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Measurement of mixing and CP violation parameters in two-body charm decays

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Abstract

A study of mixing and indirect CP violation in D0 mesons through the determination of the parameters y_CP and A_\Gamma is presented. The parameter y_CP is the deviation from unity of the ratio of effective lifetimes measured in D0 decays to the CP eigenstate K+K- with respect to decays to the Cabibbo favoured mode K-\pi+. The result measured using data collected by LHCb in 2010, corresponding to an integrated luminosity of 29 pb^-1, is y_CP = (5.5+/-6.3_{stat}+/-4.1_{syst}) x 10^-3. The parameter A_\Gamma is the asymmetry of effective lifetimes measured in decays of D0 and anti-D0 mesons to K+K-. The result is A_\Gamma = (-5.9+/-5.9_{stat}+/-2.1_{syst}) x 10^-3. A data-driven technique is used to correct for lifetime-biasing effects.

Figures and captions

Variation of the decay-time acceptance function for a two-body $ D ^0$ decay when moving the primary vertex along the $ D ^0$ momentum vector. The shaded, light blue regions show the bands for accepting a track impact parameter. While the impact parameter of the negative track (IP2) is too low in (a) it reaches the accepted range in (b). The actual measured decay time, $t_{\rm meas}$, lies in the accepted region which continues to larger decay times (c).

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$\Delta m$ vs $m_{ D ^0 }$ distribution for $ D ^0 \rightarrow K ^- \pi ^+ $ candidates. The contribution of random slow pions extends around the signal peak in the vertical direction while background is visible as a horizontal band.

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$\Delta m$ fit projections of (left) $ D ^0 \rightarrow K ^- \pi ^+ $ and (right) $ D ^0 \rightarrow K ^+ K ^- $ candidates. Shown are data (points), the total fit (green, solid) and the background component (blue, dot-dashed).

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$\ln\chi^2( {\rm IP}_ D )$ fit projection of $ D ^0 \rightarrow K ^+ K ^- $ candidates in logarithmic scale. Shown are data (points), the total fit (green, solid), the prompt signal (blue, short-dashed), and the secondary signal (purple, long-dashed).

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Proper-time fit projections of (left) $ D ^0 \rightarrow K ^+ K ^- $ and (right) $\overline{ D }{} ^0 \rightarrow K ^+ K ^- $ candidates after application of the $\ln\chi^2( {\rm IP}_ D )<2$ cut. Shown are data (points), the total fit (green, solid), the prompt signal (blue, short-dashed), and the secondary signal (purple, long-dashed).

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Animated gif made out of all figures.

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Tables and captions

Summary of systematic uncertainties.

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Created on 17 December 2018.Citation count from INSPIRE on 18 December 2018.