Measurement of the Time-Integrated 𝐶⁢𝑃 Asymmetry in 𝐷0→𝐾−⁢𝐾+ Decays

The time-integrated 𝐶⁢𝑃 asymmetry in the Cabibbo-suppressed decay 𝐷0→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 𝐷0 mesons are required to originate from promptly produced 𝐷*+→𝐷0⁢𝜋+ decays, and the charge of the companion pion is used to determine the flavor of the charm meson at production. The time-integrated 𝐶⁢𝑃 asymmetry is measured to be 𝒜𝐶⁢𝑃⁡(𝐾−⁢𝐾+)=[6.8±5.4±1.6]×10−4 where the first uncertainty is statistical and the second systematic. The direct 𝐶⁢𝑃 asymmetries in 𝐷0→𝐾−⁢𝐾+ and 𝐷0→𝜋−⁢𝜋+ decays, 𝑎𝑑𝐾−⁢𝐾+ and 𝑎𝑑𝜋−⁢𝜋+, are derived by combining 𝒜𝐶⁢𝑃⁡(𝐾−⁢𝐾+) with the time-integrated 𝐶⁢𝑃 asymmetry difference, Δ⁢𝐴𝐶⁢𝑃=𝒜𝐶⁢𝑃⁡(𝐾−⁢𝐾+)−𝒜𝐶⁢𝑃⁡(𝜋−⁢𝜋+), and other inputs, giving 𝑎𝑑𝐾−⁢𝐾+=(7.7±5.7)×10−4,𝑎𝑑𝜋−⁢𝜋+=(23.2±6.1)×10−4,with a correlation coefficient corresponding to 𝜌=0.88. The compatibility of these results with 𝐶⁢𝑃 symmetry is 1.4 and 3.8 standard deviations for 𝐷0→𝐾−⁢𝐾+ and 𝐷0→𝜋−⁢𝜋+ decays, respectively. This is the first evidence for direct 𝐶⁢𝑃 violation in a specific 𝐷0 decay.