Observation of the Mass Difference Between Neutral Charm-Meson Eigenstates

A measurement of mixing and CP 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 $D^0 → K_S^0 π^+ π^-$ decays are analyzed using a method optimized for the measurement of the mass difference between neutral charm-meson eigenstates. Allowing for CP violation in mixing and in the interference between mixing and decay, the mass and decay-width differences are measured to be $x_{CP}$=[3.97±0.46(stat)±0.29(syst)]×10$^{-3}$ and $y_{CP}$=[4.59±1.20(stat)±0.85(syst)]×10$^{-3}$, respectively. The CP-violating parameters are measured as $Δx$=[-0.27±0.18(stat)±0.01(syst)]×10$^{-3}$ and $Δy$=[0.20±0.36(stat)±0.13(syst)]×10$^{-3}$. This is the first observation of a nonzero mass difference in the $D^0$ meson system, with a significance exceeding seven standard deviations. The data are consistent with CP symmetry and improve existing constraints on the associated parameters