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Measurements of charm mixing and $C P$ violation using $D^0 \to K^\pm \pi^\mp$ decays

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Abstract

Measurements of charm mixing and $C P$ violation parameters from the decay-time-dependent ratio of $ D^0 \to K^+ \pi^- $ to $ D^0 \to K^- \pi^+ $ decay rates and the charge-conjugate ratio are reported. The analysis uses $\overline{B}\to D^{*+} \mu^- X$, and charge-conjugate decays, where $D^{*+}\to D^0 \pi^+$, and $D^0\to K^{\mp} \pi^{\pm}$. The $pp$ collision data are recorded by the LHCb experiment at center-of-mass energies $\sqrt{s}$ = 7 and 8 TeV, corresponding to an integrated luminosity of 3 fb$^{-1}$. The data are analyzed under three hypotheses: (i) mixing assuming $C P$ symmetry, (ii) mixing assuming no direct $C P$ violation in the Cabibbo-favored or doubly Cabibbo-suppressed decay amplitudes, and (iii) mixing allowing either direct $C P$ violation and/or $C P$ violation in the superpositions of flavor eigenstates defining the mass eigenstates. The data are also combined with those from a previous LHCb study of $D^0\to K \pi$ decays from a disjoint set of $ D^{*+} $ candidates produced directly in $pp$ collisions. In all cases, the data are consistent with the hypothesis of $C P$ symmetry.

Figures and captions

The time-integrated $ D ^0 \pi^+_s$ invariant mass distributions, after same-sign subtraction, for (a) RS decays and (b) WS decays. Fit projections are overlaid. Below each plot are the normalized residual distributions.

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Efficiency corrected and SS background subtracted ratios of WS/RS decays and fit projections for the DT sample. The top plot shows the $ D ^0$ $(R^+(t))$ sample. The middle plot shows the $\overline{ D }{} {}^0$ $(R^-(t))$ sample. The bottom plot shows the difference between the top and middle plots. In all cases, the error bars superposed on the data points are those from the $ \chi^2 $ minimization fits with no accounting for additional systematic uncertainties. The projections shown are for fits assuming $ C P$ symmetry (solid blue line), allowing no direct $CPV$ (dashed-dotted green line), and allowing all forms of $CPV$ (dashed magenta line). Bins are centered at the average value of $t/\tau$ of the bin.

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Efficiency-corrected data and fit projections for the DT (red open circles) and prompt (black filled circles) samples. The top plot shows the $ D ^0$ $(R^+(t))$ samples. The middle plot shows the $\overline{ D }{} {}^0$ $(R^-(t))$ samples. The bottom plot shows the difference between the top and middle plots. In all cases, the error bars superposed on the data points are those from the $ \chi^2 $ minimization fits without accounting for additional systematic uncertainties. The projections shown are for fits assuming $ C P$ symmetry (solid blue line), allowing no direct $CPV$ (dashed-dotted green line), and allowing all forms of $CPV$ (dashed magenta line). Bins are centered at the average $t/\tau$ of the bin.

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Animated gif made out of all figures.

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Tables and captions

Summary of systematic uncertainties for the DT analysis for each of the three fits described in the text.

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Fitted parameters of the DT sample. The first uncertainties include the statistical uncertainty, as well as the peaking backgrounds and the $K\pi$ detection efficiency, and the second are systematic.

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Simultaneous fit result of the DT and prompt samples. The prompt-only results from [1] are shown on the right for comparison. Statistical and systematic errors have been added in quadrature.

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Systematic uncertainties for the simultaneous fits of the DT and prompt datasets.

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Correlation matrix for the no $CPV$ fit to the DT data.

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Correlation matrix for the no direct $CPV$ fit to the DT data.

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Correlation matrix for the all $CPV$ allowed fit to the DT data.

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Correlation matrix for the no $CPV$ simultaneous fit to the prompt + DT data sets.

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Correlation matrix for the no direct $CPV$ simultaneous fit to the prompt + DT data sets.

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Correlation matrix for the all $CPV$ allowed simultaneous fit to the prompt + DT data sets.

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Supplementary Material [file]

Supplementary material full pdf

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Created on 08 August 2020.