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Measurement of the $C P$ violation parameter $A_\Gamma$ in $D^0 \to K^+K^-$ and $D^0 \to \pi^+\pi^-$ decays

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Abstract

Asymmetries in the time-dependent rates of $D^0 \to K^+K^-$ and $D^0 \to \pi^+\pi^-$ decays are measured in a $pp$ collision data sample collected with the LHCb detector during LHC Run 1, corresponding to an integrated luminosity of $3 \mathrm{fb}^{-1}$. The asymmetries in effective decay widths between $D^0$ and $\overline{D}^0$ decays, sensitive to indirect $CP$ violation, are measured to be $A_\Gamma(K^+ K^-) = (-0.30 \pm 0.32 \pm 0.10)\times 10^{-3}$ and $A_\Gamma(\pi^+\pi^-) = (0.46 \pm 0.58 \pm 0.12)\times 10^{-3}$, where the first uncertainty is statistical and the second systematic. These measurements show no evidence for $C P$ violation and improve on the precision of the previous best measurements by nearly a factor of two.

Figures and captions

Results from $ A_{\Gamma} $ fits in each subsample, before (solid red squares) and after (empty black dots) the asymmetry correction. Fit qualities ($\chi^2 /$number of degrees of freedom) are also reported to the right of each graph. The weighted average of the four $ A_{\Gamma}$ values is indicated before (red hatched band) and after (black hatched band) the correction. The numerical values for the averages are $ A_{\Gamma} ( K ^- \pi ^+ ) =(0.41 \pm 0.10)\times 10^{-3}$, $ A_{\Gamma} ( K ^+ K ^- ) =(0.93 \pm 0.31)\times 10^{-3}$ and $ A_{\Gamma} (\pi ^+ \pi ^- ) =(1.77 \pm 0.57)\times 10^{-3}$ before the correction, and $ A_{\Gamma} ( K ^- \pi ^+ ) =(0.16 \pm 0.10)\times 10^{-3}$, $ A_{\Gamma} ( K ^+ K ^- ) =(-0.30 \pm 0.32)\times 10^{-3}$ and $ A_{\Gamma} (\pi ^+ \pi ^- ) =(0.46 \pm 0.58)\times 10^{-3}$ after the correction. The label 2011 (2012) is abbreviated 11 (12) and Mag Up ( MagDown ) is abbreviated $\mathrm{U}(\mathrm{D})$.

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(Left) Sum and (right) asymmetry of distributions of positive and negative soft pions in the $(k, q_{s}\theta_x)$ plane for the 2011 Mag Up $ D ^0 \rightarrow K ^- \pi ^+ $ subsample, after integration over $\theta_y$.

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Measured asymmetry $A(t)$ in bins of $t/\tau_D$, where $\tau_D = 0.410 {\mathrm{ ps}} $ [16], for (top) $ D ^0 \rightarrow K ^+ K ^- $ and (bottom) $ D ^0 \rightarrow \pi ^+ \pi ^- $ , averaged over the full Run 1 data sample. Solid lines show the time dependence with a slope equal to the best estimates of $- A_{\Gamma} $.

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Distribution of $\ln(\chi^2_{\text{IP}} ( D ^0 ))$ for the $ D ^0 \rightarrow K ^+ K ^- $ candidates selected in the second of the three 2012 data taking periods with magnetic field pointing downwards. The unbinned maximum likelihood fit results are overlaid. Gaussian kernels are used to smooth the combinatorial and partially reconstructed backgrounds.

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Animated gif made out of all figures.

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Tables and captions

Signal yields in millions after all selection requirements.

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Supplementary Material [file]

Supplementary material full pdf

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This ZIP file contains supplementary material for the publication LHCb-PAPER-2016-063. The files are: supplementary.pdf : An overview of the extra figures *.pdf, *.png, *.eps : The figures in various formats

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Created on 08 August 2020.