25 Days of Particles: Day 11

Gluon

Classification: boson
Fundamental: yes
Mass: 0
Interactions: Strong
Spin: 1
Lifetime: stable

So, we have a bunch of particles that spend their merry lives zipping around and interacting with each other. But what does it mean for particles to interact? In our current theories of particle physics, we understand that particles can absorb or emit force-carrying particles, which changes the charges and spins and momentum of the particles involved. Each type of interaction has its own force carrier(s), and each particle will only interact with specific force carriers. These force carriers exist in matter but don't make up matter, and all the known force carriers have spin of one. This makes them vector bosons (with non-zero integer spin) instead of fermions with spin-1/2 like all the matter particles.

To illustrate how important the force carriers can be, consider the proton. The proton has a mass of 938.3 MeV. We know that the proton is made up of three quarks, two ups and a down. It's a little tricky to establish quarks masses, but the highest estimates for the masses of these quarks yields a sum of 12.4 MeV. So where does the other 925.9 MeV of mass come from?

The answer is binding energy, or the energy in the force holding the quarks together. The force carrier for the strong force is the gluon, a massless, spin 1 particle. Since the gluon is massless, it doesn't cost quarks much energy to emit these guys, and the quarks in a proton live in a sea of them; that sea contains most of the energy/mass we associate with protons. Quarks can absorb or emit gluons without changing their identity. A quark and its anti-quark can annihilate into gluons, or gluons can split into quark + anti-quark pairs.

Knowing the properties of gluons explains the properties of the strong force. Since gluons are massless, quarks can emit gluons easily, which means that quarks around each other are always emitting and absorbing gluons and so quarks are going to undergo strong force interactions all the time. But gluons also carry color-charge; there are technically eight types of gluon, for all the possible color + anti-color combinations a gluon can exist as. This means that gluons interact with each other via the strong force, emit more gluons, pull on each other, and in general have a huge party doing particle-type stuff.

So, not only are the quarks in a hadron interacting with all the gluons, but the gluons are also interacting with each other. If you start to pull on something inside the hadron, you introduce more energy, which produces more gluons and quarks. It's like trying to stretch a web that builds itself more connections as you pull--the pull won't get weaker and it would take an infinite amount of energy to separate the pieces completely. Practically, if you kick a hadron that hard, the sea of gluons will start producing quark + anti-quark pairs that will locally neutralize the color charge and the hadron will break into many more hadrons, a stream of which is called a jet.

It is also possible to have a quark radiate a gluon out of a hadron. This is how gluons were formally discovered in 1979. PETRA was colliding electrons with positrons and produced three jet events. Electrons and positrons are both leptons that carry no color charge, so to produce jets at all the leptons must have annihilated and produced a quark + anti-quark pair. The quark pair explained two jets, but the third must have come from a gluon radiated by one of the quarks.

With the current high energy accelerators that collide hadrons, gluons and jets aren't nearly so difficult to come by. Colliders like the LHC and the Tevatron produce them in spades, and anything else you want to study has to be picked out of the mess. Such is the fun of hadrons--you never can really get just one.