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Updated determination of $D^0$-$\overline{D}{}^0$ mixing and CP violation parameters with $D^0\to K^+\pi^-$ decays

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Abstract

We report measurements of charm-mixing parameters based on the decay-time-dependent ratio of $D^0\to K^+\pi^-$ to $D^0\to K^-\pi^+$ rates. The analysis uses a data sample of proton-proton collisions corresponding to an integrated luminosity of $5.0$ fb$^{-1}$ recorded by the LHCb experiment from 2011 through 2016. Assuming charge-parity (CP) symmetry, the mixing parameters are determined to be $x'^2=(3.9 \pm 2.7) \times10^{-5}$, $y'=(5.28 \pm 0.52) \times 10^{-3}$, and $R_D=(3.454 \pm 0.031)\times10^{-3}$. Without this assumption, the measurement is performed separately for $D^0$ and $\overline{D}{}^0$ mesons, yielding a direct CP-violating asymmetry $A_D =(-0.1\pm9.1)\times10^{-3}$, and magnitude of the ratio of mixing parameters $1.00< |q/p| <1.35$ at the $68.3\%$ confidence level. All results include statistical and systematic uncertainties and improve significantly upon previous single-measurement determinations. No evidence for CP violation in charm mixing is observed.

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) direct and indirect $ C P$ violation are overlaid. The last two curves overlap. The abscissa of each data point 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 levels (CL), respectively, and the point indicates the best-fit value.

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Tables and captions

Results of fits for different $ C P$ -violation hypotheses. The first contribution to the uncertainties is statistical and the second systematic. Correlations include both statistical and systematic contributions.

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Supplementary Material [file]

This ZIP file contains supplemetary material for the publication LHCb-PAPER-2017-046. The files are: Supplementary-LHCb-PAPER-2017-046.pdf : An overview of the extra figures *.pdf, *.png, *.eps, *.C : The figures in various formats

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Created on 11 July 2020.