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Abstract

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 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_K−K+^d and a_π−π+^d, 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_K−K+^d=(7.7±5.7)×10⁻⁴,a_π−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.