See subject.
It’s hard to keep up, particularly after the Higgs craziness that penetrated down to tabloid journalism explanations and declarations of profundity.
For decades, scientists have been looking for so-called “glueballs”. Now it seems they have been found at last. A glueball is an exotic particle, made up entirely of gluons – the “sticky” particles that keep nuclear particles together. Glueballs are unstable and can only be detected indirectly, by analyzing their decay. This decay process, however, is not yet fully understood.
Professor Anton Rebhan and Frederic Brünner from TU Wien (Vienna) have 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.
It’s not as exciting a discovery as the Higgs was, but it’s still pretty exciting. On the layman’s level, this is a massive particle which is composed entirely of massless particles, and that’s kind of cool. On a deeper level, glueballs represent the purest manifestation imaginable of the Strong Force (or rather, of the Color Force, of which the Strong Force is only a limited special case), and so study of glueballs could potentially give us a lot of insight into that force, which we still understand only relatively poorly.
Also, perhaps a more answerable question:
What is “a resonance,” and why is it (or a trace of, which is what it sounds like) an actual constituent of matter?
A “resonance” is easy. When you slam particles together, you get certain outcomes of the collision. Some particular outcomes become much more likely, at certain specific energies of the collision. It’s like how a tuning fork vibrates more strongly when exposed to just the right frequency of sound, hence why it’s called a resonance. Our current models explain particle resonances as being due to other intermediate particles being created at those energies: The particles themselves don’t last long enough to be easily detected directly, but they make the reaction as a whole much easier, and so show up in that way.
As for “an actual constituent of matter”, that’s more tricky, as it depends on what you mean by “constituent”. Quarks are certainly real, and they make up protons and neutrons, which are also certainly real, and they in turn make up a large portion of the matter we know and love. They’re always interacting with each other, by means of exchanging gluons, but the gluons so exchanged are virtual, not real. And those gluons aren’t generally all tangled up into discrete glueballs.
A glueball isn’t per se a constituent of matter, rather, it’s something you get if you stick some ‘constituents’ of matter, namely gluons, together, and is itself much too short-lived to have any impact on everyday physics. (I say ‘constituents’ because the gluons really are force-carrying particles, that is, they mediate the interaction that makes the actual constituents of matter stick together.)
A resonance, for all practical purposes, is a short-lived particle that’s actually so short-lived people feel uneasy attributing the particle moniker to it; the name derives from the fact that they show up as peaks in the cross section when you ramp up the energy, meaning that when the system is ‘in resonance’, you’ll create the new particle. EDIT: In other words, what Chronos said.
And the glueball discovery is exciting because the strong force is very difficult to handle mathematically—the reason is precisely its strength: for weak enough forces, one can treat the interaction as just being a very small effect, a perturbation of the no-interaction case, but this breaks down for the strong force, necessitating computationally intractable simulations and the like. The discovery of the glueball helps to nail down the parameters in these simulations.