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.)
HENRI BECQUEREL ANNOUNCED THE DISCOVERY OF RADIOACTIVITY at a series
of scientific meetings during 1896. Thus the radioactivity centennial comes
quickly on the heels of the x-ray centennial. Becquerel's first evidence
(presented on February 24) consisted of photographic plates exposed by
the rays issuing from a uranium compound. A fuller understanding of radioactivity
developed over successive months and years. Only in 1902/03, for instance,
did Ernest Rutherford establish that radioactivity was the tangible product
of the transmutation of one element into another. Others, such as Marie
Curie, added more pieces to the puzzle. Eclipsed for some years by the
greater fame of Roentgen's x rays, radioactivity came to have a profound
effect on the 20th century through the use of unstable elements in nuclear
reactors, nuclear weapons, and in medicine. By the way, the element uranium,
discovered in 1789, takes its name from the planet Uranus, which itself
had been discovered only a few years earlier in 1781. (Physics Today, February
1996.)
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