What exactly is the higgs boson? If it exists, what role does it play in particle physics? And if it exists, what will that mean?
Sorry, I don't know much about Higgs boson. Higgs boson is an elementary particle, like photon, electron etc. In the Standard Model, Higgs boson gives mass to everything. If it exists, it will mean that the Standard Model passed a very important test.
This is a very loaded question Please Register or Log in to view the hidden image! How detailed of an answer do you want? (I could go for pages!)
In particle physics symmetries are very important. Imposing them on a particle physics theory has important consequences, for instance translational and rotational symmetries give rise to the conservation of linear and angular momentum. More abstract symmetries give rise to all the known forces, however as the Standard Model is understood, these symmetries are known not to hold at ordinary energy scales. It is similar to the idea that at high temperatures magnets lose their mangetism (the domains get aligned randomly), but at low temperatures spontaneous magnetisation breaks this symmetry of the magnet. The Higgs boson is needed to explain how the symmetries involved with the high energy "electroweak" force are spontaneously broken into regular electromagnetism and the weak nuclear force, during the process of which all the particles aquire masses from the Higgs field. If the LHC discovers the Higgs boson it means it will also be able to directly probe the high-energy unified "electroweak" force which will be very interesting, and it will be a triumph of pure theoretical physics. If it doesn't find the Higgs boson then our understanding of electroweak symmetry breaking is all wrong and the world of particle physics will receive a fairly serious shakeup.
As detailed as you feel tyou want to get. As long as you can keep it relatively simple. I'm a amateur. I've read lot fo books about physics, but I still struggle with a lot of it.
Analogy would be that empty space has full rotational symmetry, but near Earth's surface this symmetry is broken by the gravitational field (or the walls and furniture if you are in a room). Higgs is like the gravitational field (or the room walls). I learned this from Gross's talk.
Hi Kurros, are you in particle physics? I wasn't aware of the standard model symmetries breaking down at higher energies. I know there are GUTs which suggest they are a subset of a larger group of symmetries such as SU(5), but within the Standard Model I don't think there's any breaking of the SU(3)xSU(2)xU(1) symmetry. The way I learned it was that the vacuum expectation value of the Higgs is introduced so as to break this symmetry, and then it's combined with mass terms which also break the symmetry, in such a way that the combination as a whole still preserves the symmetry. In plain English, introducing the Higgs particle allows the mathematics of the standard model to be tweaked so that particle masses can be introduced. Without the Higgs mechanism, the fundamental symmetries underlying the theory forbid the introduction of particle masses. As I understand it, all the particle masses in the universe depend on the vacuum expectation value of the Higgs field, and I'm not sure if this value can be tweaked or if it depends at all on the Higgs field's own self-interactions. What I do know is that when you introduce the Higgs mechanism into the standard model, the resulting Higgs particle(s) it describes will have an effect on ordinary processes, making small changes to the probabilities for various particle reactions to occur.
I agree with everything up to here. We understand at least one other good way to break electroweak symmetries. If we don't find the higgs, then we'll find something LIKE the higgs, such as technicolor. We like the higgs because it fits the models we have, and it really is the simplest option. But Nature doesn't care what we think is simple Please Register or Log in to view the hidden image!
There's generally little argument that the Higgs mechanism will be seen with regard to the electroweak sector. What precisely mediates that is another question, some people think the Higgs boson, others some kind of quark condensate or technicolour. The notion of a fundamental scalar doesn't appeal to some.
