Scales tilt against five-quark particles.

May the best data win. In the past 2 years, evidence has quickly mounted for the existence of pentaquarks, never-before-seen subatomic particles theoretically composed of five quarks or antiquarks, which are fundamental constituents of matter. Lately, however, evidence against pentaquarks has been mounting even faster, reports Curtis A. Meyer of Carnegie Mellon University in Pittsburgh.

Physicists have long known that types of quarks and antiquarks combine in twos and threes to form more-complex particles such as protons and neutrons. Until recently, however, no one had ever observed combinations of more than three of the quark or antiquarks.

That all changed when a Russian theorist and his colleagues convinced a team at the Japanese particle accelerator SPring-8 in Hyogo to reexamine data from a 2001 study whose conditions might have generated pentaquarks called theta+.

In 2003, that reanalysis yielded the first signs of a pentaquark (SN: 7/5/03, p. 3). Subsequently, a dozen teams around the world sifted through old data and found similar buried evidence that pentaquarks had been inadvertently created (SN: 10/18/03,p. 245). At SPring-8, researchers also carried out a new experiment explicitly to search for a specific pentaquark, and they seemed to have found it.

Nonetheless, Meyer points out, at least 18 teams in as many months have come up empty-handed in their retrospective searches for such particles.

"The negative evidence to date is quite overwhelming," Meyer says. Most telling, he adds, is that the data against pentaquarks is stronger by a factor of 10 than the pro-pentaquark data. That's because the studies with negative outcomes have included many more particle collisions in their tallies than have studies finding pentaquarks.

Still, the search goes on. A team from the Thomas Jefferson National Accelerator Laboratory in Newport News, Va., reported finding no sign of a pentaquark in the first experiment at that lab explicitly searching for such particles.

In another fresh outcome, preliminary data from Brookhaven National Laboratory in Upton, N.Y., have prompted speculation that a new pentaquark, called theta++, and its antimatter counterpart might have made appearances.

It will take more time and experiments to settle the question of whether pentaquarks are real or not, says Moskov Amarian of Old Dominion University in Norfolk, Va.