Observation of the mass difference between neutral charm-meson eigenstates

Abstract

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${\text{fb}}^{-1}$. A total of $30.6$ million $D^0\to K_{\mathrm{S}}^0{\pi }^{+}{\pi }^{-}$ 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 $x_{CP}=\left[3.97\pm 0.46\text{(stat)}\pm 0.29\text{(syst)}\right]\times 10^{-3}$ and $y_{CP}=\left[4.59\pm 1.20\text{(stat)}\pm 0.85\text{(syst)}\right]\times 10^{-3}$, respectively. The $CP$-violating parameters are measured as $\Delta x=\left[-0.27\pm 0.18\text{(stat)}\pm 0.01\text{(syst)}\right]\times 10^{-3}$ and $\Delta y=\left[0.20\pm 0.36\text{(stat)}\pm 0.13\text{(syst)}\right]\times 10^{-3}$. 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.

All figures and tables, along with machine-readable versions and any supplementary material and additional information, are available at https://cern.ch/lhcbproject/Publications/p/LHCb-PAPER-2021-009.html (LHCb public pages). Updated to correct sign of reported value of phi

References in corpus (3)

• Implications of D^0-\bar D^0 Mixing for New Physics
• Measurement of the mass difference between neutral charm-meson eigenstates
• Measurement of $D^0$-$\overline{D}{}^0$ Mixing and Search for Indirect $CP$ Violation Using $D^0\to K_S^0{\pi }^{+}{\pi }^{-}$ Decays

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