Non-technical summary

"Properties of Photon Plus Two-Jet Events in pbar-p collisions at
root s = 1.8 TeV"

The CDF collaboration recently reported the results of studying the
properties of events containing a high energy photon and two "jets",
energetic bundles of particles whose parents are quarks and gluons.
"Photon" is the name given to the quantum particle-like nature of light but
the photons in the CDF study have much higher energy than visible light
or medical x-rays. The photons are important because they carry 
information directly from the fundamental interaction where protons and
antiprotons collide, in contrast to the quarks and gluons which undergo
the transformation into jets. The theory that describes this fundamental
interaction is called Quantum ChromoDynamics, or QCD for short. QCD 
makes definite predictions about the behavior of the photons and
jets and the CDF study checked several of these predictions. The
distribution of photon energies is especially important since previous
measurements had shown that the photons were, on average, not as
energetic as QCD would predict if the photons came from
the "direct" interaction. There is another possibility--quarks carry
electrical charge and any charged particle can emit photons when it
feels a force. This second type of radiated photon is called 
"bremsstrahlung" after the German word for "acceleration radiation."
If there were many more bremsstrahlung photons than expected, it
might explain why the average photon energy was low.

CDF studied the photon and jet energy distributions and the angular
distribution between the photons and jets. Most distributions agreed
reasonably well with the QCD predictions but the photon energy distribution
was indeed measured to be less energetic than predicted. The angular
distribution is sensitive to the relative contributions of direct
and bremsstrahlung photons. CDF exploited this fact to measure
the bremsstrahlung fraction at 55% with an uncertainty of 18%. This
agrees well with the 57% fraction predicted by QCD calculation so
the discrepancy in the photon energy distribution does not seem to
be due to an unusually large fraction of bremsstrahlung.