by Steve Bryson
The electroweak theory is the theory that describes both the electromagnetic and the weak nuclear force. Therefore electroweak theory contains quantum electrodynamics (QED) as a subtheory. In fact, electroweak theory ways that the electromagnetic force is not really a force by itself, but is rather a part a more general force, the other part of which is the weak nuclear force. In this way, the electroweak theory is really a unified field theory, unifying the electromagnetic force with the weak nuclear force. This in itself is remarkable enough, but there is more. We must, of course, explain why the electromagnetic force looks so different from the weak force, and in so doing we discover such things as why particles have mass, why the weak force appears as a short range, not a long range force, and we predict the existence of previously unknown particles (three of which have since been found!). We also find hints to the answers to such questions as why are there different flavors of quarks and what is the difference between an electron and a neutrino.
We spent the last two classes talking about the electromagnetic force and QED, the quantum theory that describes it. So let us introduce ourselves to the weak nuclear force. Unlike the other forces that we have discussed, the weak force shows up in two ways. This force gives very small pushes to particles (thus the name weak), and the weak force changes the identity of particles. Thus the weak force is responsible for the decay of, for example, a muon into an electron, an electron anti-neutrino, and a muon neutrino. Another example is the decay of a down quark into an up quark and an electron and an electron anti-neutrino. This last example is observed when a neutron (made of an up and two downs) decays into a proton (made of two downs and an up) and an electron and an electron anti-neutrino. There are many more examples of such changes in identities.
These changes in identities are now understood (via the full electroweak theory) to be due to two particles called the W+ and the W-. These W particles play the same role in the weak interaction that the photon plays in QED. In other words the W particles carry the weak 'force', and they do so by changing the identity of particles (there is a third particle which also carries the weak force, the Z0, but more about that later). Basically, the W's will turn an electron into an electron neutrino, or a muon into a muon neutrino, or a tau into a tau neutrino. The W's will also turn a down (or strange or top)quark into an up (or charm or bottom) quark. All of these changes can be reversed.
The other particle that carries the weak force is the Z0 particle, which acts more like the photon in that it changes the motion of the particles that it interacts with. In this way, neutrinos (which have neither electromagnetic charge nor color charge, remember) can interact and 'push' on the other particles (though this push is very weak). The push due to the Z0 particle is called 'the weak neutral current'.
This completes the summary of the effects of the weak nuclear force. To understand just how and why this force acts the way it does, we must get into the full blown electroweak theory.
As I said above, the W+, W-, and the Z0 particles are analogous to the photon of QED in that they carry the (in this case) weak force. These three particles are quite different from the photon, however, in that they are massive (about 90 times heavier than the proton) and the W+ and the W- carry an electromagnetic charge (positive and negative, respectively). It is the massiveness of these particles that is responsible for the fact that the weak force is a short range force. Due to their large masses, these particles are very short lived particles, living only on the order of 10-25 seconds.
It turns out that what makes these force carrying particles so massive is the same thing that gives all the massive particles their masses! This is the structure of the electroweak force. In this way we understand the masses of all the particles to be related and perhaps due to the electroweak interaction. In other words, if there were no electroweak force, it may be that none of the particles in nature would have any mass!
To get an account of what it is about the structure of the electroweak theory that gives all of these remarkable features, you should now read over the Gauge Theories section of the handout from session 3. Here you will find a description of the concept of gauge symmetry which is the fundamental theoretical underpinning of all the theories that we have discussed. This symmetry is not as complete in the electroweak model as it is in, for example, QCD, and this lack of symmetry is directly responsible for all of the features of the electroweak theory described above. The symmetry is said to be 'spontaneously broken', spontaneous because nobody knows why it happens in the electroweak theory and not elsewhere.
In order to describe the spontaneous symmetry breaking of the electroweak interactions, physicists had to introduce a new particle called the Higgs boson. This particle has not been observed, and there are some theoretical reasons to doubt that it is fundamental -- it might be a bound state of as yet unknown fermions. There are no predictions as to what its mass is, nor are there any specific predictions as to its properties aside from its being a boson. For this reason it is very difficult to say that it does not exist (it may simply be too heavy to make in current accelerators), so finding it would settle a lot of questions.
Using spontaneous symmetry breaking and the Higgs boson we have been able to predict such things as the existence and masses of the three particles that carry the weak force (W+, W-, and Z0) before they were discovered in 1983. This is a spectacular confirmation of the electroweak theory.