What is an Axion?

[IDMclean]

Long the Fixation of Physicists Worldwide, a Tiny Particle Is Found

I was prompted on reading the apparent discovery of the Axion to research what an Axion is.

 

I have found that like Neutrinos, the information for the axion is rare and scattered. Also there seems to be several different kinds Axions, though the information of them is likewise scattered.

 

One thing that peaked my interest is that Cosmological Axions are supposed to be Pseudo-Scalar.

 

They are part of Quantum Chromodynamics (QCD).

 

Anyone know more?

[Erasmus00]

Whenever you have a continuous symmetry that gets broken, you develop what are called gapless excitations. This sounds complicated, but I'll try to explain.

 

Lets say the energy of your system is naturally translationally invariant (if we moved the whole glob somewhere else, it would have the same energy). This is true for instance, of electromagnetic interactions. You normally expect your system has the same translational invariance as the energy equation. For example, a liquid is pretty much the same everywhere: it has the translation invariance.

 

Now, lets say that our liquid freezes and forms a crystal. Now, all of a sudden our system isn't translationally invariant. Some parts (the crystal lattice points) are special. We say the symmetry has been broken. Now that we have lost the symmetry we notice somethings: first, our crystal has a property the old liquid didn't, it is rigid.

 

There is also one new important property: because its rigid it can support a new type of wave called a transverse or shear wave. You can think of it like this: both solids and liquids can support what are called compression waves. Think of it like squeezing the object and it pulsates in and out. A shear wave is like when you take a piece of jello and you wiggle it, the wiggle propagates down the jello. You can't "wiggly" a liquid like that.

 

There is a physical importance of shear waves: you can make shear waves with minimally small energies, this is what we mean by the fact that they are "ungapped." There is no minimal threshhold energy to make one.

 

What does this have to do with axions? Anytime a symmetry gets broken (like translational invariance) we get a new type of ungapped wave in our system. In quantum mechanics, these ungapped waves become particles (particles are the same as waves.) Axions are this type of "ungapped" particle for a certain symmetry that gets broken in the theory of nuclear matter QCD.

-Will

[InfiniteNow]

Will, thank you. Those are terms I can follow.

[InfiniteNow]

From wiki:

 

A pseudoscalar, when multiplied by an ordinary vector, becomes a pseudovector or axial vector; a similar construction creates the pseudotensor.

Hence, one can presume, the decision to call it an axion.