This letter reports the full reconstruction of B mesons through the decay chain ${\mathit{B}}^{\pm }$→J/ψ ${\mathit{K}}^{\pm }$, J/ψ→${\mathrm{\mu }}^{+}$${\mathrm{\mu }}^{\mathrm{-}}$, using data obtained at the Collider Detector at Fermilab in p¯p collisions at √s =1.8 TeV. This exclusive sample, the first observed at a hadron collider, is then used to measure the B-meson cross section, from which we extract the b-quark cross section. We obtain σ=2.8±0.9 (stat) ±1.1(syst) μb for ${\mathit{B}}^{\mathrm{-}}$ mesons with ${\mathit{P}}_{\mathit{T}}$>9.0 GeV/c and rapidity ‖y‖<1.0. We obtain σ=6.1±1.9(stat) ±2.4(syst) μb, for b quarks with transverse momentum ${\mathit{P}}_{\mathit{T}}$>11.5 GeV/c and rapidity ‖y‖<1.0.

• Received 28 January 1992

DOI:https://doi.org/10.1103/PhysRevLett.68.3403