London: Scientists may have discovered the long-sought-after exotic particle ‘glueball’ which is made up entirely of gluons – the “sticky” particles that keep nuclear particles together. Glueballs are unstable and can only be detected indirectly, by analysing their decay. This decay process, however, is not yet fully understood.
Professor Anton Rebhan and Frederic Brunner from Vienna University of Technology, TU Vienna, have now employed a new theoretical approach to calculate glueball decay. Their results agree extremely well with data from particle accelerator experiments. This is strong evidence that a resonance called “f0(1710),” which has been found in various experiments, is in fact the long-sought glueball. Further experimental results are to be expected in the next few months.
Protons and neutrons consist of even smaller elementary particles called quarks. These quarks are bound together by strong nuclear force. “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 Rebhan.
Gluons can be seen as more complicated versions of the photon. The massless photons are responsible for the forces of electromagnetism, while eight different kinds of gluons play a similar role for the strong nuclear force. However, there is one important difference: gluons themselves are subject to their own force, photons are not. This is why there are no bound states of photons, but a particle that consists only of bound gluons, of pure nuclear force, is in fact possible.
In 1972, shortly after the theory of quarks and gluons was formulated, the physicists Murray Gell-Mann and Harald Fritsch speculated about possible bound states of pure gluons (originally called “gluonium,” today the term “glueball” is used).
Several particles have been found in particle accelerator experiments which are considered to be viable candidates for glueballs, but there has never been a scientific consensus on whether or not one of these signals could in fact be the mysterious particle made of pure force. Instead of a glueball, the signals found in the experiments could also be a combination of quarks and antiquarks.
“Unfortunately, the decay pattern of glueballs cannot be calculated rigorously,” said Rebhan. Simplified model calculations have shown that there are two realistic candidates for glueballs: the mesons called f0(1500) and f0(1710).
For a long time, the former was considered to be the most promising candidate. The latter has a higher mass, which agrees better with computer simulations, but when it decays, it produces many heavy quarks (the so-called “strange quarks”).
The calculated decay pattern into two lighter particles agrees extremely well with the decay pattern measured for f0(1710), researchers said.