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Measurement of $D^{0}-\bar{D}^0$ mixing parameters and search for CP violation using $D^{0} \to K^{+}\pi^{-}$ decays

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Abstract

Measurements of charm mixing parameters from the decay-time-dependent ratio of D0->K+pi- to D0->K-pi+ rates and the charge-conjugate ratio are reported. The analysis uses data, corresponding to 3 fb^{-1} of integrated luminosity, from proton-proton collisions at 7 and 8 TeV center-of-mass energies recorded by the LHCb experiment. In the limit of charge-parity (CP) symmetry, the mixing parameters are determined to be x'^2=(5.5 +- 4.9)x10^{-5}, y'= (4.8 +- 1.0)x10^{-3}, and R_D=(3.568 +- 0.066)x10^{-3}. Allowing for CP violation, the mixing parameters are determined separately for D0 and D0bar mesons yielding A_D = (-0.7 +- 1.9)%, for the direct CP-violating asymmetry, and 0.75 < |q/p|< 1.24 at the 68.3% confidence level, where q and p are parameters that describe the mass eigenstates of the neutral charm mesons in terms of the flavor eigenstates. This is the most precise determination of these parameters from a single experiment and shows no evidence for CP violation.

Figures and captions

Distribution of $ M( D ^0 \pi_{\rm s} ^+)$ for selected (a) right-sign $ D ^0 \rightarrow K^-\pi^+$ and (b) wrong-sign $ D ^0 \rightarrow K^+\pi^-$ candidates.

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Efficiency-corrected ratios of WS-to-RS yields for (a) $ D ^{*+}$ decays, (b) $ D ^{*-}$ decays, and (c) their differences as functions of decay time in units of $ D ^0$ lifetime. Projections of fits allowing for (dashed line) no $ C\!P$ violation, (dotted line) no direct $ C\!P$ violation, and (solid line) full $ C\!P$ violation are overlaid. The abscissa of the data points corresponds to the average decay time over the bin; the error bars indicate the statistical uncertainties.

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Two-dimensional confidence regions in the $(x'^2,y')$ plane obtained (a) without any restriction on $ C\!P$ violation, (b) assuming no direct $ C\!P$ violation, and (c) assuming $ C\!P$ conservation. The dashed (solid) curves in (a) and (b) indicate the contours of the mixing parameters associated with $\overline{ D }{} ^0$ ( $ D ^0$ ) decays. The best-fit value for $\overline{ D }{} ^0$ ( $ D ^0$ ) decays is shown with an open (filled) point. The solid, dashed, and dotted curves in (c) indicate the contours of $ C\!P$ -averaged mixing parameters at 68.3%, 95.5%, and 99.7% confidence level (CL), respectively. The best-fit value is shown with a point.

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Tables and captions

Results of fits to the data for different hypotheses on the $ C\!P$ symmetry. The reported uncertainties are statistical and systematic, respectively; ndf indicates the number of degrees of freedom. See App. A for fits results including correlation coefficients.

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Detailed fit results. Reported uncertainties and correlation coefficients include both statistical and systematic sources.

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Supplementary Material [file]

Supplementary material full pdf

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Created on 11 December 2018.Citation count from INSPIRE on 11 December 2018.