For decades, physicists have expected exotic particles called glueballs to spring from the collisions of particles in high-energy accelerators. But these beasts--which consist entirely of subatomic particles called gluons--are hard to distinguish from other, more mundane, particles. As a result, there have been plenty of candidates over the years, but no definitive sightings. Now, the claim of one contender--a particle spotted 11 years ago--may have gotten stronger.
Gluons bind quarks together to make protons and neutrons, but they are also supposed to be able to stick to themselves and form a glueball. Physicists are, however, vague on exactly what glueballs should look like, and that makes spotting one rather difficult. It takes "heroic computer calculations" to figure out what mass they should have, says Michael Chanowitz, a theorist at Lawrence Berkeley National Laboratory in Berkeley, California.
In 1986, an experiment at Brookhaven National Laboratory in Upton, New York, found a particle that had about the right mass for a glueball. But some physicists argued that the particle was simply an excited meson, masquerading as a glueball. Then last year, Andrew Foland and Daniel Bliss, both part of the 200-member team of physicists working with the CLEO detector at Cornell, realized that they might be able to tell if the Brookhaven particle had been an excited meson.
To do that, Bliss and Foland analyzed events in which two photons collide, which could make an excited meson. The pair used CLEO's large tracking chamber to reconstruct the trajectories of kaons--the decay product of an excited meson--produced in the trillions of collisions. They collected all events that produced two kaons and looked to see if the mass of these remnants added up to a parent particle with the mass of the Brookhaven particle. They didn't. "If it was a meson, we would have seen it," says Bliss.
The finding, reported in the 17 November Physical Review Letters, makes the existence of the Brookhaven glueball more likely, says Chanowitz, although he'd like to make sure that other features of the particle match theory. Robert Jaffe, a theorist at the Massachusetts Institute of Technology, also agrees, but cautions that the Brookhaven particle itself is controversial.