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Observation of $C P$ violation in charm decays

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Abstract

A search for charge-parity ($C P$) 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 $D^*(2010)^+ \to D^0 \pi^+$ decays or from the charge of the muon in $\overline{B} \to D^0 \mu^-\bar{\nu}_\mu X$ decays. The difference between the $C P$ asymmetries in $D^0 \to K^- K^+$ and $D^0 \to \pi^- \pi^+$ decays is measured to be $\Delta A_{C P} = [ -18.2 \pm 3.2 (\rm stat.) \pm 0.9 (\rm syst.) ] \times 10^{-4}$ for $\pi$-tagged and $\Delta A_{C P} = [ -9 \pm 8 (\rm stat.) \pm 5 (\rm syst.) ] \times 10^{-4} $ for $\mu$-tagged $D^0$ mesons. Combining these with previous LHCb results leads to $$\Delta A_{C P} = ( -15.4 \pm 2.9) \times 10^{-4},$$ 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 $C P$ violation in the decay of charm hadrons.

Figures and captions

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

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Systematic uncertainties on $\Delta A_{ C P }$ for $\pi$- and $\mu$-tagged decays (in $10^{-4}$). The total uncertainties are obtained as the sums in quadrature of the individual contributions.

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Created on 25 May 2019.Citation count from INSPIRE on 25 May 2019.