Internal quark-quark EM forces of particles?

[ianhoolihan]

Hi all,

So quarks are given fractional charges, which then add to the total charge of the particle they constitute. My question is if the electromagnetic forces between quarks are taken into account? I was thinking that such things might be automatically taken care of via Feynman diagrams etc, but I don't think QFT deals well with bound states...

As a side point, presumably there is then a charge distribution inside particles, depending on the charges of the quarks. Is there empirical evidence showing effects of this? I presume it does not show up in things such as the hydrogen spectrum as the electron is sufficiently far from the proton for it to be negligible...?

All thoughts appreciated.

Ianhoolihan

 

[Rooted]

Hi,

I think the internal structure of hadrons is comparatively poorly understood and there are still many unanswered questions about it. Quarks interact with each other through the strong force, and this is two orders of magnitude greater than the electomagnetic force at the fermi scale, so has a much smaller role to play.

From a distance of a few fermi away a nucleus has a saxon-woods type potential, but the further you get into the core of a hadron the stranger things get. There are lots of experiments ongoing and future experiments planned to discover more about this very subject...

 

[tom.stoer]

My question is if the electromagnetic forces between quarks are taken into account?

afaik the el.-mag. forces are always neglected when calculating QCD bound states, simply the strong interactions dominate other forces

I was thinking that such things might be automatically taken care of via Feynman diagrams etc ...

not automatically, but if you draw and calculate *all* diagrams ...

... but I don't think QFT deals well with bound states...

I guess you mean *perturbative* QFT; QFTs can be defined non-perturbatively and can deal with bound states, of course; there are calculations regarding color confinement using non-perturbative, canonical quantization, and there is of course the lattice gauge quantization dealing with hadron masses, form factors and things ike that (afaik with an error less than 5%)

... there is then a charge distribution inside particles, depending on the charges of the quarks. Is there empirical evidence showing effects of this?

yes, but not in the ordinary position space distribution; one studies nucleon structure functions in deep inelastic electron-nucleon scattering; form factors are - roughly speaking - the Fourier transform of charge and current densities measured in elastic scattering whereas the structure functions are measured in inelastic scattering.

 

[ianhoolihan]

I think the internal structure of hadrons is comparatively poorly understood and there are still many unanswered questions about it. Quarks interact with each other through the strong force, and this is two orders of magnitude greater than the electomagnetic force at the fermi scale, so has a much smaller role to play.

Fair point.

From a distance of a few fermi away a nucleus has a saxon-woods type potential, but the further you get into the core of a hadron the stranger things get. There are lots of experiments ongoing and future experiments planned to discover more about this very subject...

But the saxon-woods potential is still spherically symmetric...?

Nonetheless, I'm glad I've asked a question which doesn't have a silly answer and make me feel like a fool!

Crikey, banana cake has been in the oven way to long!

 

[ianhoolihan]

not automatically, but if you draw and calculate *all* diagrams ...

Yes, by "automatically" I mean "if you include enough diagrams".

I guess you mean *perturbative* QFT; QFTs can be defined non-perturbatively and can deal with bound states, of course; there are calculations regarding color confinement using non-perturbative, canonical quantization, and there is of course the lattice gauge quantization dealing with hadron masses, form factors and things ike that (afaik with an error less than 5%)

Actually, I'm just beginning QFT, so what you've said is beyond me. However I do always hear that QFT generally works only for initial and final states that are of free particles. Clearly I have a lot to learn.

yes, but not in the ordinary position space distribution; one studies nucleon structure functions in deep inelastic electron-nucleon scattering; form factors are - roughly speaking - the Fourier transform of charge and current densities measured in elastic scattering whereas the structure functions are measured in inelastic scattering.

Glad to know there is evidence, though again, understanding what you've said is beyond me.

 

[tom.stoer]

no problem; you should simply be aware of the fact that QFT in textbooks and lectures sometimes means "perturbative QFT" and that there is something beyond ...

... you may want to start here http://en.wikipedia.org/wiki/Lattice_QCD

 

[mfb]

As a side point, presumably there is then a charge distribution inside particles, depending on the charges of the quarks.

The uncertainty in the theoretic prediction for the hyperfine structure in hydrogen is dominated by the (incomplete) knowledge of the charge distribution in a proton.

 

[ianhoolihan]

The uncertainty in the theoretic prediction for the hyperfine structure in hydrogen is dominated by the (incomplete) knowledge of the charge distribution in a proton.

That makes sense. Thanks for pointing that out.