Physics News Update
The American Institute of Physics Bulletin of Physics News
Number 258 February 13, 1996 by Phillip F. Schewe and Ben Stein

DO QUARKS HAVE SUBSTRUCTURE? An experiment at Fermilab has now gathered data which could be interpreted as evidence for the existence of a new level of matter. Monitoring high-energy proton-antiproton collisions, physicists on the CDF collaboration are on the lookout for a variety of interesting phenomena. Last year their vigilance paid off handsomely with the discovery of the top quark. Now they are reporting on a class of violent interactions in which a quark inside a proton scatters directly with a quark inside one of the oncoming antiprotons, sending jets of particles sideways out of the interaction area. A plot of the frequency of such events as a function of "transverse energy" (that portion of the collision energy attributed to the transverse motion of debris particles away from the beam axis) shows that theory and experimental data agree closely except at the highest of energies.

CDF has amassed a sample of 1200 events above a transverse energy of 200 GeV, significantly more events than expected. Several possible explanations have been given to account for the events: e.g., that the detector is not correctly calibrated for this most interesting of event configurations or that estimates of jet production have not been properly calculated. But the observed excess of high-transverse-energy events is exactly what one would expect if one of the incoming quarks had scattered from something hard inside the other quark.

This can be compared with two other notable chapters in physics history. In 1911 Ernest Rutherford surmised that atoms had a substructure (consisting of a nucleus surrounded by electrons) by scattering alpha particles from a thin foil. Many years later at SLAC, and with a higher-energy beam (which allows one to study matter at a much smaller size scale), physicists discovered that protons also had a substructure; that is, that they consist of constituent quarks. The 1990 physics Nobel Prize went to the leaders of this experiment. Now at Fermilab the energy scale is higher still and the distance scale probed, 10**-19 m, is perhaps the smallest scale ever studied in a particle physics experiment. If the CDF result is verified, it would constitute a departure from the venerable "standard model" of particle physics and would be an indication that quarks, which we currently regard as the most basic of building blocks, would themselves have constituents. Just as the recent observation of extrasolar planets has stretched our perceptual universe outward, so the resolution of a finer scale for matter would comparably stretch our view in the inner direction. (A CDF article, F. Abe et al., has been submitted to Physical Review Letters, July 1996, but a news account of the subject appears in Science, 9 February 1996.) "