As most of you are aware by now, two weeks ago the elusive Higgs boson was detected at the Large Hadron Collider. Since that time I have been sent several questions about this great discovery, which I will try to answer in this and subsequent blog entries. 

What is the Higgs mechanism/field/particle/boson?

I have written about this in several previous entries, so I won't repeat the details here.   Suffice it to say, that the Standard Model of particle physics only works if all matter started out massless, and gained its mass later. The Higgs field fills the Universe, and by slowing down matter it gives the illusion of mass. This idea has been discussed for decades, but now the experimentalists have detected small fluctuations in the field, and that is what we call the Higgs boson.

Exactly what is a boson?

It is really just another name for a certain type sub-atomic particle. All fundamental particles are classified as fermions or bosons, depending on their spin, interactions with magnetic fields, and ability to co-exist in the same quantum mechanical state. The Higgs is a spin-0 boson, which means it doesn't spin, and infinite numbers of Higgs particles can be in the same place with the same energy.

What is this symmetry that requires massless particles? 

 Its formal name is the SU(2) gauge symmetry. In the Standard Model, three of the four fundamental forces of nature are actually predicted to exist because of symmetry properties of particles. In this case, allowing certain pairs of particles to be interchangeable in the laws of physics automatically produces the weak nuclear forces that cause some forms of radioactive decay. But the symmetry in only present if the particles are initially massless. The Higgs mechanism assumes the particles begin massless, but the difficulty of moving through the Higgs field gives the appearance of mass.

Why is the photon massless?

This is a little hard to explain properly without using a lot of mathematics, but it is a requirement of the Higgs model. As I wrote in the answer to the previous question, the electromagnetic and weak nuclear forces are generated in the Standard Model by requiring certain pairs of particles to be interchangeable (I will use X,Y as generic particles for the arguments). This requires four particles to transmit the force (one that changes a type X particle into type Y, one that change Y into X, and two that leave X and Y unchanged). It also requires two separate Higgs fields, so that the two Higgs fields can be interchanged and maintain the same symmetries. But to generate the masses, the two Higgs fields must lose this symmetry property, and change to one field (technically a combination of the original two). The result is that only three of the force carriers interacts with this new Higgs and gain mass, and the fourth one remains massless. It is this fourth particle that we call the photon. But it is worth noting that the electric charges and nuclear charges depend on how the Higgs fields collapsed, and are truly random selection.