Measurement of $CP$ violation in $B^0 \rightarrow D^+ D^-$ decays

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The $CP$ violation observables $S$ and $C$ in the decay channel $B^0 \rightarrow D^+ D^-$ are determined from a sample of proton-proton collisions at center-of-mass energies of 7 and 8 TeV, collected by the LHCb experiment and corresponding to an integrated luminosity of 3 fb$^{-1}$. The observable $S$ describes $CP$ violation in the interference between mixing and the decay amplitude, and $C$ parametrizes direct $CP$ violation in the decay. The following values are obtained from a flavor-tagged, decay-time-dependent analysis: \begin{align*} S &= -0.54 ^{+0.17}_{-0.16} \text{(stat)} \pm 0.05 \text{(syst)} ,\newline C &= \phantom{-}0.26 ^{+0.18}_{-0.17} \text{(stat)} \pm 0.02 \text{(syst)} . \end{align*} These values provide evidence for $CP$ violation at a significance level of 4.0 standard deviations. The phase shift due to higher-order Standard Model corrections is constrained to a small value of \begin{align*} \Delta\phi = -0.16 ^{+0.19}_{-0.21} \text{rad} . \end{align*}

Figures and captions

Distribution of the reconstructed mass of all $B^0 \rightarrow D^+ D^-$ candidates (a) and background-subtracted decay time distribution for tagged candidates (b). In plot (a) besides the data points and the projection of the full PDF (solid black) the projections of the $B^0$ signal (dashed blue), the $B^0_s \rightarrow D^+D^-$ background (short-dash-dotted turquoise), the $B^0 \rightarrow D^+_s D^-$ background (dotted green), the $B^0_s \rightarrow D^-_s D^+$ background (long-dash-three-dotted red) and the combinatorial background (long-dash-dotted purple) are shown.

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Decay-time-dependent signal yield asymmetry. The solid curve is the projection of the signal PDF given in \cref{eq:fullpdf}.

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Masses of $B^0 \rightarrow D^+_s D^-$ candidates and projected PDFs, shown with a linear scale on the vertical axis (a) and a logarithmic scale (b). The solid line is the PDF projection, the blue dashed line represents the $B^0 \rightarrow D^+_s D^-$ component, while the dash-dotted cyan (dotted green) line represents the combinatorial ($B^0_s \rightarrow D^-_s D^+$) background.

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Raw mixing asymmetry as a function of the $B^0$ decay time for events tagged by (a) the OS tagger and (b) the SS tagger. The solid line represents the PDF projection.

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Animated gif made out of all figures.


Created on 14 January 2021.