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, aK-K+d and aπ-π+d, 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 aK-K+d=(7.7±5.7)×10-4,aπ-π+d=(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.