Decays of the $\Xi_b^$ and $\Omega_b^$ baryons to the charmless final states $p h^{}h^{\prime }$, where $h^{(\prime)}$ denotes a kaon or pion, are searched for with the LHCb detector. The analysis is based on a sample of protonproton collision data collected at centreofmass energies $\sqrt{s} = 7$ and $8 TeV$, corresponding to an integrated luminosity of $3 {\rm fb}^{1}$. The decay $\Xi_b^ \to p K^K^$ is observed with a significance of $8.7$ standard deviations, and evidence at the level of $3.4$ standard deviations is found for the $\Xi_b^ \to p K^\pi^$ decay. Results are reported, relative to the $B^\to K^+K^K^$ normalisation channel, for the products of branching fractions and $b$hadron production fractions. The branching fractions of $\Xi_b^ \to p K^\pi^$ and $\Xi_b^ \to p \pi^\pi^$ relative to $\Xi_b^ \to p K^K^$ decays are also measured.
(Left) tree and (right) loop diagrams for the $\Xi ^_ b \rightarrow p K ^ K ^ $ decay channel. 
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Mass distributions for $ b $ hadron candidates in the (top left) $ p K ^ K ^ $, (top right) $ p K ^ \pi ^ $, (bottom left) $ p \pi ^ \pi ^ $ and (bottom right) $ K ^+ K ^ K ^ $ final states. Results of the fits are shown with dark blue solid lines. Signals for $\Xi ^_ b $ and $ B ^ $ ($\POmega ^_ b $) decays are shown with pink (light green) dashed lines, combinatorial backgrounds are shown with grey longdashed lines, crossfeed backgrounds are shown with red dotdashed lines, and partially reconstructed backgrounds are shown with dark blue doubledotdashed lines. 
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Efficiencycorrected and backgroundsubtracted [38] $m( p K ^ )_{\rm min}$ distribution from $\Xi ^_ b \rightarrow p K ^ K ^ $ candidates. 
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Animated gif made out of all figures. 
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Fitted yields, efficiencies and relative branching fractions multiplied by fragmentation fractions ($R_{ p h^ h^{\prime }}$). The two uncertainties quoted on $R_{ p h^ h^{\prime }}$ are statistical and systematic. Upper limits are quoted at 90 (95) % confidence level for modes with signal significance less than $3 \sigma$. Uncertainties on the efficiencies are not given as only the relative uncertainties affect the branching fraction measurements. 
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Created on 20 February 2021.