Dijet Angular Distributions in $p\bar{p}$ Collisions at $\sqrt{s} = 1.8$ TeV

Abstract

We present measurements of diet angular distributions for events with two or more jets produced in $p\bar{p}$ collisions at the Fermilab Tevatron Collider at $\sqrt{s}$ = 1.8 TeV. The distribution of the scattering angle of a parton in the center-of mass reflects the dynamics of the interaction of two partons. This distribution was measured for different fixed regions of the diet invariant mass, in particular: 260 < $M_{jj}$ < 425, 475 < $M_{jj}$ < 635, $M_{jj}$ > 550, and $M_{jj}$ > 635 GeV / $c^2$ • Because the dijet angular distribution is expected to be relatively insensitive to the parton distribution within a proton, we can measure the properties of parton parton interactions without being concerned with the inability to calculate quantitative aspects of the substructure of the proton. We find that the next-to-leading order predictions of Perturbative Quantum Chromodynamics (QCD) show good agreement with the observed dijet angular distributions for all four mass ranges. At dijet invariant masses of greater than $ \sim 500$ GeV / $c^2$, the dijet angular distribution becomes sensitive to any new contact interactions originating from possible quark compositeness. In order to search for evidence that quarks are not point-like objects, but are rather bound states of more fundamental particles, we compared the predicted angular distribution with that measured in data for the mass range of $M_{jj}$ > 550 GeV /$c^2$. There was no departure from the expectations of next-to-leading-order Perturbative QCD, and we have used that to place a limit on the interaction scale for quark compositeness as $\Lambda$ > 1.8 TeV.