25 Feb 09
Originally posted by DeepThoughtBut quarks cannot exist in isolation, so their mass is merely of theoretical interest. In any case a proton is not merely the "sum" of 2 up quarks and a down quark.
According to the particle data-group the mass of the up quark is between 1.5 and 3.3 MeV/c² the mass of the down quark is 2.5 to 5.0 MeV/c². From this you´d expect the mass of a proton to be, at most, 11.6 MeV/c². The actual mass is 938 MeV/c². So here is a clear example of the whole being a couple of orders of magnitude larger than the sum of the parts.
25 Feb 09
Originally posted by KazetNagorraThey can exist in isolation, you just have to get a temperature higher than the deconfining transition.
But quarks cannot exist in isolation, so their mass is merely of theoretical interest. In any case a proton is not merely the "sum" of 2 up quarks and a down quark.
The mass is of experimental interest, in small x scattering experiments the quarks behave essentially like free particles. In order to compare the experimental results with the theory you need to know where the pole of the propagator is, although I´ll grant you that a lot of the time the energy is so high that you can neglect the mass anyway. The particle data group information comes from this type of experiment.
The other stuff in protons are virtual particles. They are not physical. Gluons are massless in the theory anyway.
25 Feb 09
Originally posted by DeepThoughtOkay, I should have said they cannot exist in isolation in a bound state.
They can exist in isolation, you just have to get a temperature higher than the deconfining transition.
The mass is of experimental interest, in small x scattering experiments the quarks behave essentially like free particles. In order to compare the experimental results with the theory you need to know where the pole of the propagator is, although I ...[text shortened]... rotons are virtual particles. They are not physical. Gluons are massless in the theory anyway.
