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Evidence for CP violation in time-integrated $D^0 \rightarrow h^-h^+$ decay rates

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Abstract

A search for time-integrated CP violation in D0 -> h-h+ (h=K, pi) decays is presented using 0.62 fb^-1 of data collected by LHCb in 2011. The flavor of the charm meson is determined by the charge of the slow pion in the D*+ -> D0 pi+ and D*- -> D0bar pi- decay chains. The difference in CP asymmetry between D0 -> K-K+ and D0 -> pi-pi+, Delta ACP = ACP(K-K+) - ACP(pi-pi+), is measured to be [-0.82 \pm 0.21(stat.) \pm 0.11(syst.)]. This differs from the hypothesis of CP conservation by 3.5 standard deviations.

Figures and captions

Fits to the (a) $m( K ^- K ^+ )$ and (b) $m(\pi ^- \pi ^+ )$ spectra of $ D ^{*+}$ candidates passing the selection and satisfying $0<\delta m<15$ MeV$/c^2$. The dashed line corresponds to the background component in the fit, and the vertical lines indicate the signal window of 1844--1884 MeV$/c^2$.

 fitMass.pdf [250 KiB]
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fitMass.pdf

Fits to the $\delta m$ spectra, where the $D^0$ is reconstructed in the final states (a) $K^- K^+$ and (b) $\pi^- \pi^+$, with mass lying in the window of 1844--1884 MeV$/c^2$. The dashed line corresponds to the background component in the fit.

 fitDel[..].pdf [224 KiB]
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fitDeltaMass.pdf

Time-dependence of the measurement. The data are divided into 19 disjoint, contiguous, time-ordered blocks and the value of $\Delta A_{ C P }$ measured in each block. The horizontal red dashed line shows the result for the combined sample. The vertical dashed line indicates the technical stop referred to in Table I.

 dacp_r[..].pdf [14 KiB]
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dacp_runblock.pdf

Animated gif made out of all figures.

 PAPER-2011-023.gif
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thumbnail_PAPER-2011-023.gif

Tables and captions

Values of $\Delta A_{ C P }$ measured in subsamples of the data, and the $\chi^2/\mathrm{ndf}$ and corresponding $\chi^2$ probabilities for internal consistency among the 27 bins in each subsample. The data are divided before and after a technical stop (TS), by magnet polarity (up, down), and by the sign of $p_x$ for the slow pion (left, right). The consistency among the eight subsamples is $\chi^2/\mathrm{ndf}=6.8/7$ (45%).

 Table_1.pdf [46 KiB]
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tex code 		Table_1.pdf

Summary of absolute systematic uncertainties for $\Delta A_{ C P }$.

 Table_2.pdf [17 KiB]
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tex code 		Table_2.pdf

Created on 27 April 2024.