A How can quarks exist if they are confined?

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Avodyne

Quarks and gluons most definitely "exist" in the sense that the quantum field theory that we believe describes hadrons, QCD, has quark fields and gluon fields in it, in exactly the same way that quantum electrodynamics (QED) has electron/positron fields and photon fields in it. If we set the QCD coupling constant to zero, we get a QFT of free quarks and gluons. Though there is not yet a fully mathematically rigorous construction of QCD, there is ample evidence that we can start with a lattice version and take the zero-spacing limit, with the result being the correct QFT of hadrons. In this sense, QCD is on a much better mathematical footing than QED, which is expected to be a trivial theory that cannot survive removal of the cutoff (equivalently, taking the lattice spacing to zero).

Haelfix

I don’t understand the statement that the SU(3) lattice gauge theory doesnt include quarks and gluons...

I also don’t understand which *physical Hilbert space* we’re talking about. Clearly we aren’t discussing the perturbative Hilbert space of QCD, which is the only one I know how to rigorously construct given a certain regularization scheme and which is only valid where we take a certain asymptotic limit after carefully modding out the various unphysical degrees of freedom. Still this is a valid physical sector of the as yet to be discovered full theory, so you can’t just ignore it, and this sector clearly includes Quarks and Gluons and is verified by deep inelastic scattering amongst many other experiments.

So presumably the statements in this thread are about the as yet to be defined full nonperturbative Hilbert space of the theory. But again which part? QCD exhibits different critical behavior at various phases of the theory, which is usually captured by a variety of different ad hoc methods. Most of which includes quarks and gluons at some level, but some of the resulting theories don’t necessarily agree on their overlap.

I think a lot of the ‘what are the fundamental DOF of QCD’ debates have sort of ended with the duality revolutions, where it became clear that vastly different descriptions could lead to the same underlying physical predictions

A. Neumaier

QCD, has quark fields and gluon fields in it, in exactly the same way that quantum electrodynamics (QED) has electron/positron fields and photon fields in it.
But in QED, the transversal photon field is physical, while in QCD, quark fields cannot be made physical by taking the transversal part.
I don’t understand the statement that the SU(3) lattice gauge theory doesnt include quarks and gluons...
The claim by DarMM was only that it does not contain ghosts.
presumably the statements in this thread are about the as yet to be defined full nonperturbative Hilbert space of the theory. But again which part?
Yes. Most of it refers to the vacuum sector (zero density and temperature), for which Wightman's axioms are expected to hold.
I think a lot of the ‘what are the fundamental DOF of QCD’ debates have sort of ended with the duality revolutions, where it became clear that vastly different descriptions could lead to the same underlying physical predictions
But the physical Hilbert space defining the vacuum representation should still be the same, in the sense of carrying an isomorphic representation of the algebra of observables.

This is analogous to the well-known fact from representation theory of finite-dimensional Lie algebras that the same representation can be obtained in very different ways. Nevertheless, the irreducible unitary representations are uniquely determined by a few quantum numbers.

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DarMM

Gold Member
Quarks and gluons most definitely "exist" in the sense that the quantum field theory that we believe describes hadrons, QCD, has quark fields and gluon fields in it, in exactly the same way that quantum electrodynamics (QED) has electron/positron fields and photon fields in it. If we set the QCD coupling constant to zero, we get a QFT of free quarks and gluons.
That's not just some simple $g \rightarrow 0$ limit though. Removing the coupling prevents it from being a gauge theory as it removes the gluon self-coupling terms, so it's a completely different (non-gauge) theory, a singular limit as such, just think of it in terms of the SU(3) fiber bundle. It's quite unlike QED, where sending $e$ to zero just decouples electrons.

Also, related to what @A. Neumaier said, some of the photon's degrees of freedom are physical, however the gluon and quark fields the entirety of their degrees of freedom map out of the Hilbert space.

DarMM

Gold Member
I don’t understand the statement that the SU(3) lattice gauge theory doesnt include quarks and gluons...
It doesn't include ghosts.

