The reclusive glueball may soon be forced out of hiding, thanks to a Berkeley Lab theoretical physicist who found the odd signature these subatomic particles leave behind when they decay.

Michael Chanowitz is a theoretician in Berkeley Lab’s Physics Division. (Photo
Roy Kaltschmidt)

This work, conducted by Michael Chanowitz and published in the October 21, 2005 issue of Physical Review Letters, is an important step in the 30-year hunt for a ghostly particle that serves as one of the lynchpins of modern particle physics.

Glueballs are so named because they are composed of subatomic particles called gluons, which have the all-important job of gluing together particles called quarks, which in turn combine to form protons, neutrons, and other particles. They are an essential prediction of quantum chromodynamics, a theory that physicists use to explain the strong nuclear force, which holds the nuclei of atoms together.

But despite their starring role in nature’s most powerful force, physicists have yet to experimentally verify the existence of a glueball. They’ve come across some likely suspects, but they don’t know enough about glueballs to definitively know one when they see one. Now, that could change.

“People thought that glueballs decay equally to pairs of three different types of quarks: up, down, and strange. But I show that the lightest glueball decays preferentially to strange quark pairs. We can use this signature to hunt for them,” says Chanowitz, a theoretician with Berkeley Lab’s Physics Division, who relied on a technique called all-orders perturbation theory to arrive at his results.

Chanowitz’s conclusions are buttressed by puzzling results from supercomputer calculations conducted ten years ago by Donald Weingarten and colleagues at the IBM Watson Research Center. Contrary to conventional wisdom, they found that the lightest glueball decays more frequently to pairs of K mesons, which are particles that are partly composed of strange quarks, than to much lighter pi mesons, which are composed of up and down quarks.

Based on his recent work, Chanowitz believes that Weingarten’s results are due to the lightest glueball’s preference to decay to strange quark pairs rather than to pairs of up and down quarks. In addition, he predicts that this preference only applies when the lightest glueball decays to two particles, not three or more.

“My work adds credence to Weingarten’s seemingly counterintuitive results and provides an explanation for what was then an unexpected finding,” says Chanowitz. “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 particle, f(1710), possesses 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,” says Chanowitz. “It strengthens the hypothesis that f(1710) is indeed the scalar glueball, and emphasizes that the particle deserves a much closer look.”

According to Chanowitz, a closer look means using today’s vastly more powerful supercomputers to conduct theoretical space-time lattice calculations to verify the results obtained by Weingarten and colleagues a decade ago. In addition, definitive experimental studies of f(1710) can be performed during the next few years at the Beijing Electron-Positron Collider and the Cornell Electron Storage Ring.

Physicists are eager to experimentally verify glueballs because their existence, which to this day has only been hypothesized, is one of the key features that distinguish quantum chromodynamics from quantum electrodynamics — two theories that are used to explain the strong nuclear force and the electromagnetic force, respectively.

At the heart of this prediction is a difference between photons and gluons. Under the laws of quantum electrodynamics, photons carry the electric force but are themselves charge neutral. In other words, there is no positive or negative photon, meaning two photons don’t bind to form a “lightball.” But under the theory of quantum chromodynamics, which governs gluons, it is believed that gluons carry a charge, in this case a “color” charge. Because they have color charge, two gluons can bind to form a color-neutral particle, a glueball.

Gluons bind quarks into particles like protons or neutrons. Here, three quarks — two ups and a down — are depicted forming a proton, held together by the exchange of (invisible) gluons. Because gluons carry color charge they can interact with one another to form glueballs, which decay into particles made of quarks and antiquarks. (Illustration courtesy Jefferson Lab)

“Although quantum chromodynamics is a very successful theory, this simple and dramatic prediction has resisted experimental verification for three decades because glueballs are not easily distinguished from ordinary particles made of quark-antiquark pairs,” says Chanowitz.

Chanowitz believes that the scalar glueball’s unique decay pattern could be just the clue physicists need to obtain this experimental verification. He adds that the scalar glueball exhibits this decay pattern because the strong nuclear force barely changes the spin direction of very light quarks, a phenomenon that reveals itself in the scalar glueball’s inclination to decay to quark-antiquark pairs of the heavier strange quark.

“The experimental consequence is that for scalar glueball decays to two particles, but not for decays to three or more particles, the decay products will predominantly contain strange quarks,” says Chanowitz. “This is a novel signature that can be used to search for the scalar glueball.”

Chanowitz’s research on scalar glueballs was supported in part by the Office of High Energy and Nuclear Physics within the Office of Science of the U.S. Department of Energy.

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