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→𝐾0S⁢𝜋+⁢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 𝑥𝐶⁢𝑃=[3.97±0.46⁢(stat)±0.29⁢(syst)]×1⁢0−3 and 𝑦𝐶⁢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.