Measurement of the mass and production rate of $\Xi_b^-$ baryons

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 $p_{\rm T}<20$~GeV/\it{c}, we measure \begin{align*} \frac{f_{\Xi_b^-}}{f_{\Lambda_b^0}}\frac{\mathcal B(\Xi_b^-\to J/\psi\Xi^-)}{\mathcal B(\Lambda_b^0\to J/\psi\Lambda)} &= (10.8\pm0.9\pm0.8)\times10^{-2}~~[\sqrt{s}=7,8~TeV],\\ \frac{f_{\Xi_b^-}}{f_{\Lambda_b^0}}\frac{\mathcal B(\Xi_b^-\to J/\psi\Xi^-)}{\mathcal B(\Lambda_b^0\to J/\psi\Lambda)} &= (13.1\pm1.1\pm1.0)\times10^{-2}~~[\sqrt{s}=13~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(\Xi_b^-) &= 5796.70\pm0.39\pm0.15\pm0.17~MeV/\it{c}^2, \end{align*} \noindent 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.