I also don’t understand which *physical Hilbert space* we’re talking about.
Same as in QED, the physical Hilbert space is a subset of the whole unphysical (Hilbert-Krein) space selected by a BRST condition or similar.

Still this is a valid physical sector of the as yet to be discovered full theory, so you can’t just ignore it, and this sector clearly includes Quarks and Gluons and is verified by deep inelastic scattering amongst many other experiments.
I don't think perturbation theory is a physical sector, it's a method of approximating calculations which is very accurate (once you don't include too many terms) within a certain range of momenta and energy, but that's not a physical sector to me.

The whole point of the thread is that it is not so clear it includes Quarks and Gluons, as they map out of the Hilbert Space and thus aren't well-defined operators on the physical Hilbert space. You can use Quark and Gluon fields, but it's not so clear it contains them any more than it contains Faddeev-Poppov ghosts

Haelfix

It doesn't include ghosts.
Sorry I apologize I misread your statement, but to be fair it was a little hard to follow b/c Lattice QCD *is* a nonperturbative approach and it manifestly does include quarks and gluons. So i'm failing to square the premise a little. How can a nonperturbative approach explicitly calculate the masses of an object quite precisely, and then the very same theory in principle somehow not include those objects? Note that this is more general than even lattice qcd... Other nonpertubative objects (like Wilson loops) can also be interpreted as extended quark anti quark pairs undergoing some sort of displacement.

Same as in QED, the physical Hilbert space is a subset of the whole unphysical (Hilbert-Krein) space selected by a BRST condition or similar.
Sure but that's the physical Hilbert space of the pertubative approach. It's not really applicable in the same way in lattice qcd for instance.

I don't think perturbation theory is a physical sector, it's a method of approximating calculations which is very accurate (once you don't include too many terms) within a certain range of momenta and energy, but that's not a physical sector to me.
If pertubation theory didn't map out that corner of the theory (high momentum transfer/deep UV), we wouldn't be able to say much about QCD at all and we wouldn't know anything about asymptotic freedom. Instead I view the pertubative method as the solution for the problem Avodyne mentioned above, namely the limiting behaviour of the theory when you send the QCD coupling constant to zero. So those states must clearly be included in the full Hilbert space of the theory, whatever else you may think of it.

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Haelfix

But the physical Hilbert space defining the vacuum representation should still be the same, in the sense of carrying an isomorphic representation of the algebra of observables.

This is analogous to the well-known fact from representation theory of finite-dimensional Lie algebras that the same representation can be obtained in very different ways. Nevertheless, the irreducible unitary representations are uniquely determined by a few quantum numbers.
This should definitely be true but note that in more general dualities, you can have a homomorphism of Hilbert spaces where each end carries very different quantum numbers, so the map can be quite nontrivial in general.

DarMM

Gold Member
Sorry I apologize I misread your statement, but to be fair it was a little hard to follow b/c Lattice QCD *is* a nonperturbative approach and it manifestly does include quarks and gluons. So i'm failing to square the premise a little. How can a nonperturbative approach explicitly calculate the masses of an object quite precisely, and then the very same theory in principle somehow not include those objects?
No worries about the misreading, it's a long thread even I'm not certain what I've said at times!

What objects does Lattice QCD calculate the masses of that we are saying might not exist here. The discussion here is about quarks and gluons, I don't believe lattice QCD calculates their masses, but takes them as inputs (in the quark case).

Sure but that's the physical Hilbert space of the pertubative approach. It's not really applicable in the same way in lattice qcd for instance.
No, it is also a nonperturbative statement, see for example Nakanishi's book on the Covariant Operator formalism for Gauge theories or Strocchi's book "An Introduction to Non-Perturbative Foundations of Quantum Field Theory". It works there just as well. I don't see what is explicitly perturbative about it unless I'm missing something.

