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 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.
Highlights
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
Neutral charm mesons propagating freely can change into their own antiparticles, as the mass eigenstates are linear combinations of the flavor eigenstates. These flavor-changing neutral currents do not occur at tree level in the standard model (SM) and allow for hypothetical particles of arbitrarily high mass to contribute significantly to the process
This can affect the mixing of mesons and antimesons such that measurements of these processes can probe physics beyond the SM [1,2,3,4]
Summary
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. The measured signal yields are corrected for two effects that do not cancel in the ratio: experimentally induced correlations between the phase space and decay time and charge-dependent efficiencies (detection asymmetries). The effect depends predominantly on the invariant mass of two pions from the D0 decay, with the efficiency to reconstruct the candidates at low values decreasing significantly at low D0 decay times This can bias the measured yield ratios and produce mixing-like trends. The kinematics of the pions produced in the D0 decay depend on the Dalitz-plot coordinate and D0 flavor This can result in asymmetric efficiency variations for D0 and D 0 candidates that imitate CP violation. They are included as constraints in the fit along with the associated covariance matrix ΔVasym describing uncertainties coming from the limited size of the calibration samples
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