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

The time-integrated CP asymmetry in the Cabibbo-suppressed decay D-0 -> 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*(+) -> 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 A(CP) (K- K+) = [6.8 +/- 5.4 +/- 1.6] x 10(-4) where the first uncertainty is statistical and the second systematic. The direct CP asymmetries in D-0 -> K- K+ and D-0 -> pi(-) pi(+) decays, a(K-K+)(d) and a(pi-pi+)(d), are derived by combining A(CP) (K- K+) with the time-integrated CP asymmetry difference, Delta A(CP) = A(CP) (K- K+) - A(CP) (pi(-) pi(+)), and other inputs, giving a(K-K+)(d) = (7.7 +/- 5.7) x 10(-4); a(pi-pi+)(d) = (23.2 +/- 6.1) x 10(-4); 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 -> K- K+ and D-0 -> pi(-) pi(+) decays, respectively. This is the first evidence for direct CP violation in a specific D-0 decay.