If pertubation theory didn't map out that corner of the theory (high momentum transfer/deep UV), we wouldn't be able to say much about QCD at all and we wouldn't know anything about asymptotic freedom. Instead I view the pertubative method as the solution for the problem Avodyne mentioned above, namely the limiting behaviour of the theory when you send the QCD coupling constant to zero. So those states must clearly be included in the full Hilbert space of the theory, whatever else you may think of it.
First, I wasn't saying perturbation theory was invalid, just that it's not a sector. It's a method useful for calculating in a given regime of momenta, provided you don't sum too many terms.

Secondly, they simply aren't included in the Hilbert Space of the theory, since their states have negative norm. I'm not following the argument why states existing in a singular limit where the fundamental nature of the theory is lost, e.g. $g \rightarrow 0$ losing the gauge/fiber bundle structure, means those states have to be in the Hilbert space of the theory in general. From the analysis of Strocchi, Balaban and Nakinishi they don't seem to.

Haelfix

The discussion here is about quarks and gluons, I don't believe lattice QCD calculates their masses, but takes them as inputs (in the quark case).
I'm far from an expert on QCD, but my understanding is that they can go both ways. The standard way is to take Quark masses as an input and to compute the mass of some Hadron, but conversely you can take the Hadronic spectrum and compute a Quark 'mass'. Where 'mass' as you might expect is a bit of a fuzzy scheme dependent concept inside a strongly interacting composite object (it is certainly not the usual pole in the propagater, considering that there are strongly divergent infrared effects at play). In any event this is an active area of research (see the pdg section on this (p726))

No, it is also a nonperturbative statement, see for example Nakanishi's book on the Covariant Operator formalism for Gauge theories or Strocchi's book "An Introduction to Non-Perturbative Foundations of Quantum Field Theory". It works there just as well.
Ok, I will try to check those books out if I get the time.
Cards on the table, my cursory understanding of confinement/infrared slavery basically begins and mostly ends with this paper:

https://journals.aps.org/prd/abstract/10.1103/PhysRevD.10.2445

as well as some of Seiberg and Witten's work on the subject in the context of deformed N=2 Supersymmetry where they explicitly show the analog of the 'dual Meissner effect' which leads to the emergence of an explicit flux tube via Monopole and Dyon condensation. This is afaik, one of the only known examples of a theory where the mechanism of confinement is understood to some extent (maybe some of the experts here know of others).
Note that in this picture, it's somewhat subtle to ask question about the nature of the fundamental degrees of freedom, analogous to the situation in string theory where you can ask a similar sort of question about strings vs Dbranes. In any event, while this picture is known to not be quite right for QCD, it explains many of the desired features (Regge behavior, the linear confining potential etc).

"I'm not following the argument why states existing in a singular limit where the fundamental nature of the theory is lost, e.g. g→0 losing the gauge/fiber bundle structure, means those states have to be in the Hilbert space of the theory in general. From the analysis of Strocchi, Balaban and Nakinishi they don't seem to. "
Regarding this singular limit, you are probably better served by the discussion in the other thread about asymptotic states in QCD, and the experts there can help you better than I can. In the cartoon version of the flux tube picture this limit corresponds to stretching the tube infinitely far to where it breaks, but when it breaks, it always breaks into smaller pieces of string (b/c you have to put so much energy in that you create a virtual quark-antiquark pair that then rebinds into smaller 'mesons') and thus you would never observe something other than a color singlet state (albeit one that you can make arbitrarily close to 'looking' like a free quark at high momentum transfer for the purposes of scattering experiments). Nevertheless, the quarks are still the constituent probes of that theory as well as what appears in the classical Lagrangian, and the only reason you can never see them must completely lie within the quantum dynamics of the extra QCD self interaction term.

All this to say that I completely agree that quarks as elementary particles are of a very different nature than electrons. However they are also of a different nature than virtual particles and certainly of ghost states so I don't think it's correct to imagine a final nonperturbative theory completely excising them from their description. How could that even possibly work and preserve all of the successes of the eight fold way, deep inelastic scattering and so forth..

