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Measurement of the $\Xi_b^-$ and $\Omega_b^-$ baryon lifetimes

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Abstract

Using a data sample of pp collisions corresponding to an integrated luminosity of $3 \rm fb^{-1}$, the $\Xi_b^-$ and $\Omega_b^-$ baryons are reconstructed in the $\Xi_b^- \rightarrow J/\psi \Xi^-$ and $\Omega_b^- \rightarrow J/\psi \Omega^-$ decay modes and their lifetimes measured to be $\tau (\Xi_b^-) = 1.55 ^{+0.10}_{-0.09} {\rm(stat)} \pm 0.03 {\rm(syst)}$ ps, $\tau (\Omega_b^-) = 1.54 ^{+0.26}_{-0.21} {\rm(stat)} \pm 0.05 {\rm(syst)}$ ps. These are the most precise determinations to date. Both measurements are in good agreement with previous experimental results and with theoretical predictions.

Figures and captions

Efficiency for triggering simulated $\Xi^-_ b $ events as a function of the reconstructed decay time under the 2012 data-taking conditions. Only a restricted decay-time range is shown in order to emphasize the region where the effect is large. The left panel shows the efficiency for events passing the trigger with only the requirement on the $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ vertex displacement. The right panel shows the efficiency for events passing the trigger with requirements on both the $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ vertex displacement and the muon impact parameter and required to not pass the other trigger. The results of fits with functions proportional to that given in Eq. 2 are overlaid.

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Efficiency due to the detector acceptance, reconstruction and selection of simulated $\Xi^-_ b $ (left) and $\POmega^-_ b $ (right) events as a function of the reconstructed decay time under the 2012 data-taking conditions. The results of fits with functions proportional to that given in Eq. 3 are overlaid.

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Distributions of the reconstructed invariant mass (top) and decay time (middle and bottom) of the $\Xi^-_ b \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \Xi^- $ (left) and $\POmega^-_ b \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \POmega^- $ (right) candidates. The middle (bottom) panels show the decay time distributions of the candidates in the signal (background) mass regions. The signal mass region is defined as 5773--5825 $ {\mathrm{ Me V /}c^2}$ for $\Xi^-_ b $ and 6028--6073 $ {\mathrm{ Me V /}c^2}$ for $\POmega^-_ b $ candidates, as shown by the vertical dotted lines in the mass distributions, whereas the background mass regions include all other candidates. The results of the fits are overlaid.

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Animated gif made out of all figures.

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Tables and captions

Fitted $\beta$ values (in ps$^{-1}$) for the efficiency described in Eq. 3 extracted from simulated $\Xi^-_ b $ and $\POmega^-_ b $ decays. The quoted uncertainties are statistical only.

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Fitted parameters with statistical uncertainties for the $\Xi^-_ b $ and $\POmega^-_ b $ signals.

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Systematic uncertainties on the lifetime measurements in fs. For the total uncertainty, all the contributions are summed in quadrature.

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Created on 17 August 2019.Citation count from INSPIRE on 23 August 2019.