Measurement of the Time-Integrated CP 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^*+ → 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 A_CP (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, a^d_K−K+ and a^d_π−π+, are derived by combining A_CP(K⁻K⁺) with the time-integrated CP asymmetry difference, ΔA_CP = A_CP(K⁻K⁺) − A_CP(π⁻π⁺), and other inputs, giving a^d_K−K+ = (7.7 ± 5.7) × 10⁻⁴, a^d_π−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.