Measurement of the mass and production rate of ${\Xi }_b^{-}$ baryons

Abstract

The first measurement of the production rate of ${\Xi }_b^{-}$ baryons in $pp$ collisions relative to that of ${\Lambda }_b^0$ baryons is reported, using data samples collected by the LHCb experiment, and corresponding to integrated luminosities of $1$, $2$ and $1.6$ fb${}^{-1}$ at $\sqrt{s}=7,8$ and $13$ TeV, respectively. In the kinematic region $2<\eta <6$ and $p_{\mathrm{T}}<20$ GeV/$c$, we measure \begin{align*} \frac{f_{Ξ_b^-}}{f_{Λ_b^0}}\frac{\mathcal{B}(Ξ_b^-\to J/ψΞ^-)}{\mathcal{B}(Λ_b^0\to J/ψΛ)} &= (10.8\pm0.9\pm0.8)\times10^{-2}~~[\sqrt{s}=7,8~{\rm TeV}],\\ \frac{f_{Ξ_b^-}}{f_{Λ_b^0}}\frac{\mathcal{B}(Ξ_b^-\to J/ψΞ^-)}{\mathcal{B}(Λ_b^0\to J/ψΛ)} &= (13.1\pm1.1\pm1.0)\times10^{-2}~~[\sqrt{s}=13~{\rm TeV}], \end{align*} where $f_{{\Xi }_b^{-}}$ and $f_{{\Lambda }_b^0}$ are the fragmentation fractions of $b$ quarks into ${\Xi }_b^{-}$ and ${\Lambda }_b^0$ baryons, respectively, $\mathcal{B}$ represents branching fractions, and the uncertainties are due to statistical and experimental systematic sources. The values of $f_{{\Xi }_b^{-}}/f_{{\Lambda }_b^0}$ are obtained by invoking SU(3) symmetry in the ${\Xi }_b^{-}\to J\psi {\Xi }^{-}$ and ${\Lambda }_b^0\to J/\psi \Lambda$ decays. Production asymmetries between ${\Xi }_b^{-}$ and ${\bar{\Xi }}_b^{+}$ baryons are also reported. The mass of the ${\Xi }_b^{-}$ baryon is also measured relative to that of the ${\Lambda }_b^0$ baryon, from which it is found that \begin{align*} m(Ξ_b^-) &= 5796.70\pm0.39\pm0.15\pm0.17~{\rm MeV}/c^2, \end{align*} where the last uncertainty is due to the precision on the known ${\Lambda }_b^0$ mass. This result represents the most precise determination of the ${\Xi }_b^{-}$ mass.

arXiv comment10 pages, 1 figure; All figures and tables, along with any supplementary material and additional information, are available at https://cern.ch/lhcbproject/Publications/p/LHCb-PAPER-2018-047.html

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