# Observation of the $\varXi^{-}_{b}\to J/\psi\varLambda K^{-}$ decay

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## Abstract

The observation of the decay $\varXi^{-}_{b}\to J/\psi\varLambda K^{-}$ is reported, using a data sample corresponding to an integrated luminosity of $3 \mathrm{fb}^{-1}$, collected by the LHCb detector in $pp$ collisions at centre-of-mass energies of $7$ and $8 \mathrm{TeV}$. The production rate of $\varXi_{b}^{-}$ baryons detected in the decay $\varXi_{b}^{-}\to J/\psi\varLambda K^{-}$ is measured relative to that of $\varLambda_{b}^{0}$ baryons using the decay $\varLambda_{b}^{0}\to J/\psi \varLambda$. Integrated over the $b$-baryon transverse momentum $p_{\rm T}<25 \mathrm{GeV/}c$ and rapidity $2.0<y<4.5$, the measured ratio is \begin{equation*} \frac{f_{\varXi_{b}^{-}}}{f_{\varLambda_{b}^{0}}}\frac{\mathcal{B}(\varXi_{b}^{-}\to J/\psi\varLambda K^{-})}{\mathcal{B}(\varLambda_{b}^{0}\to J/\psi \varLambda)}=(4.19\pm 0.29 (\mathrm{stat})\pm0.15 (\mathrm{syst}))\times 10^{-2}, \end{equation*}where $f_{\varXi_{b}^{-}}$ and $f_{\varLambda_{b}^{0}}$ are the fragmentation fractions of $b\to\varXi_{b}^{-}$ and $b\to\varLambda_{b}^{0}$ transitions, and $\mathcal{B}$ represents the branching fraction of the corresponding $b$-baryon decay. The mass difference between $\varXi_{b}^{-}$ and $\varLambda_{b}^{0}$ baryons is measured to be \begin{equation*} M(\varXi_{b}^{-})-M(\varLambda_{b}^{0})=177.08\pm0.47 (\mathrm{stat})\pm0.16 (\mathrm{syst} ) \mathrm{MeV/}c^{2}. \end{equation*}

## Figures and captions

 Feynman diagrams $\Xi ^-_ b$ and $\Lambda ^0_ b$ decays. Diagram (a) contributes to both $\Lambda ^0_ b \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} p K ^-$ decays and $\Xi ^-_ b \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \Lambda K ^-$ decays, diagram (b) contributes only to the $\Xi ^-_ b$ decay. Fig1.pdf [44 KiB] HiDef png [25 KiB] Thumbnail [4 KiB] *.C file Reconstructed (left) $\Xi ^-_ b \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \Lambda K ^-$ and (right) $\Lambda ^0_ b \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \Lambda$ candidates using (top) LL and (bottom) DD $\Lambda$ types. The solid (blue) lines show the full fit functions, the dashed (red) lines the signal components, the dot-dashed (purple) lines the $\Xi ^-_ b \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \Sigma^{0} K ^-$ background and the dotted (black) lines the combinatorial background. At the bottom of each figure the differences between the data and the fit divided by the uncertainty in the data are shown. Fig2a.pdf [38 KiB] HiDef png [531 KiB] Thumbnail [192 KiB] *.C file Fig2b.pdf [37 KiB] HiDef png [402 KiB] Thumbnail [158 KiB] *.C file Animated gif made out of all figures. PAPER-2016-053.gif Thumbnail

## Tables and captions

 Relative systematic uncertainty for the ratio $R_{\Xi ^-_ b /\Lambda ^0_ b }$. Table_1.pdf [47 KiB] HiDef png [117 KiB] Thumbnail [17 KiB] tex code

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