Observation of the decay $\Xi_b^- \to p K^-K^-$

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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 proton-proton collision data collected at centre-of-mass 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.

Figures and captions

(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 long-dashed lines, cross-feed backgrounds are shown with red dot-dashed lines, and partially reconstructed backgrounds are shown with dark blue double-dot-dashed lines.

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Efficiency-corrected and background-subtracted [38] $m( p K ^- )_{\rm min}$ distribution from $\Xi ^-_ b \rightarrow p K ^- K ^- $ candidates.

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Tables and captions

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.