Pairs of particles or larger groups are not usually considered to be real unless they separate by distance of more than a wave length and become measurable in isolation.
In space vacuum virtual particles do not seem to scatter photons or other real particles. Some researchers say that virtual particles do not exist physically, which in context may be true.
Quarks do appear to scatter small particles, but cannot be separated by more than a wave length. One possible explanation is that polarization potential of vacuum is quantized and does not accommodate fractional charges far from mates.
Are Quarks Virtual Particles Or Real Particles?
They're real particles-the distance between them isn't a useful measure. The reason they're real is because they're described that way in QCD, which can describe the experiments that involve them. The vacuum of space doesn't have anything to do with the subject.
That they're confined in hadrons simply means that, when hadrons are weakly coupled, the quarks are strongly coupled-to the point that they don't appear, as free particles, when hadrons are detected.
Virtual particles don't have well-defined mass (one Lorentz invariant that characterizes real particles); quarks, in QCD, do.
Dear Jerry,
Your question is quite subtle and perhaps it would be good to try to explain things deeply as a direct answer for understanding what is involved within such question.
It is well known that Quantum Mechanics allows , temporary violations of conservation of energy, in such a form that one particle can form a pair of heavier particles (called virtual), which quickly rejoin into the original particle as if they had never existed. If this were all then we could think that this is just a trick allowed by Uncertainty Principle of Quantum Mechanics. But Physics needs more experiments for calling the virtual particles to be particles.
The first experiments stated with the Hydrogen atom that has two energy levels that coincidentally seem to have the same energy. But when the atom is in one of those levels it interacts differently with the virtual electron and positron than when it is in the other, so their energies are shifted a tiny bit because of those interactions. This is the Lamb shift between the states 2S1/2 and 2P1/2 measured in 1947 and which opened the necesity of QED with its special vacuum. That is to say it corresponds to one-loop effect of quantum electrodynamics where there are the creation of what has called the virtual pair of particles: electron-positron. The atom interacts differently for the virtual electron than the virtual positron which justified the existance of virtual particles for the first time.
On the other hand, quarks are particles like electrons of Electrodynamics, interacting besides via the short ranged Strong Force. Two of the lighter quarks, the so-called "up" and "down" quarks, bind together to make up protons and neutrons. At the LEP collider at the European particle physics laboratory CERN, millions of Z bosons--the particles that mediate neutral weak interactions--were produced and their mass was very accurately measured. While the Standard Model predicted the mass of the Z boson, but the measured value differed a little as it happened for the photons in Lamb effect of QED. This small difference could be explained in terms of the time the Z spent as a virtual top-antitop quarks if such a top quark had a certain mass. When the top quark mass was directly measured a few years later at the Tevatron collider at Fermilab, the value agreed with that obtained from the virtual particle pairs analysis, providing a dramatic test of our understanding of virtual particles.
Summarizing the electron can be a virtual particle as the top quark, although they are usually real particles.
I hope this can helps.
Dear Jerry,
Anthony Duncan in his book on the Conceptual Foundation of Quantum Field Theory has a nice distinction between elementary particles and elementary fields. Electrons are elementary particles and are described by elementary fields in Quantum Electrodynamics. Quarks are NOT elementary particles. However, they are elementary fields in the sense that one uses them for writing down the fundamental Lagrangian of the strong interactions, the Quantum Chromodynamics Lagrangian:
"Quantum chromodynamics (QCD) ... provides a particularly stark example of the perils of the nave dictum 'for every eld, a particle'. In con ning theories such as QCD, the dynamics is speci ed via a Lagrangian involving quark and gluon elds which do not interpolate for any of the physical particles ('hadrons') in the theory. In fact, these elds strictly speaking are not de ned on the physical Hilbert space at all! However, multi-local combinations of the quark and gluon elds, appropriately coupled to be invariant under the local gauge symmetry of the theory, form well-de ned almost local operators which do interpolate for physical particles, and are well de ned on the physical state space." (p. 300)
Thanks everyone for comments.
Daniel Baldomirets in the question I am looking for the limits of vacuum space. In QCD are given limits on quarks with asymptotic freedom and internal models, I look for an external explanation to agree with the internal restraints. Without an external constraint the particle in a box example would allow free quarks to be occasionally found.
One suggestion is that vacuum is composed of quantum states that in 4D cannot contain fractional charges separated from other fractional charges. This supports the models where force increases with distance, and free quarks do not appear. It also gives a chance to find something new about vacuum quantum states.
Theories are large enough to allow suggestion that vacuum had other quantum states in the early universe in which free quarks are a possibility.
