# Measurement of $CP$ violation in $B^0\to D^{*\pm} D^{\mp}$ decays

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## Abstract

The decay-time-dependent $CP$ asymmetry in $B^0 \to D^{*\pm}D^{\mp}$ decays is measured using a data set corresponding to an integrated luminosity of $9$fb$^{-1}$ recorded by the LHCb detector in proton-proton collisions at centre-of-mass energies of 7, 8 and 13 TeV. The $CP$ parameters are measured as $S_{D^*D} = -0.861 \pm 0.077 \text{(stat)} \pm 0.019 \text{(syst)} ,$ $\Delta S_{D^*D} = 0.019 \pm 0.075 \text{(stat)} \pm 0.012 \text{(syst)} ,$ $C_{D^*D} = -0.059 \pm 0.092 \text{(stat)} \pm 0.020 \text{(syst)} ,$ $\Delta C_{D^*D} = -0.031 \pm 0.092 \text{(stat)} \pm 0.016 \text{(syst)} ,$ ${\cal A}_{D^*D} = 0.008 \pm 0.014 \text{(stat)} \pm 0.006 \text{(syst)} .$ The analysis provides the most precise single measurement of $CP$ violation in this decay channel to date. All parameters are consistent with their current world average values.

## Figures and captions

 Mass distributions for the $B ^0 \rightarrow D ^{*\pm} D ^\mp$ decay with (left) $D ^0 \rightarrow K ^- \pi ^- \pi ^+ \pi ^+$ and (right) $D ^0 \rightarrow K ^- \pi ^+$ for (top) Run 1 and (bottom) Run 2 data samples. Besides the data points and the full PDF (solid black) the projections of the $B ^0$ signal (dashed blue), the $B ^0_ s \rightarrow D ^{*\pm} D ^\mp$ background (dotted green), the $B ^0 \rightarrow D ^+_ s D ^{*-}$ background (dash-dotted turquoise), the $B ^0 \rightarrow D ^{*+} D ^{*-}$ background (long-dash-dotted magenta), the $B ^0_ s \rightarrow D ^{*+} D ^{*-}$ background (dash-three-dotted red) and the combinatorial background (long-dashed green) are shown. Fig1a.pdf [98 KiB] HiDef png [365 KiB] Thumbnail [318 KiB] *.C file Fig1b.pdf [95 KiB] HiDef png [330 KiB] Thumbnail [272 KiB] *.C file Fig1c.pdf [97 KiB] HiDef png [338 KiB] Thumbnail [280 KiB] *.C file Fig1d.pdf [97 KiB] HiDef png [318 KiB] Thumbnail [258 KiB] *.C file Decay-time distribution of the $B ^0 \rightarrow D ^{*\pm} D ^\mp$ signal candidates, summed over all data samples, where the background contribution is subtracted by means of the sPlot technique. The projection of the PDF is represented by the full line. Fig2.pdf [56 KiB] HiDef png [198 KiB] Thumbnail [188 KiB] *.C file Asymmetry between $\overline{ B } {}^0$ and $B ^0$ signal yields as a function of the decay time, for (blue full dot) $D ^{*+}$ $D ^-$ and (red empty dot) $D ^{*-}$ $D ^+$ signal candidates with a non null tagging decision. The background contribution is subtracted by means of the sPlot technique. The corresponding projections of the PDF are represented by the blue dashed (red continuous) line. The $B ^0$ flavour is determined by the combination of all flavour-tagging algorithms. Fig3.pdf [16 KiB] HiDef png [141 KiB] Thumbnail [117 KiB] *.C file Animated gif made out of all figures. PAPER-2019-036.gif Thumbnail

## Tables and captions

 Flavour-tagging parameters obtained as a weighted average of the values measured in the two control channels. The quoted uncertainties are statistical and systematic. Table_1.pdf [39 KiB] HiDef png [154 KiB] Thumbnail [64 KiB] tex code Tagging efficiency and tagging power for $B ^0 \rightarrow D ^{*\pm} D ^\mp$ signal candidates in the four data samples, computed using the event-by-event predicted mistag $\eta$ and the calibration parameters obtained from control channels. The quoted uncertainties are statistical only. Table_2.pdf [47 KiB] HiDef png [91 KiB] Thumbnail [46 KiB] tex code Instrumental asymmetries of $D ^\pm$ , $D ^{*\pm}$ and their combination for $B ^0 \rightarrow D ^{*\pm} D ^\mp$ decays obtained from prompt $D$ meson decays. The quoted uncertainties are statistical, in the last column statistical and systematic. Table_3.pdf [64 KiB] HiDef png [35 KiB] Thumbnail [17 KiB] tex code Summary of the systematic uncertainties. The total systematic uncertainties are computed as quadratic sum of individual contributions. Table_4.pdf [80 KiB] HiDef png [151 KiB] Thumbnail [67 KiB] tex code

## Supplementary Material [file]

 Supplementary material full pdf Supple[..].pdf [189 KiB]

Created on 21 February 2020.