Terms to describe the strange world of quantum physics have come to be quite common in our lexicon. Who, for instance, hasn't at least heard of a quark, or a gluon or even Schrodinger's cat? Now there's a new name to remember: "Glueball." A long sought-after exotic particle, and recently claimed to have been detected by researchers at TU Wien, the glueball's strangest characteristic is that it is composed entirely of gluons. In other words, it is a particle created from pure force.

First mooted as a particle in 1972 when physicists Murray Gell-Mann and Harald Fritsch wondered about possible bound states of recently-discovered gluons, scientists have sought the particle in the intervening decades. Originally dubbed "gluonium," but now called glueballs, these strange particles of pure force are exceptionally unstable and can only be indirectly detected by monitoring their decay as they disassemble into lesser particles.


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More recently, physics Professor Anton Rebhan and his PhD student Frederic Brünner from TU Wien have theorized that a strong nuclear decay resonance, called f0(1710), observed in the data from a number of particle accelerator experiments is strong evidence for the elusive glueball particle.

Quarks are small elementary particles that make up such things as neutrons and protons. Binding these quarks together is the strong nuclear force which, in turn, couples the larger particles.

"In particle physics, every force is mediated by a special kind of force particle, and the force particle of the strong nuclear force is the gluon," said Professor Rebhan.

Elementary particles come in two kinds: those that carry force (bosons), such as photons, and those that make up matter (fermions), such as electrons. In this context, gluons may be viewed as more complex forms of the photon. However, as photons are the force carriers for electromagnetism, gluons exhibit a similar role for the strong nuclear force. The major difference between the two, however, is that gluons are able to be influenced by their own forces, whereas photons are not. As a result, photons cannot exist in force-bound states, though gluons, which are attracted by force to each other, make a particle of pure nuclear force possible.

In this way, many researchers believe that many of the unexplained particles discovered in particle accelerator experiments could indicate the presence of pure nuclear force particles, or glueballs. Contentiously, however, some scientists are of the opinion that the signals detected in the experiments may also just be some sort of conglomeration of quarks and antiquarks. This is particularly difficult to prove either way, though, as whatever the mysterious particle is it is too short-lived to be directly detectable.

Nevertheless, two mesons (a meson is a subatomic particle composed of one quark and one antiquark), entitled f0(1500) and f0(1710) have been determined via calculations to be the most likely candidates for the glueball particle. For some time, scientists believed that f0(1500) met many of the mathematical criteria for being the front-runner as the glueball particle, although much of this bias was also largely due the fact that many researchers believed that the production of heavy (strange) quarks in the decay of f0(1710) was implausible because gluon interactions do not normally distinguish between heavier and lighter quarks.

"Unfortunately, the decay pattern of glueballs cannot be calculated rigorously," said Professor Rebhan. "Our calculations show that it is indeed possible for glueballs to decay predominantly into strange quarks."

Despite the inconsistencies to accepted quark behavior, the decay pattern calculated by the two TU Wien researchers, which shows disassembly into two lighter particles, actually lines-up exceptionally well with the pattern measured for f0(1710). The researchers have shown that other decay patterns into two particles or more is possible, and have also calculated their decay rates.

Though these alternative glueball decays have yet to be measured, two experiments to be conducted at the Large Hadron Collider at CERN (TOTEM and LHCb) and one accelerator experiment in Beijing (BESIII) over the next few months are expected to produce data that will hopefully support the TU Wien researcher's hypothesis.

"These results will be crucial for our theory," said Professor Rebhan. "For these multi-particle processes, our theory predicts decay rates which are quite different from the predictions of other, simpler models. If the measurements agree with our calculations, this will be a remarkable success for our approach."

If the measurements and calculations do, in fact, agree, the evidence for f0(1710) being a glueball would be highly credible. Such a confirmation would also once again demonstrate that higher dimensional gravity research can be effectively utilized to solve particle physics problems. According to the researchers, this would be one more overarching support of Einstein’s theory of general relativity, the centenary of which occurs next month.

The results of this research were recently published in the journal Physical Review Letters.

Source: TU Wien