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First observations of $\bar{B}_s^0\to D^+D^-$, $D_s^+D^-$ and $D^0\bar{D}^0$ decays

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Abstract

First observations and measurements of the branching fractions of the $\bar{B}_s^0\to D^+D^-$, $\bar{B}_s^0\to D_s^+D^-$ and $\bar{B}_s^0\to D^0\bar{D}^0$ decays are presented using 1.0 fb$^{-1}$ of data collected by the LHCb experiment. These branching fractions are normalized to those of $\bar{B}^0\to D^+D^-$, $B^0\to D_s^+D^-$ and $B^-\to D^0D_s^-$, respectively. An excess of events consistent with the decay $\bar{B}^0\to D^0\bar{D}^0$ is also seen, and its branching fraction is measured relative to that of $B^-\to D^0D_s^-$. Improved measurements of the branching fractions ${\cal{B}}(\bar{B}_s^0\to D_s^+D_s^-)$ and ${\cal{B}}(B^-\to D^0D_s^-)$ are reported, each relative to ${\cal{B}}(B^0\to D_s^+D^-)$. The ratios of branching fractions are {-0.2in} {center} {align*} {{\cal{B}}(\bar{B}_s^0\to D^+D^-)\over {\cal{B}}(\bar{B}^0\to D^+D^-)} &= 1.08\pm 0.20\pm0.10, {{\cal{B}}(\bar{B}_s^0\to D_s^+D^-)\over {\cal{B}}(B^0\to D_s^+D^-)} &= 0.050\pm 0.008\pm0.004, {{\cal{B}}(\bar{B}_s^0\to D^0\bar{D}^0)\over {\cal{B}}(B^-\to D^0D_s^-)} &= 0.019\pm 0.003\pm0.003, {{\cal{B}}(\bar{B}^0\to D^0\bar{D}^0)\over {\cal{B}}(B^-\to D^0D_s^-)} &= 0.0014\pm 0.0006\pm0.0002,{{\cal{B}}(\bar{B}_s^0\to D_s^+D_s^-)\over {\cal{B}}(B^0\to D_s^+D^-)} &= 0.56\pm 0.03\pm0.04, {{\cal{B}}(B^-\to D^0D_s^-)\over {\cal{B}}(B^0\to D_s^+D^-)} &= 1.22\pm 0.02\pm0.07, {align*} {center} \noindent where the uncertainties are statistical and systematic, respectively.

Figures and captions

Feynman diagrams contributing to the double-charm final states discussed in this paper. They include (a) tree, (b) $W$-exchange and (c) penguin diagrams.

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Invariant mass distributions for (left) $\overline{ B }{} ^0_ s \rightarrow D ^+_ s D ^-_ s $ and (right) $ B ^0 \rightarrow D ^+_ s D ^- $ candidates in the data with the loose BDT selection applied to the latter. The signal and background components are indicated in the legend. The $\Lambda_b^0\rightarrow \Lambda _ c ^+ D ^-_ s $, $\overline{ B }{} ^0_ s \rightarrow D ^+_ s K ^- K ^+ \pi ^- $ and $ B ^0 \rightarrow D ^- K ^+ K ^- \pi ^+ $ background components are too small to be seen, and are excluded from the legends.

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Invariant mass distribution for $ B ^0 \rightarrow D ^+_ s D ^- $ and $\overline{ B }{} ^0_ s \rightarrow D ^+_ s D ^- $ candidates in the data, with the tight BDT selection applied. The distribution is plotted on a (left) linear and (right) logarithmic scale to highlight the suppressed $\overline{ B }{} ^0_ s \rightarrow D ^+_ s D ^- $ signal. Signal and background components are indicated in the legend.

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Invariant mass distributions for (left) $\overline{ B }{} ^0 _{(s)}\rightarrow D ^+ D ^- $ and (right) $\overline{ B }{} ^0 _{(s)}\rightarrow D ^0 \overline{ D }{} ^0 $ candidates in the data. Signal and background components are indicated in the legend.

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Invariant mass distribution for $ B ^- \rightarrow D ^0 D ^-_ s $ candidates in the data. Signal and background components are indicated in the legend. The $ B ^- \rightarrow D ^0 K ^- K ^+ \pi ^- $ background components are too small to be seen, and are excluded from the legend.

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Animated gif made out of all figures.

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Tables and captions

Individual contributions to the efficiency for selecting the various $B\rightarrow D\overline{ D }{} ^{\prime}$ final states. Shown are the efficiencies to reconstruct and trigger on the final state, and to pass the charm cross-feed veto, the VS $\chi^2$ and BDT selection requirements. The total selection efficiency is the product of these four values. The relative uncertainty on the selection efficiency for each decay mode due to the finite simulation samples sizes is 2%. Entries with a dash indicate that the efficiency factor is not applicable.

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Summary of the observed signal and normalization mode yields and their relative efficiencies, as used in the measurements of the ratios of branching fractions. The quoted uncertainties are statistical only.

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Sources of systematic uncertainty and their values (in %) for the ratios of branching fractions of the indicated decays. For $\cal{B}(\overline{ B }{} ^0 _{(s)}\rightarrow D ^0 \overline{ D }{} ^0 )/\cal{B}( B ^- \rightarrow D ^0 D ^-_ s )$, the error on $f_d/f_s$ only applies to the $\overline{ B }{} ^0_ s \rightarrow D ^0 \overline{ D }{} ^0 $ decay, as indicated by the values in parentheses.

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Created on 16 August 2019.Citation count from INSPIRE on 23 August 2019.