Measurement of the time-integrated $CP$ asymmetry in $D^0\to K^{-}{K}^{+}$ decays

Abstract

The time-integrated $CP$ asymmetry in the Cabibbo-suppressed decay $D^0\to K^{-}{K}^{+}$ is measured using proton-proton collision data, corresponding to an integrated luminosity of 5.7 fb${}^{-1}$ collected at a center-of-mass energy of 13 TeV with the LHCb detector. The $D^0$ mesons are required to originate from promptly produced $D^{\ast +}\to D^0{\pi }^{+}$ decays and the charge of the companion pion is used to determine the flavor of the charm meson at production. The time-integrated $CP$ asymmetry is measured to be \begin{align*} \mathcal{A}_{C\!P}(K^-K^+) = \left[6.8 \pm 5.4 \text{ (stat)} \pm 1.6 \text{ (syst)} \right]\times 10^{-4}. \end{align*} The direct $CP$ asymmetries in $D^0\to K^{-}{K}^{+}$ and $D^0\to {\pi }^{-}{\pi }^{+}$ decays, $a_{K^{-}{K}^{+}}^d$ and $a_{{\pi }^{-}{\pi }^{+}}^d$, are derived by combining ${\mathcal{A}}_{CP}(K^{-}{K}^{+})$ with the time-integrated $CP$ asymmetry difference, $\Delta {\mathcal{A}}_{CP}={\mathcal{A}}_{CP}(K^{-}{K}^{+})-{\mathcal{A}}_{CP}({\pi }^{-}{\pi }^{+})$, giving \begin{alignat*}{2} a_{K^-K^+}^{d} &= (\phantom{2}7.7 \pm 5.7) \times 10^{-4}, \end{alignat*} \begin{alignat*}{2} a_{π^-π^+}^{d} &= (23.2 \pm 6.1) \times 10^{-4}, \end{alignat*} with a correlation coefficient corresponding to $\rho =0.88$. The compatibility of these results with $CP$ symmetry is 1.4 and 3.8 standard deviations for $D^0\to K^{-}{K}^{+}$ and $D^0\to {\pi }^{-}{\pi }^{+}$ decays, respectively. This is the first evidence for direct $CP$ violation in a specific $D^0$ decay.

arXiv commentAll figures and tables, along with any supplementary material and additional information, are available at https://cern.ch/lhcbproject/Publications/p/LHCb-PAPER-2022-024.html (LHCb public pages)

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