Observation of the Mass Difference between Neutral Charm-Meson Eigenstates

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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 D0→K0Sπ+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 xCP=[3.97±0.46(stat)±0.29(syst)]×10−3 and yCP=[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 D0 meson system, with a significance exceeding seven standard deviations. The data are consistent with CP symmetry and improve existing constraints on the associated parameters.

Original languageEnglish
Article number111801
Number of pages12
JournalPhysical Review Letters
Issue number11
Publication statusPublished - 7-Sep-2021

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