Inside protons: origin of the sea quarks

[QuasiParticle]

I have seen in many places, for example here, the statement that the proton consists of valence quarks and sea quarks. I am somewhat confused as to where this picture comes from. The sea quarks are virtual quark-antiquark pairs. I have encountered virtual particles only in the context of perturbation theory. Is this where the idea for the sea quarks is coming from? I find this strange, because the proton is a bound state and it should not be possible to describe it by using perturbation theory. Also, perturbative approaches to QCD only work at high energy.

I know that in lattice QCD they sometimes take into account the sea quarks. This puzzles me even more, since LQCD is advertised as non-perturbative. So how do the virtual particles arise there. I tried to see what they do in LQCD. First of all, the action is replaced, for example, by the Wilson action. Secondly, in the path integral formalism the quark fields are integrated out (in the continuum case) and a fermion determinant remains in the expression for the expectation value. However, this matrix is quite tedious, so they use some tricks to reintroduce the quark fields back into the action. Setting the determinant equal to one corresponds to neglecting the sea quarks. But I'm still lost about how virtual particles enter into the LQCD.

Finally, shouldn't there also be all the other particles "inside the proton"? I mean virtual photons, electrons, and even W & Z, not just virtual quarks. To me the "old picture" of the proton with only three quarks, which interact through the gluon fields, is clearer than trying to include an infinite amount (?) of all kinds of virtual particles. All the peculiarities of the system can be buried into the gluon fields, right? That's where the sea quarks originate in the first place.

Any thoughts?

 

[mfb]

Is this where the idea for the sea quarks is coming from?

The concepts are related. If you have some non-perturbative field, you can still try to "find" particles in it.

Finally, shouldn't there also be all the other particles "inside the proton"?

Sure, but their weaker coupling and/or high mass makes them negligible.

Without the concept of sea quarks, it is hard to explain results from deep inelastic scattering where you see those individual quark contributions.

 

[RGevo]

If you want to resolve point like particles in hadrons, you need high energies.

At this energy, qcd factorisation is the correct approach, and describing evolving partons with probability distributions works well.

There are constraints on quarks, gluons and even photon probability distribution functions. One can even do the same for leptons inside hadrons (it's very small).

Below a GeV or so it doesn't make sense to think about an evolving Parton because the non-factorisable contributions (which are very large when alpha_s is large) would be missed, and multi Parton interactions would be necessary I guess.

Anyway, if your not at energy scales large enough to resolve partons, does it make sense to talk about it in such a way. Thoughts?

 

[QuasiParticle]

The concepts are related. If you have some non-perturbative field, you can still try to "find" particles in it.

Can you elaborate on this?

Sure, but their weaker coupling and/or high mass makes them negligible.

Right, but if we are talking about the constituents of protons on a "fundamental" level, then I suppose everything should be included, regardless of their "amount". But if we do this, is the conclusion that everything is made out of everything..?

Without the concept of sea quarks, it is hard to explain results from deep inelastic scattering where you see those individual quark contributions.

This comment indeed brings up one possible source for the idea of antiquarks in a proton. I had to refresh my knowledge on the subject. If one analyzes the experimental data of deep inelastic scattering experiments in the framework of the quark-parton model, then yes, one is forced to conclude that the proton consists also of a small amount of antiquarks (and actually some mysterious other stuff, which turns out to be the gluons).

The quark-parton model assumes that the proton consists of some number of valence quarks and any mixture of quark-antiquark pairs. All of the quarks are assumed to be non-interacting. One then computes the cross-sections for various lepton-proton collisions as functions of the quark momenta distributions. Comparing these to the experimental data on the cross-sections, the various quark distribution functions can be deduced. The result is that there are contributions from antiquarks as well. However, this does not make any notion of their character being virtual in any way.

In any case, I would perhaps not consider this as a valid justification for the existence of virtual quark-antiquark pairs inside the proton (whatever this actually means), because this result has been obtained by assuming the quark system to be non-interacting, i.e. neglecting the gluon fields.

My intuition says that the quark-antiquarks are the result of some kind of computational approximation/method for the effects of the gluon fields, and this picture of the proton should not be taken too literally. I feel this way, because it is difficult to understand what it would mean if there "really were virtual particles inside the proton". I have always viewed virtual particles as nothing more than a computational/visualizational tool. So my feeling is that the sea quarks are more of a tool to describe the effects of the gluon fields than anything "real".Another thing that came to my mind regarding the lattice QCD: In LQCD do the virtual pairs arise directly from the path integral quantization? Because in the amplitude one needs to integrate over all field configurations. I quess in the lattice computation this might translate to adding virtual particle pairs somehow. But if this is the case then I don't quite understand why they are called virtual. When using the path integral in QM we don't call the integrated spatial variables virtual.
I'm sorry RGevo, I didn't quite capture your point. When you say "factorization" do you mean using perturbation theory?

 

[mfb]

Right, but if we are talking about the constituents of protons on a "fundamental" level, then I suppose everything should be included, regardless of their "amount". But if we do this, is the conclusion that everything is made out of everything..?

That sounds like philosophy.

You have sea quarks from charm and bottom (and a tiny top contribution), but they certainly cannot be real due to their mass.

because this result has been obtained by assuming the quark system to be non-interacting, i.e. neglecting the gluon fields.

Not neglecting the gluon fields, but their interaction with the other partons. Those interactions do not happen at the scale of the deep inelastic scattering. It is like shooting a relativistic particle into a solid object: sure the atoms there have some thermal motion, but this is not relevant for the interactions of the relativistic particle.

No picture of subatomic objects should be taken too literally.

 

[QuasiParticle]

That sounds like philosophy.

Well, I suppose it is philosophy to some degree It just seems a little inconsistent to declare the proton to consist of quark-antiquark pairs and then to neglect all the other particles, which should also exist following the same logic.

Not neglecting the gluon fields, but their interaction with the other partons.

Ah, yes. And the sea quarks in fact arise from the gluons.

No picture of subatomic objects should be taken too literally.

Yes, I suppose so. Sometimes it just seems that these pictures are made to sound too literal even among physicists.Thanks for your contribution, by the way!

 

[Riad]

Why no one is mentioning what the experiment say.. I read somewhere that when you use a low energy probe you sea only three constituents- the quarks, but when you use high energy probes you can see many other centers and these are the sea quarks. Now the question is how many!

 

[mfb]

At low energy you only see hadrons. Once you see quarks you always see the contributions from all (and also gluons) - but the sea quark contribution becomes more important for higher energies.

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