Who's the Smallest of Them All?

Don't worry, the quark still reigns as the universe's smallest object. Findings to appear in the journal Physical Review Letters upend recent suggestions that these subatomic particles might harbor even tinier entities.

After smashing together protons and antiprotons with 1.8 trillion electron volts of energy earlier this year, the Collider detector (CDF) group at the Fermi National Accelerator Laboratory saw an unexpectedly large number of "hard," or large-angle, collisions. According to their calculations, these more energetic collisions might have indicated something even smaller lurking inside the sea of quarks and gluons that comprise the protons and antiprotons.

However, a finer analysis of the smashups "doesn't at all encourage the interpretation of [them] being due to new physics," says Giorgio Bellettini of the University of Pisa in Italy and CDF co-spokesperson. That's because the group went beyond its original work and has now sorted the debris by both its energy level and scattering angle. This analysis allowed them to reconstruct with greater detail the physics of the collisions.

The results showed that if quarks were to contain an even smaller particle, it would probably have to be less than a thousandth the size of a proton. "In that situation," says William Carithers of the Lawrence Berkeley National Laboratory and the other CDF co-spokesperson, "one usually resorts to Occam's razor and says, 'Why invoke something which is very radical when something much more prosaic will do?' " Instead, he says, it's more likely that gluons carry more kinetic energy during the collisions than standard calculations had suggested. This, Bellettini says, might explain the greater number of hard collisions without having to invoke the "new physics" of quark substructure.

Still, Carithers says the data on energy rearrangement within protons has not been shown to be compatible with data from hundreds of other accelerator experiments and could knock other data out of line with theory. If so, he says, "we're back in trouble again."