DarMM

Gold Member
All this to say that I completely agree that quarks as elementary particles are of a very different nature than electrons. However they are also of a different nature than virtual particles and certainly of ghost states so I don't think it's correct to imagine a final nonperturbative theory completely excising them from their description. How could that even possibly work and preserve all of the successes of the eight fold way, deep inelastic scattering and so forth..
They seem to not be in the nonpertuabtive Hilbert space of QCD due to having negative norm and mapping out of the Hilbert space into the surrounding Krein space. It seems it is possible to write QCD completely in terms of hardonic-glueball operators on a hadronic-glueball Hilbert space*. Thus the confusion/point of this thread is exactly your final sentence.

*See: Taichiro Kugo, Izumi Ojima; Local Covariant Operator Formalism of Non-Abelian Gauge Theories and Quark Confinement Problem, Progress of Theoretical Physics Supplement, Volume 66, 1 February 1979, Pages 1–130

In essence if quarks are states in the Hilbert space, they can't be confined. See here as well: https://arxiv.org/abs/hep-ph/0105142

A. Neumaier

you can take the Hadronic spectrum and compute a Quark 'mass'. Where 'mass' as you might expect is a bit of a fuzzy scheme dependent concept inside a strongly interacting composite object (it is certainly not the usual pole in the propagater, considering that there are strongly divergent infrared effects at play). In any event this is an active area of research (see the pdg section on this (p726))
p.726, which you cite, says that the quark masses determined by lattice QCD are bare masses (i.e., parameters of the bare Lagrangians in a suitable approximation) and not physical masses (poles of the propagator). Trying to get the latter by fits to experiment produces values that violate causality: The Kallen-Lehmann decomposition of the propagator contains complex conjugate quark poles where the squared mass $m^2$ has a negative real part, while causality requires masses to be real and nonnegative. This implies that a quasi-free space based on the Fock construction but with the fitted Kallen-Lehmann decomposition leads to an indefinite inner product. Therefore it is not a Fock space in the sense in which the term is used by mathematical physicists (where a Hilbert space must result) but only an ''indefinite Fock space'' of the kind @samalhayat mentioned in a related thread.

As a consequence, the (non-free) field operators for quarks can also be defined only on a Krein space - i.e., a generaliziation of a Hilbert space obtained by replacing the definiteness condition of the inner product by the weaker nondegeneracy condition.

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A. Neumaier

many observations (e.g. Deep Inelastic Scattering) look most natural in terms of quarks.
There is an interesting paper by Casher, Kogut, and Susskind in Phys. Rev. D19 (1974), 732-745. They discuss the exactly solvable toy model of QED_2 in 1+1 dimensional spacetime, which exhibits formal asymptotic freedom and confinement of electrons. They show that some sort of ''deep inelastic scattering of electrons'' occurs although the physical Hilbert space contains no electrons. They extend QED_2 to a model with 3 abelian quarks, leading to mesons and baryons.

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Demystifier

2018 Award
Indeed! In our perception of physical reality there are neither Hilbert and Fock spaces, Lie and other groups in QT, nor configuration and phase spaces, no fiberbundles, Minkowksi and pseudo-Riemannian manifolds in classical physics. These are all description of our perceptions of Nature. It is an astonishing empirical fact that we can order our perceptions (at least the "objective" ones) using these mathematical entities.
Yes. In the formulation of QM I present in the paper linked in my signature, the main object of concern is neither observable (as in standard QM) nor beable (as in the usual formulation of Bohmian mechanics), but a perceptible.

Demystifier

2018 Award
In what sense?
At a more fundamental level, I think all particles of the Standard Model should be thought of as quasiparticles, in the same sense in which a phonon (the quantum of sound) is a quasiparticle. See the paper linked in my signature.

king vitamin

Gold Member
At a more fundamental level, I think all particles of the Standard Model should be thought of as quasiparticles, in the same sense in which a phonon (the quantum of sound) is a quasiparticle. See the paper linked in my signature.
I agree; in a strongly interacting quantum many-body system, whether it's a QFT or whether it's the Hubbard model, the excitations one sees at low energy will not have any simple relation to the "fundamental" degrees of freedom which are specified in the Hamiltonian.