Measurement of the Time-Integrated CP Asymmetry in D0→K−K+ Decays

The time-integrated CP 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*+→D0π+ 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 ACP(K−K+)=[6.8±5.4±1.6]×10−4 where the first uncertainty is statistical and the second systematic. The direct CP asymmetries in D0→K−K+ and D0→π−π+ decays, adK−K+ and adπ−π+, are derived by combining ACP(K−K+) with the time-integrated CP asymmetry difference, ΔACP=ACP(K−K+)−ACP(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 CP 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 CP violation in a specific D0 decay.