A High-Five for Quarks

Quark quirk. Zapping a nucleus with enough energy seems to yield a five-quark monster.

Three quarks for muster Mark? Every physicist's favorite passage from Finnegans Wake might need a little updating. Several experiments around the world seem to have created an exotic particle containing five quarks rather than the two or three that make up all other quarky matter. If true, this new particle, dubbed the theta-plus , might help physicists banish the last remaining shadows in quantum chromodynamics (QCD), the theory that describes quarks and the forces that bind them together.

QCD does not forbid five-quark particles. But all known quarky matter is made up of three-quark ensembles known as baryons or quark-antiquark pairs known as mesons. Years of looking for bizarre four- and five-quark ensembles left scientists empty-handed and puzzled.

Now scientists at three laboratories think they finally have spotted a five-quark beastie. The first experiment, at the SPring-8 accelerator facility near Osaka, zaps a carbon target with high-energy light. A second, at the Jefferson National Accelerator Facility in Newport News, Virginia, sends light into deuterium or hydrogen targets. The third, at the Institute for Theoretical and Experimental Physics (ITEP) in Moscow smashes mesons into xenon nuclei.

In each case, researchers hope that jolted quarks inside atomic nuclei will fleetingly recombine into new species of particles. When these decay, the resulting baryons and mesons should betray their exotic heritage. All three groups report that the debris from the collisions point back toward particles. "The fact that all the labs are reporting similar results is a relief," says Takashi Nakano, who heads the Japanese effort, which published its findings in the 4 July issue of Physical Review Letters. The Russian results will be submitted there shortly, and the U.S. data were unveiled at a conference in May.

According to Terrance Goldman, a physicist at Los Alamos National Laboratory in New Mexico, figuring out the precise shapes of exotic particles like the can "fill in the last major chink" in the armor of quantum chromodynamics. Physicists know that quark-matter particles aren't always spherical, yet they routinely ignore that change of curvature in their QCD calculations. If "pentaquark" states are not spherical, Goldman says, then physicists can finally figure out what their models were getting wrong and fill in the missing details.

Related sites
T. Nakano et al.'s Physical Review Letters abstract
Conference on the Intersections of Particle and Nuclear Physics