Posted: Oct 03, 2011 12:39 pm
by twistor59
zaybu wrote:
mizvekov wrote:By the way, I forgot to add to the last post that I think there is another potential source of confusion going on around here.
QFT has a definition of real particles, which is contrasted with virtual particles, and the former are what correspond to ins and outs of feynman diagrams, while the later are the internal arrows.



If you take that position, as I have already pointed out to Twistor, then quarks, gluons, W's and Z bosons are never in these in and out states, and by that arguments, they are not real. Ditto with the Higgs boson.

I believe that "virtual" in QFT means "cannot be seen while the exchange takes place". It doesn't mean they are fictional. They are as real as any other "particles", and that's the gist of my arguments: at higher energies, what exist are just particles.



Looking at the electroweak theory, there are two types of scenario:
1) A case where a W-interaction takes place as an internal line, like this.


Muon_Decay_j.jpg


Muon_Decay_j.jpg (9.01 KiB) Viewed 808 times





Here, the phrase “a virtual W- is produced, which subsequently decays to electron and antineutrino” is often used. These are the cases where I’m claiming that the description of the internal line as a “virtual W particle” can be a bit misleading.

The second type of scenario is
2) A case where W particles are produced in the out-state. In this case the W’s are real and their properties can be measured.


WProdn.jpg


WProdn.jpg (9.27 KiB) Viewed 808 times




(the thing that came out as a question mark was meant to be a gamma, i.e a virtual photon).

Same for the Higgs. We have our EWSB models where the Higgs interacts with other particles giving them mass, but these are “virtual Higgs” interactions. That’s why we need to go to all the expense of trying to produce a real Higgs, which will appear in an out-state.

The situation with quarks and gluons is a bit trickier. I don’t have a working knowledge of QCD, but the impression I get is that you can’t do perturbative S-matrix calculations in the usual way because the in and out-states would have to contain free quarks and/or free gluons and this is not possible. Instead of free quarks/gluons what you see in the lab is hadronization/jets. There are, however, a set of tricks that you can use to apply perturbation theory to compute jet cross sections.

Although you can’t isolate a quark or gluon, you can do experiments to estimate their properties. They are real particles, by any reasonable criteria for the definition of “real”.