Well, at energies below about 100 GeV, the symmetry IS SU(3)xU(1). ``Spontaneously broken'' means that the symmetry of the vacuum doesn't respect the symmetry of the full theory. This is analogous to, say, SU(5) breaking to the SM. Somehow, some particular vacuum of SU(5) was chosen such that exactly an SU(3)xSU(2)xU(1) subgroup survived. How this happened is not known---I think my advisor did some work on this in the 1980's. The best way to think of it is in terms of perturbed harmonic oscillators, which is exactly what we're dealing with. Suppose you add a small bump, proportional to some (small) number m, at the bottom of your harmonic oscillator. Now do quantum mechanics. You quantize the thing and calculate the spectrum. What you should find is that the energy levels look like E_n + a_n m, for some dimensionless numbers a_n which can be positive or negative. The perturbation looks is proportional to m, which means that as E gets bigger and bigger, you can neglect the perturbation altogether. This is what happens in the very early universe, when the higgs mass (100 GeV) is much smaller than the radiation temperature. The particles in the primordial soup don't know about the bump at the bottom of the well because it's too small. Now fast forward some. As the energy levels become comparable to m, the wavefunction starts to notice that there's a little bump at the bottom of the well. Eventually, if we take enough energy out of the system, the wavefunction has to find a ground state, which will be some linear combination of ``to the left of the bump'' and ``to the right of the bump''. This is what happens to the higgs field: as the universe cooled, the higgs field realized that there was a little bump at the bottom of the well, and it had to choose a ground state. The ground state that it chose only respects SU(3)xU(1), and not the full SU(3)xSU(2)xU(1) (or even SU(5) ) of the standard model (or GUT). Of course, if you start to probe energies above 100 GeV, or distances below 1/(100 GeV), you'll start to notice new symmetries that you hadn't seen before.
What I heard is that scientists used to called it the "Goddamn Particle" because they were having trouble finding it in their detectors. Later it got shortened to "God" particle, and in popular books these days they probably think the "God" part has something to do with it generating masses.
WARNING TO BISHADI Bishadi--- You will not post as an expert on topics that you have no expertise in. Future offenses will result in a ban.
Ok so maybe I was a little melodramatic with that one Please Register or Log in to view the hidden image!. You mean SU(2)xU(1) of course. I am in particle physics, but I can't claim to be too much of an expert because I'm still learning a lot of this myself. I'm doing my PhD in particle phenomenology, currently looking at what kinds of supersymmetry theories are most likely to be observable at the LHC and other upcoming experiments. As for the symmetry breaking in the SM, BenTheMan said it pretty well, except for getting the groups slightly mixed up, by accident I am sure Please Register or Log in to view the hidden image!. This has actually been confusing me for a little while but the way I think it goes is that at high energies the SM respects the gauge group SU(3)xSU(2)xU(1). These are Lie groups (i.e. continuous symmetry groups), and associated with them are what are known as 'generators', which one can use to 'generate' all the elements of the groups. SU(3) has 8 generators, which are associated with the 8 gluons of the strong force (I haven't figured out exactly how yet...), but the correspondence is not so direct for the SU(2)xU(1) electroweak group. At high energies the conserved charge associated with the U(1) group is 'weak hypercharge', and the gauge bosons associated with these groups are all massless. During electroweak symmetry breaking the SU(2)xU(1)_Y (Y for hypercharge) group is broken down to the U(1)_EM group (EM for electromagnetic, the conserved charge is the regular electric charge), which is a different 'version' of U(1). The SU(2) gauge bosons acquire masses during the process and you get the massive W and Z bosons and the massless photon. In other words the 4 gauge bosons associated with the electromagnetic and weak forces (photon, W^+/- and Z^0) are actually funny mixtures of the higher energy SU(2)xU(1)_Y gauge bosons. So at high energies you don't really have the photon, W^+/- and Z^0, you have 4 different, massless, gauge bosons instead. If someone wants to correct me I welcome it very much because I need to know this stuff Please Register or Log in to view the hidden image!.
After EWSB, the symmetry of the vacuum is SU(3) x U(1). QCD x EM. You only know that SU(3)xSU(2)xU(1) is the symmetry of the full theory if you probe shorter length scales/higher energy scales. This is what spontaneous symmetry breaking means: the symmetry of the vacuum is different from the symmetry of the full theory. That sounded pretty much right. The Z and the photon are mixtures of the neutral SU(2) gauge boson (typically called W_3) and the U(1) gauge boson, typically called B.