If the big bang can be thought of as collapse of a previous false vacuum and destruction of everything it contained, a mechanism can be suggested in which hadrons were being destroyed a little bit faster than the components they contained. A while later as energy density decreased in expansion and hadrons were beginning to be created it is conceivable that the component parts formed shortly before the hadrons were assembled. In both cases free quarks are conceivable. That epoch probably occurred in space of changing quantum states.
The topic is important at this time related to proposals for deep space transport at high speed, where local vacuum is expected to have quantum states similar to the early universe based on energy density. There seems to be a possibility that travelers might encounter free quarks unless quarks are virtual particles.
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Readers please notice RG is losing bits of messages today.
case quarks were virtual particles a higgs field should be present IN the particle they build- it is not, so quarks must be real particles. so the theory :))
Respected Jerry Sir,
Your question is quit interesting and I can summed my answer in the following way.
The deep inelastic experiment or counter experiment of classical Rutherford experiment performed by a team from MIT and Stanford University, led by Jerome Friedman, Henry Kendall, and Richard Taylor by investigating the scattering of electrons from a proton, and, analyzing the data using a theory by James Bjorken, concluded that there are point-like scatterers inside the nucleons. Also the resulting angular distribution of the scattered electrons in the above mentioned experiments can be explained only if there are tiny constituents inside the proton as well as neutron. On the basis of this they found that the nucleons has “parts” i.e. it composed of partons. Initially people treat them as quarks of the eightfold way.
Later on, the discovery of J/ψ particle which is also famous as “November revolution” established the reality of quarks. Its a vector meson having mass about 3.1 GeV (three times that of the protons) and its unusual lifetime is a thousand times longer than typical life-time of hadrons. It later on identify as charmonium (bound state of charm and anti charm). The c-quark is sufficiently , massive that its description can easily be found using non-relativistic quantum mechanics. The above explanation indicates the reality of quarks. Some people treat the J/ψ particle hydrogen atom of quark physics.
Dear Jerry,
Quarks are particles. In fact the matter is classified in leptons and quarks as elementary particles which form it. Perhaps the difficulty is that a free quark is not observed because its energy is far above the pair production energy for quark-antiquark pairs. For the up and down quarks the masses are tens of MeV so pair production would occur for distances much less than a fermi. You would expect a lot of mesons (quark-antiquark pairs) in very high energy collision experiments and that is what is observed.
A visualization of quark confinement is the "bag model". One can imagine the quarks as contained in an elastic bag which allows the quarks to move freely around, as long as you don't try to pull them further apart because their energy increases very highly with distance at difference of what happens with gravitation and electrodynamics. Thus if you try to pull a quark out, the bag stretches and resists.The vacuum of QCD is formed with quark-antiquark pairs forming virtual gluons in parallel to the electrodynamic vacuum formed by pairs electron-positron associted to virtual photons.
I don't know if this can give you an answer to your question.
Dear Daniel,
I am willing for quarks to be particles, but require something more than the popular explanation concerning how they are asymptotically contained. Many researchers were disappointed to not find quarks in a free state occasionally.
Consider mesons that are far from each other and not protected by additional particles. All are made from pairs of quarks with fractional charges. Mesons are unstable and decay, but never into fractional charges. My question works best in this situation.
Mesons have binding energy of quarks in some cases larger than the total decay energy. It says to me that vacuum is providing assistance to the binding of quarks in mesons, much like it does for containment of virtual particles in pairs.
Dear Jerry,
You wrote:
Consider mesons that are far from each other and not protected by additional particles. All are made from pairs of quarks with fractional charges. Mesons are unstable and decay, but never into fractional charges. My question works best in this situation.
The idea is well explained in the Standard Model where the baryon number is conserved. And an integer baryon number implies an integer electric charge (in terms of elementary charges). The same derivation also works for the U(1) weak hypercharge.
I don't know any case where it would be true what you say and by definition vacuum is the fundamental state which prevents to give energy:
Mesons have binding energy of quarks in some cases larger than the total decay energy.
One direct reference for your question could be
http://www-pnp.physics.ox.ac.uk/~barra/teaching/subatomic.pdf
Quarks are neither real or virtual particles; they are imaginary particles!
Quarks are not real (so irrelevant to ask whether they may be particles, "virtual" or otherwise), they are inventions of the human mind (i.e., "imaginary" ... mathematical abstractions that {like numbers and ideal forms of geometry such as circles, triangles and lines} can 'exist' only in the human mind) and have no basis in reality (aka "in Nature").
When you are able to point-out (observe or measure) a quark ("free" or otherwise) in Nature (reality) then please let me know ... I will be open-minded enough to look at your quark and reexamine my answer to this question, at that time *chuckle*
How can you judge "particles" you never saw in your biological lifetime?
Evolution created organisms to perceive what they need in social and environmental interactions?
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