Measurement of the Mass and Production Rate of Ξb− Baryons

The first measurement of the production rate of Ξb− baryons in pp collisions relative to that of Λ0b 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 √s = 7, 8 and 13 TeV, respectively. In the kinematic region 2 < η < 6 and p_T < 20 GeV/c, we measure f_Ξb−/f_Λb0 B(Ξb− → J/ψ Ξ−)/B(Λb0 → J/ψ Λ) = (10.8 ± 0.9 ± 0.8) × 10^−2 [√s = 7, 8 TeV], f_Ξb−/f_Λb0 B(Ξb− → J/ψ Ξ−)/B(Λb0 → J/ψ Λ) = (13.1 ± 1.1 ± 1.0) × 10^−2 [√s = 13 TeV], where f_Ξb− and f_Λ0b are the fragmentation fractions of b quarks into Ξb− and Λ0b baryons, respectively; B represents branching fractions; and the uncertainties are due to statistical and experimental systematic sources. The values of f_Ξb−/f_Λb0 are obtained by invoking SU(3) symmetry in the Ξb− → J/ψ Ξ− and Λ0b → J/ψ Λ decays. Production asymmetries between Ξb− and anti-Ξb+ baryons are also reported. The mass of the Ξb− baryon is also measured relative to that of the Λ0b baryon, from which it is found that m(Ξb−) = 5796.70 ± 0.39 ± 0.15 ± 0.17 MeV/c^2, where the last uncertainty is due to the precision on the known Λ0b mass. This result represents the most precise determination of the Ξb− mass.