Testing QCD at Large Total Transverse Energies

Within the framework of leading-order perturbative QCD we expect events with large total transverse energies to arise from 2 -> 2 scattering, and hence to contain two jets in the final state. Higher order corrections to the leading-order process can give rise to additional jets. The analysis of the rate and properties of events with large total transverse energy produced in proton-antiproton collisions therefore provides a test of QCD that complements the traditional inclusive-jet and two-jet analyses. In particular, the rate and properties of multijet events in the large total transverse energy event sample provides a test of the higher-order corrections to the leading-order QCD matrix element.

The CDF analyses of events with large total transverse energies have addressed the following questions:

  • Are there events in the CDF data sample that have large total transverse energies but do not have a jet-like structure ?

    • This question is addressed in F. Abe et al (CDF Collaboration), Phys. ReV. D45, 2249 (1992), which describes results from the analysis of the CDF 1988/9 data sample. With the exception of a few photon + jet(s), Z + jet(s), and electron + jet(s) events, all of the observed large total transverse energy events are multijet events containing two-or-more high transverse momentum jets.

  • Is the CDF event rate as a function of the total transverse energy well described by QCD predictions ?

    • This question is addressed in F. Abe et al (CDF Collaboration), Phys. ReV. D45, 2249 (1992), which describes results from the analysis of the CDF 1988/9 data sample. The observed uncorrected total transverse energy spectrum was shown to be reasonable well described by the QCD prediction obtained from the HERWIG parton shower Monte Carlo plus detector simulation provided the predictions were scaled upwards by a K-Factor of 1.12. An analysis is currently in progress of the much larger 1992/3 data sample. The fully corrected spectrum is compared with both HERWIG and next-to-leading order matrix element predictions. With the more precise high statistics data sample preliminary results suggest that the present QCD calculations underestimate the event rate at the highest transverse energies.

  • Is there any evidence for new physics at the highest transverse energies ?

    • Current QCD predictions for the inclusive-jet transverse momentum differential cross-section, and the two-jet mass and total-transverse-energy differential cross-sections underestimate the preliminary CDF measurements at large energies. "Preliminary" means that the experimental systematics are still under study. We will have to wait until these systematic studies have been completed before we know whether the QCD calculations are in agreement or disagreement with the measurements. Note however that in prl 75, 608 (1995) we showed that both the leading-jet angular distributions and the jet multiplicity distribution seem to be in agreement with QCD predictions at the highest total transverse energies.

  • Are the rates and properties of multijet events observed in the large total transverse energy event sample consistent with leading-order QCD 2 -> N matrix element predictions ?

    • In prl 75, 608 (1995) we show that the multijet mass distributions, leading-jet-angular distributions, jet transverse-momentum distributions, and the mass-dependent jet-multiplicity distributions are well described by the NJETS LO QCD predictions for two-, three-, four-, and five-jet events.

  • Do parton shower Monte Carlo calculations give a reasonable description of the properties of multijet events with large total transverse energies ?

    • This question is addressed in F. Abe et al (CDF Collaboration), Phys. ReV. D45, 2249 (1992), which describes results from the analysis of the CDF 1988/9 data sample, and in prl 75, 608 (1995) which describes results from the analysis of the 1992/3 data sample. In general, it appears the the HERWIG parton shower Monte Carlo gives an excellent description of the properties of multijet events with large total transverse energies. The properties looked at include jet rates, jet transverse-momentum and pseudorapidity distributions, single-jet shapes, multijet mass distributions, leading-jet-angular distributions, and mass-dependent jet-multiplicity distributions. Of these distributions, only the observed jet transverse momentum distributions for three- and four-jet events seem to discriminate between the matrix element and parton shower predictions, the data favoring the matrix element calculation.