Measurement of the $\Lambda_b^0$, $\Xi_b^-$ and $\Omega_b^-$ baryon masses

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Bottom baryons decaying to a J/\psi meson and a hyperon are reconstructed using 1.0 fb^{-1} of data collected in 2011 with the LHCb detector. Significant \Lambda_b^0 \rightarrow J/\psi \Lambda, \Xi_b^-\rightarrow J/\psi \Xi^- and \Omega_b^- \rightarrow J/\psi \Omega^- signals are observed and the corresponding masses are measured to be M(\Lambda_b^0) = 5619.53 \pm 0.13 (stat) \pm 0.45 (syst) MeV/c^2, M(\Xi_b^-) = 5795.8 \pm 0.9 (stat) \pm 0.4 (syst) MeV/c^2, M(\Omega_b^-) = 6046.0 \pm 2.2 (stat) \pm 0.5 (syst) MeV/c^2, while the differences with respect to the \Lambda_b^0 mass are M(\Xi_b^-)-M(\Lambda_b^0) = 176.2 \pm 0.9 (stat) \pm 0.1 (syst) MeV/c^2, M(\Omega_b^-)-M(\Lambda_b^0) = 426.4 \pm 2.2 (stat) \pm 0.4 (syst) MeV/c^2. These are the most precise mass measurements of the \Lambda_b^0, \Xi_b^- and \Omega_b^- baryons to date. Averaging the above \Lambda_b^0 mass measurement with that published by LHCb using 35 pb^{-1} of data collected in 2010 yields M(\Lambda_b^0) = 5619.44 \pm 0.13 (stat) \pm 0.38 (syst) MeV/c^2.

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Average momentum scale bias $\alpha$ determined from the reconstructed mass of various decay modes after the momentum calibration procedure. The $ K ^0_{\rm\scriptscriptstyle S} $ decays are divided into two categories according to whether both daughter tracks (a) have hits or (b) do not have hits in the vertex detector. The black error bars represent the statistical uncertainty whilst the (yellow) filled areas also include contributions to the systematic uncertainty from the fitting procedure, the effect of QED radiative corrections, and the uncertainty on the mass of the decaying meson [2]. The (red) dashed lines show the assigned uncertainty of $\pm 0.3 \times 10^{-3}$ on the momentum scale.

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Invariant mass distribution for (a) $\Lambda^0_ b \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \Lambda $, (b) $\Xi^-_ b \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \Xi^- $ and (c) $\POmega^-_ b \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \POmega^- $ candidates. The results of the unbinned maximum likelihood fits are shown with solid lines.

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Results of the fits to the invariant mass distributions. The quoted uncertainties are statistical. The $\Lambda^0_ b $ signal is described by a double Gaussian function with widths $\sigma_1$ and $\sigma_2$; the fraction of the yield described by the first component is $0.58 \pm 0.11$.

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Systematic uncertainties (in $ {\mathrm{ Me V /}c^2}$ ) on the mass measurements and their differences. The total systematic uncertainty is obtained from adding all uncertainties in quadrature.

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Comparison of the $b$-baryon mass measurements using the full 2011 data sample with the single most precise results from the ATLAS [22], CDF [11,23] and D0 [10,12] collaborations, and with the PDG averages [2]. The PDG averages contain the results from CDF and D0 as well as the $\Lambda^0_ b $ measurement from LHCb performed with the 2010 data sample. The quoted errors include statistical and systematic uncertainties. All values are in $ {\mathrm{ Me V /}c^2}$ .

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Supplementary Material [file]

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This ZIP file contains supplemetary material for the publication LHCb-PAPER-2012-048. The files are: Supplementary.pdf : An overview of the extra figures *.pdf, *.png, *.eps : The figures in variuous formats

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Created on 19 February 2021.