Comparison of three-jet and two-jet cross sections in pp̄ collisions at the CERN SPS pp̄ collider

Abstract Results are presented on two-jet and three-jet cross sections, measured in the UA1 experiment at the CERN Super Proton Synchrotron (SPS) pp Collider, at the highest available subprocess cms energies ( s >150 GeV ). Precise measurements of the two-jet angular distribution are consistent with previous results but show significant scale-breaking effects. The three-jet Dalitz plot and the three-jet angular distributions show evidence for final- and initial-state bremsstrahlung processes, in agreement with the leading-order QCD predictions. A comparison of the yield of wide-angle three-jet events with the yield of two-jet events at smaller scattering angles gives for the strong interaction coupling constant: α s ( K 3J K 2J )=0.16±0.02±0.03 at Q2≈4000 GeV2, where the factor K 3J K 2J may plausibly be assumed to be close to unity.