Sticky Stuff: Elusive Glueballs Possibly Found in Atom Smasher
Nucleons such as protons and neutrons are made up of quarks and gluons (shown left). By contrast, a glueball would be all gluon (shown right). Theoretical physicists recently announced that a particle detected at the LHC may be the long-sought glueball.
Credit: TU Wien

A long-sought subatomic particle called a glueball may have been hiding in plain sight at the world's largest atom smasher.

New calculations suggest that a particle spotted at the Large Hadron Collider (LHC) in Geneva, Switzerland, is actually a glueball, a bizarre particle made exclusively of subatomic particles known as gluons. True to their name, gluons carry the strong nuclear force that acts within the nucleus, providing a kind of glue that keeps protons and neutrons in the nucleus of an atom.

If the LHC's particle is a glueball, it would be an oddball. All of the other particles that have been observed so far combine gluons and the elementary building blocks of matter known as quarks. [In Photos: The World's Largest Atom Smasher]

"The idea is that, in principle, those gluons themselves can form a bound state, without the need of including quarks," said study co-author Frederic Brunner, a doctoral candidate in physics at the Vienna University of Technology in Austria. "That's somehow remarkable."

Long predicted, never seen

In the 1920s, physicists knew that the nucleus of the atom contained positively charged protons. But they also knew that things with the same charge repulse each other, and they couldn't figure out how those protons could be squished into the cramped quarters of the atomic nucleus without generating a humongous repulsive force.

Researchers eventually showed that the strong nuclear force that acts within the nucleus must counteract the repulsive force pushing protons apart. Later on, physicists hypothesized that protons and neutrons (collectively called nucleons) were made of yet-tinier particles called quarks. Because all forces act through other particles in particle physics, they suspected that a strong nuclear force bound these quarks together via another particle, which they dubbed the gluon.

In 1972 the physicist Murray Gell-Mann realized that a particle composed entirely of gluons was possible. Over the years, shadowy hints of the particle (now called a glueball) were found in several experiments, but no one could ever prove that what they had seen was a glueball, Brunner said.

Hints in LHC Data

But data from the LHC could already have evidence of glueballs' existence, Brunner said. Among the debris of the atom smasher's billions of proton collisions are subatomic particles called hadrons, which flit into existence briefly, only to decay into even tinier subatomic particles.

These decay patterns leave a fleeting trace on the LHC's ultrasensitive detectors. Based on the pattern of decay, physicists concluded that these last decay products were mesons, or a category of subatomic particles that mediate the strong nuclear force.

The data from the LHC hasn't revealed exactly what these mesons are, but they've given them provisional names — f0(1500) and f0(1710).

So Brunner and his doctoral advisor, theoretical physicist Anton Rebhan, wondered whether one of these particles could be the elusive glueball. To answer that question, the team developed a mathematical model using something called the holographic principle.

In essence, the holographic principle provides a method for mapping everything in the 4D world (three dimensions plus time) onto a higher-dimensional space that could theoretically lurk in the universe.

The team found that the f0(1710) meson decayed at about the right rate to be a glueball. However, the reams of data from the LHC still aren’t enough to rule out the notion that other candidate particles such as f0(1500) are in fact the glueball, Brunner said.

"We need a more thorough understanding of the decay rates of the involved particle," Brunner told Live Science.

However, that may come soon. "The data relevant for our prediction is being taken right now," and the results could be analyzed within the year, Brunner said.

The findings were reported Sept. 21 in the journal Physical Review Letters.

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