Measurement of the bb Cross Section at HERA-B

The bb̄ production cross section has been measured in collisions of 920 GeV protons off a nuclei target using the HERA‐B detector. The identification of bb̄ events was done via inclusive bottom quark decays into J/ψ by exploiting the longitudinal separation of the J/ψ → l+l− decay vertices from the primary proton nucleus interaction, where both the μ+μ− and the e+e− decay channels have been reconstructed. The first measurement, using data collected during a short period in 2000, yields a cross section in the combined analysis of σ(bb̄) = 32−12+14(stat)−7+6(sys) nb/nucleon. On the most recent data samples taken in 2002/03, a much more accurate measurement with the uncertainties reduced to 10% is expected.