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

Information

LHCb-PAPER-2016-063
CERN-EP-2017-028
arXiv:1702.06490 [PDF]

(Submitted on 21 Feb 2017)
Phys. Rev. Lett. 118 (2017) 261803
Inspire 1514549

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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 $\mathrm{U}(\mathrm{D})$.	Fig1a.pdf [13 KiB] HiDef png [277 KiB] Thumbnail [257 KiB] *.C file
	Fig1b.pdf [12 KiB] HiDef png [294 KiB] Thumbnail [278 KiB] *.C file
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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 $\theta_y$.	Fig2a.pdf [52 KiB] HiDef png [580 KiB] Thumbnail [508 KiB] *.C file
	Fig2b.pdf [58 KiB] HiDef png [955 KiB] Thumbnail [596 KiB] *.C file
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} $.	Fig3a.pdf [102 KiB] HiDef png [121 KiB] Thumbnail [98 KiB] *.C file
	Fig3b.pdf [103 KiB] HiDef png [123 KiB] Thumbnail [103 KiB] *.C file
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.	Fig4.png [30 KiB] HiDef png [223 KiB] Thumbnail [101 KiB] *.C file
Animated gif made out of all figures.	PAPER-2016-063.gif Thumbnail

Tables and captions

Signal yields in millions after all selection requirements.

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Supplementary Material [file]

Supplementary material full pdf	supple[..].pdf [3 MiB]
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