In a paper submitted to the journal Physical Review D, the CDF collaboration reported the results of a study of very complex events containing six "jets" produced in very violent high energy collisions between protons and antiprotons. Jets are tight bundles of particles seen in the detector that are the manifestation of the quarks and gluons that make up the proton. The properties and interactions of the quarks and gluons are described by a theory called Quantum ChromoDynamics, or QCD for short. In its simplest form, QCD is known to be reliable when describing how two quarks or gluons interact--for example, when they scatter off each other and end up in the detector as two jets. The picture gets more complicated when, for example, an outgoing quark or gluon "radiates" an extra gluon resulting in three jets in the final state. By adding more and more of this "strong radiation", one can build up more complex events with many jets in the final state seen by the detector. In principle, QCD can describe even such complicated events. The CDF experiment therefore sought to compare the observations of six-jet events with the QCD predictions.

The main challenge for the analysis was finding a set of metrics that could both completely characterize the six jets (it takes 20 variables!) and be calculated using the tools of QCD. The CDF analysis used a technique developed earlier for dealing with four- and five-jet systems. The trick is to pair the two "lightest" jets into a single "pseudojet" so that the new system looks like it has one jet less. Treating the pseudojet like a real jet, one pairs the next lightest pair of jets, and so on until the system is reduced to a three-jet system, for which a good choice of metrics is known.

CDF found that the QCD calculations described the six-jet variables very well. There are a couple of minor exceptions which shed some light on the limitations of the mathematical tools used to calculate the theoretical predictions. On the whole, the result is an impressive success for the theory and for our ability to make complicated predictions based on the theory. The QCD calculations described the data much better than a trivial model that assigned the six-jet variables probabilistically (i.e., without any underlying description of how quarks and gluons interact).

Postscript version of publish ed article
For more information, contact Steve Geer [sgeer@fnald.fnal.gov]