Measurement of Dijet Azimuthal Decorrelations
by the Fermilab DØ Collaboration

The theory that describes the strong nuclear force, Quantum Chromodynamics (QCD), is being actively investigated by the DØ Collaboration at the Fermilab Tevatron Collider. The proper description of QCD radiative processes is crucial for a wide range of precision measurements as well as for searches for new phenomena. Most of the interesting events produced in proton-antiproton collisions at the Tevatron contain collimated sprays of particles called jets. A clean and simple way to study QCD radiative processes is to examine their impact on the angular distributions of these jets. We find that recent theoretical calculations closely match our measured distributions. QCD-inspired models, crucial to understanding most particle and nuclear physics measurements, also match our distributions, though some tuning was required.
In the most simple case, the collision of a proton and an antiproton produces two jets with equal energies away from the beam direction and correlated azimuthal angles (Φ), such that the azimuthal difference ΔΦ is equal to π radians (180 degrees). This difference can be less than π when additional particles or additional jets are produced. A small number of low energetic additional particles produces small deviations from π, whereas smaller values of the azimuthal difference are an indication that additional high energetic jets have been produced in the event (see animation on the right). The azimuthal difference therefore provides an ideal testing ground to examine the transition between soft and hard QCD processes based on a single observable.
FIG. 1: Scetch of the azimuthal opening angle between the two leading pT jets.

FIG. 2: The distribution of the azimuthal angle between the two leading jets in four regions of the transverse momentum of the leading jet.
Our measurement of azimuthal correlations between jets produced at high transverse momenta in proton-antiproton collisions is presented on the left. Comparisons between our data and both the theoretical calculations and two QCD-inspired models can be viewed by moving the computer mouse over the figure or the colored boxes.

Please move the computer mouse over the colored boxes:
  - to see the theoretical predictions from QCD
  - to see the predictions from two different models

September 13, 2004