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Measurement of indirect CP asymmetries in $D^0 \to K^-K^+$ and $D^0 \to \pi^-\pi^+$ decays

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Abstract

Time-dependent $CP$ asymmetries in the decay rates of the singly Cabibbo-suppressed decays $D^0\rightarrow K^-K^+$ and $D^0\rightarrow \pi^-\pi^+$ are measured in $pp$ collision data corresponding to an integrated luminosity of 3.0 fb$^{-1}$ collected by the LHCb experiment. The $D^0$ mesons are produced in semileptonic $b$-hadron decays, where the charge of the accompanying muon is used to determine the initial state as $D^0$ or $\bar{D}^0$. The asymmetries in effective lifetimes between $D^0$ and $\bar{D}^0$ decays, which are sensitive to indirect $CP$ violation, are determined to be \begin{align*} A_{\Gamma}(K^-K^+) = (-0.134 \pm 0.077 \; {}^{+0.026}_{-0.034})\% \, A_{\Gamma}(\pi^-\pi^+) = (-0.092\pm 0.145 \; {}^{+0.025}_{-0.033})\% \, \end{align*} where the first uncertainties are statistical and the second systematic. This result is in agreement with previous measurements and with the hypothesis of no indirect $CP$ violation in $D^0$ decays.

Figures and captions

Invariant mass distributions for (a) $ D ^0 \!\rightarrow K ^- K ^+ $ , (b) $ D ^0 \!\rightarrow \pi ^- \pi ^+ $ and (c) $ D ^0 \!\rightarrow K ^- \pi ^+ $ candidates. The results of the fits are overlaid. Underneath each plot the pull in each mass bin is shown, where the pull is defined as the difference between the data point and total fit, divided by the corresponding uncertainty.

massal[..].pdf [21 KiB]
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*.C file
massallfullselKK_combined.pdf
massal[..].pdf [21 KiB]
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massallfullselpipi_combined.pdf
massal[..].pdf [21 KiB]
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massallfullselKpi_combined.pdf

Raw $ C\!P$ asymmetry as function of $ D ^0$ decay time for (a) $ D ^0 \!\rightarrow K ^- K ^+ $ , (b) $ D ^0 \!\rightarrow \pi ^- \pi ^+ $ and (c) $ D ^0 \!\rightarrow K ^- \pi ^+ $ candidates. The results of the $\chi^2$ fits are shown as blue, solid lines with the $\pm1$ standard-deviation ($\sigma$) bands indicated by the dashed lines. The green, dashed lines indicate one $ D ^0$ lifetime ($\tau=410.1\rm \,fs $). Underneath each plot the pull in each time bin is shown.

Agamma[..].pdf [28 KiB]
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Agamma_ACP_AllKKSimfit.pdf
Agamma[..].pdf [28 KiB]
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Agamma_ACP_AllpipiSimfit.pdf
Agamma[..].pdf [27 KiB]
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Agamma_ACP_AllKpiSimfit.pdf

Mistag probability, before subtracting the contribution from wrong-sign (WS) decays, determined with $ D ^0 \!\rightarrow K ^- \pi ^+ $ candidates. The result of the fit to the data points with an exponential function is overlaid (solid, blue line). The red, dashed line indicates the expected mistag contribution from WS decays.

momega[..].pdf [21 KiB]
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momega_Kpiomega_fullrange.pdf

Measured values of $ A_{\Gamma}$ for different magnet polarities and data-taking periods for (a) $ D ^0 \!\rightarrow K ^- K ^+ $ , (b) $ D ^0 \!\rightarrow \pi ^- \pi ^+ $ and (c) $ D ^0 \!\rightarrow K ^- \pi ^+ $ decays. The vertical line and error band indicate the average $ A_{\Gamma}$ obtained from the combined data set. The error bars indicate the statistical uncertainty only.

polari[..].pdf [13 KiB]
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polarity_KK_check.pdf
polari[..].pdf [13 KiB]
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polarity_pipi_check.pdf
polari[..].pdf [13 KiB]
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polarity_Kpi_check.pdf

Animated gif made out of all figures.

PAPER-2014-069.gif
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thumbnail_PAPER-2014-069.gif

Tables and captions

Contributions to the systematic uncertainty of $ A_{\Gamma}( K ^- K ^+ )$ and $ A_{\Gamma}(\pi ^- \pi ^+ )$ . The constant and multiplicative scale uncertainties are given separately.

Table_1.pdf [47 KiB]
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tex code
Table_1.pdf

Supplementary Material [file]

Supplementary material full pdf

supple[..].pdf [118 KiB]
supplementary.pdf

This ZIP file contains supplemetary material for the publication LHCb-PAPER-2014-053. The files are: Supplementary.pdf : An overview of the extra figures *.pdf, *.png, *.eps, *.C : The figures in variuous formats

Fig1-S.pdf [14 KiB]
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*C file
Fig1-S.pdf

Created on 09 December 2018.Citation count from INSPIRE on 09 December 2018.