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Study of beauty hadron decays into pairs of charm hadrons

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Abstract

First observations of the decays $\Lambda_b^0 \to \Lambda_c^+ D_{(s)}^-$ are reported using data corresponding to an integrated luminosity of $3\,{\rm fb}^{-1}$ collected at 7 and 8 TeV center-of-mass energy in proton-proton collisions with the LHCb detector. In addition, the most precise measurement of the branching fraction ${\mathcal{B}(B_s^0 \to D^+D_s^-)}$ is made and a search is performed for the decays $B^0_{(s)} \to \Lambda_c^+ \Lambda_c^-$. The results obtained are \begin{eqnarray*} \mathcal{B}(\Lambda_b^0 \to \Lambda_c^+ D^-)/\mathcal{B}(\Lambda_b^0 \to \Lambda_c^+ D_{s}^-) &=& 0.042 \pm 0.003({\rm stat}) \pm 0.003({\rm syst}), \left[\frac{\mathcal{B}(\Lambda_b^0 \to \Lambda_c^+ D_{s}^-)}{\mathcal{B}({}\overline{B}_d^0 \to D^+D_s^-)}\right]\big/\left[\frac{\mathcal{B}(\Lambda_b^0 \to \Lambda_c^+\pi^-)}{\mathcal{B}({}\overline{B}_d^0 \to D^+\pi^-)}\right] &=& 0.96 \pm 0.02({\rm stat}) \pm 0.06({\rm syst}), \mathcal{B}(B_s^0 \to D^+D_s^-)/\mathcal{B}({}\overline{ B}_d^0 \to D^+D_s^-) &=& 0.038\pm0.004({\rm stat})\pm0.003({\rm syst}), \mathcal{B}({}\overline{B}^0 \to \Lambda_c^+ \Lambda_c^-)/\mathcal{B}({}\overline{B}_d^0 \to D^+D_s^-) & < & 0.0022\; [95\% \; {\rm C.L.}], \mathcal{B}(B^0_{s} \to \Lambda_c^+ \Lambda_c^-)/\mathcal{B}(B_s^0 \to D^+D_s^-) & < & 0.30\; [95\% \; {\rm C.L.}]. \end{eqnarray*} Measurement of the mass of the $\Lambda_b^0$ baryon relative to the $B^0$ meson gives ${M(\Lambda_b^0) -M(B^0) = 339.72\pm 0.24({\rm stat}) \pm 0.18({\rm syst})}$ MeV$/c^2$. This result provides the most precise measurement of the mass of the $\Lambda_b^0$ baryon to date.

Figures and captions

Invariant mass distributions for (left) $ {\it \Lambda}_b^0 \rightarrow {\it \Lambda}_c^+ D^-_s$ and (right) $ {\it \Lambda}_b^0 \rightarrow {\it \Lambda}_c^+ D^-$ candidates with the fits described in the text overlaid.

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Invariant mass distributions for $D^+D_s^-$ candidates selected using BDT criteria optimized for the (left) $ {}\overline{B}{}^0 \rightarrow D^+ D^-_s$ and (right) $ B_s^0 \rightarrow D^+ D^-_s$ decay modes with the fits described in the text overlaid.

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Efficiency-corrected yield ratio of $ {\it \Lambda}_b^0 \rightarrow {\it \Lambda}_c^+ D^-_s $ to $ {}\overline{B}{}^0 \rightarrow D^+ D^-_s $ vs $p_{\rm T}$ . The points are located at the mean $p_{\rm T}$ value of the $ {\it \Lambda}_b^0$ in each bin. The curve shows the data fit with the shape of the $p_{\rm T}$ dependence measured in Ref. [34].

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Animated gif made out of all figures.

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Tables and captions

Relative systematic uncertainties on branching fraction measurements (%). The production ratio $\sigma( B ^0_ s )/\sigma( {}\overline{B}{}^0 )$ is taken from Ref. [32]. Numbers in brackets in the last column are for the $B^0_s$ decay mode.

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Systematic uncertainties for $M( {\it \Lambda}_b^0 ) - M( {}\overline{B}{}^0 )$.

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Created on 09 December 2018.Citation count from INSPIRE on 18 December 2018.