb-PAPER-2019-006.html

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