Observation of the Mass Difference Between Neutral Charm-Meson Eigenstates

A measurement of mixing and C P violation in neutral charm mesons is performed using data reconstructed in proton-proton collisions collected by the LHCb experiment from 2016 to 2018, corresponding to an integrated luminosity of 5.4 fb^-1 . A total of 30.6 million D⁰→K_S⁰π⁺π^- decays are analyzed using a method optimized for the measurement of the mass difference between neutral charm-meson eigenstates. Allowing for C P violation in mixing and in the interference between mixing and decay, the mass and decay-width differences are measured to be x_{C P}=[3.97 ±0.46 (stat)±0.29 (syst)] ×1 0^-3 and y_{C P}=[4.59 ±1.20 (stat)±0.85 (syst)] ×1 0^-3, respectively. The C P -violating parameters are measured as Δ x =[-0.27 ±0.18 (stat)±0.01 (syst)] ×1 0^-3 and Δ y =[0.20 ±0.36 (stat)±0.13 (syst)] ×1 0^-3. This is the first observation of a nonzero mass difference in the D⁰ meson system, with a significance exceeding seven standard deviations. The data are consistent with C P symmetry and improve existing constraints on the associated parameters.