Search for CP violation in D0 → K+K− decays using D0 → K01 π+π− decays as calibration mode

A search for time-dependent violation of the charge-parity (CP) symmetry, A_CP(KK), in the D^0->K^+K^- decay is performed at the LHCb experiment using proton-proton collision data recorded from 2016 to 2018 at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 5.4 fb^-1.The D^0 meson is required to originate from a D^*+->D^0π+ decay, such that its flavor at production is identified by the charge of the accompanying pion.Nuisance asymmetries are subtracted using a sample of D^*+->D^0(->K^0_1π^+π^-)π^+ decays. The blinded asymmetry is measured to be A_CP(KK) = [xx +- 5.1 +- 1.5] x 10^-4, where the first uncertainty is statistical, and the second is systematic. This is the world's most precise measurement of this quantity to date. The combination of this measurement of A_CP(KK) with previous measurements of the same quantity, the time- integrated CP asymmetry difference, ΔA_CP = A_CP(KK) - A_CP(ππ), and the time-dependent CP asymmetry, ΔY, measured with D^0->K^+K^- and D^0->π^+π^- decays, the CP asymmetries in the D^0->K^+K?- and D^0->π^+π^- decays, a^d_KK and a^d_ππ, gives a^d_KK = [yy +- 3.3] x 10^-4, a^d_ππ = [zz +- 4.3] x 10^-4, where the uncertainties include systematic and statistical contributions. Under the hypothesis that the central value of the new measurement of A_CP(KK) presented here is equal to the world average, the measured value of a^d_ππ would show a departure from zero at the level of 5 standard deviations, providing the first observation of CPV in a single decay channel of a charmed meson.