WORLD SCIENCE

"Long before it's in the papers"
December 19, 2005

Dec. 17, 2005
Special to World Science

Physicists have been on a three decades’ long search for a strange type of subatomic particle called a glueball. 

But the hunt may be almost, or already, over, a researcher claims. And if that’s true, it could clarify what nature’s most fundamental particles are.

According to the theory of quantum chromodynamics, the known components of the atomic nucleus, called protons and neutrons, are actually composed of triplets of "quarks," shown in red above. Particles known as gluons carry a force that holds the quarks together so that they are practically inseparable. (Courtesy U.S. Department of Energy)

The glueball quest is connected with a popular theory called quantum chromodynamics, which claims matter’s most basic components are tiny entities called quarks. Other particles, called “gluons,” act as a “glue” that binds quarks together to form the protons and neutrons of the atomic nucleus.

Most particle physicists consider the theory definitive; atom-smashing experiments have confirmed it, says Michael Chanowitz, a theoretical physicist at Lawrence Berkeley National Laboratory in Berkeley, Calif. 

Yet one of its most dramatic predictions, he added, has yet to be verified. That is the existence of glueballs, particles made only of gluons. 

Glueballs would be “an intriguing new form of matter,” he said. Little is known about what they’re like, and what they might be useful for—probably nothing, he added. But their discovery could raise new questions that lead to further progress in physics, and as for their practical applications, “you never know.”

Either way, he said, glueballs would certainly be unique, because they would be the only force-carrying particles known to stick together. 

This is in contrast to particles such as photons, which we see as light. Photons carry the electromagnetic force, which makes electrically charged objects attract or repel each other. But photons themselves don’t interact, as they have no charge. Gluons attract each other because do have a sort of charge, whimsically called “color charge” though it has nothing to do with color.

Beyond this basic description, the properties of glueballs are murky. That largely explains why physicists haven’t been able to find them, Chanowitz said: researchers don’t quite know what to look for.

But, he argued, a solution may be at hand.

It’s believed that newly formed glueballs would quickly decay, or fall apart, producing other particles in the process. Chanowitz says a glueball could be identified by the types of particles it decays into. He detailed his proposal in the October 21, 2005 issue of the research journal Physical Review Letters.

Traditionally, physicists “thought that glueballs decay equally [often] to pairs of three different types of quarks,” Chanowitz told Science@BerkeleyLab magazine, a publication of the laboratory.

But his calculations, he added, shows that when the glueball undergoes one common type of decay—into pairs of particles—those tend to consist of a particular type of quark, called the strange quark.

“We can use this signature to hunt” for glueballs, he told the publication, adding that he relied on a technique called all-orders perturbation theory to reach his conclusions.

The findings, he said, fit with puzzling results from supercomputer calculations done ten years ago by Donald Weingarten and colleagues at the IBM Watson Research Center in Yorktown Heights, N.Y.

Contrary to previous wisdom, they found that the lightest glueball decays more often to pairs of so-called K mesons, particles partly composed of strange quarks, than to much lighter “pi mesons.” These are composed of other types of quarks called “up” and “down.” 

Chanowitz said Weingarten’s results are due to the lightest glueball’s preference to decay to strange quark pairs. 

“My work adds credence to Weingarten’s seemingly counterintuitive results and provides an explanation for what was then an unexpected finding,” Chanowitz told Science@BerkeleyLab. “It shows that we’re on track.”

Chanowitz’s work also tightens the focus on a particle that physicists have eyed as the lightest glueball, also called the scalar glueball, for several years, the publication reported. The particle, f(1710), has many characteristics of a glueball, but it has been dismissed because it predominantly decays to K-meson pairs.

“But my analysis implies this is exactly what it should do,” Chanowitz told the magazine. He added that studies with supercomputers and particle-smashers could clear up the question in a few years.

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