The energy that would be associated with making a quark free would be infinite, so it would not work :

If you tried to pull out a quark from inside a nucleon by sheer force, the energy you'd need to achieve this would be such that first a meson would be created, along with another quark that would take the place of the original quark inside the nucleon .... As you'd keep increasing energy levels so as to break up the succession of created mesons, you'd have to go all the way to infinity before any quark could be truly freed - no can do, I'm afraid

Dear Chris,

of course one should add to your most accurate answer 'up to now'.

Confinement might break down at some scale

Confinement is a long distance (low energy) effect, so there is not really any uncertainty regarding what will happen at higher energy scales. We know that confinement breaks down at high energy. Indeed, at high energies (small distances), the quarks do not feel any confinement, which is referred to as asymptotic freedom. That doesn't change what happens at long distances though.

As Chris said, when you pull the quarks far enough apart (more than a few proton size or so, around 10^(-14) meters), the energy in the colour field between them is so large that you can enter a lower energy state by creating a new quark/antiquark pair between the two quarks moving apart. And you know nature, it loves to pop into the lower energy states. So as you add more energy to pull the quarks apart, all you will do is to create a longer string of hadrons and mesons.

Dear Christoffer , thank you for your answer.

Hi Chris Ransford,

Taking your scenario:

Can you explain how a meson is created if I was to fission a proton made up of three quarks so that the down quark with charge -1/3 was freed with the two up quarks still bound by a gluon,(if this is what you mean? Wouldn't you already have the meson structure in place for this baryon before the fission: that is, a meson created between the up quark and a down anti-quark and another created between the down quark and up anti-quark. From my understanding of the charge symmetry between a proton and anti-proton there would be only these two mesons linking the two types of subatomic particles of matter with no meson created between the up quark and up anti-quark?

Also, how can you explain another quark binding to the once proton with now two quarks to maintain the three quark baryon structure? Wouldn't this quark already be bound to a proton or are you assuming this is a 'free quark' if we take that free quarks exist?

I have only been studying particle physics for a few months so the support of your statements might make things a bit clearer for me. Thanks

Dear Marc,

If you are new to particle physics, you may not be familiar with how colour charges in the strong force work. Let me summarise quickly, although I recommend reading more in depth to understand better.

The charge associated with the strong force is referred to as "colour". This corresponds to electric charge in electromagnetism. In the strong force however, there are three different charges: red, blue, and green. A quark (such as an up or down quark) for example, can have the charge "red" (which means +1 red charge, and no blue or green charge), "blue" or "green". Similairly an antiquark (such as antiup or antidown) can have charges antired (-1 red charge, no blue, no green), antigreen or antiblue.

For example a meson is formed by a quark and an antiqaurk with oposite charge. That is, either a quark with red charge and an antiwaurk with antired, or blue + antiblue, green + antigreen. (Actually it is a quantum superposition of the three, but that is a different story.) As you notice, the total colour charge is zero. For example in the case of red + antired, the total red charge is +1 (red) -1 (antired) = 0. And both the red and antired quark are neutral in blue and green charge. This is very similar to an electromagnetic dipole, where the field lines go from one charge to the other, but no field lines escape out to infinity. Compare to a single charge, where the field lines stretch out to infinity.

Funny thing with the colour charges is that if you add up equal amounts of red green and blue, they cancel out (the analogy is RGB colours adding up to white, but the real reason lies in the mathematics of group theory). So if you have a red quark, a blue quark, and a green charge, they will together be colour neutral. This is a baryon.

Now, if you start pulling a red charge (one of the quarks) out of a baryon (such as a proton), you will get a field between the red and the blue and green charges. The blue and green charges will act as an anti-red charge, as they all together cancel out to neutral. This field will quickly get more and more energy as you pull them apart, and at a certain point you will be able to jump to a lower energy state by creating a new red + antired quark-antiquark pair, where the antired charge will couple up with the red charge you are pulling out (forming a red-antired meson) and the new red charge jumps back to the original blue and green charges in the baryon. The new red and old green and blue quarks will form a (colour neutral) baryon again.

The difference to electromagnetism is that the energy in the dipole field between two electric charges doesn't grow too much when you pull the charges apart (so not enough energy to create new particles), while the colour field grows very rapidly (reaching enough energy to create new quark-antiquark pairs when on the order of 10^(-14)m apart.

I hope that cleared it up for you. Good luck with your particle physics studies.

Thanks for this brief lesson on colour charge. Does this mean colour charge is a subsitute terminology for electric charge or is electric charge a different phenomenon that quarks are or are not governed by?

Electric charge is associated with electromagnetism, colour charge is associated with the strong force. So they are two different kinds of charges that work in parallel. Let me list the charges of some common particles, I think that will help clear it out.

electron/muon/taun: -1 electric, 0 red, 0 blue, 0 green

neutrino (any flavour): -0 electric, 0 red, 0 blue, 0 green

positron/antimuon/antitaun: +1 electric, 0 red, 0 blue, 0 green

red up-quark: +2/3 electric, +1 red, 0 blue, 0 green

green up-quark: +2/3 electric, 0 red, 0 blue, +1 green

blue up-quark: +2/3 electric, 0 red, +1 blue, 0 green

red down-quark: -1/3 electric, +1 red, 0 blue, 0 green

green down-quark: -1/3 electric, 0 red, 0 blue, +1 green

blue down-quark: -1/3 electric, 0 red, +1 blue, 0 green

antired up-antiquark: -2/3 electric, -1 red, 0 blue, 0 green

antigreen up-antiquark: -2/3 electric, 0 red, 0 blue, -1 green

antiblue up-antiquark: -2/3 electric, 0 red, -1 blue, 0 green

antired down-antiquark: +1/3 electric, -1 red, 0 blue, 0 green

antigreen down-antiquark: +1/3 electric, 0 red, 0 blue, -1 green

antiblue down-antiquark: +1/3 electric, 0 red, -1 blue, 0 green

note that the quark does not only come in different flavours (up, down, ...), but each flavour also comes in three different colours. The flavour sets the electric charge, the colour sets the strong force charge. The electrons and neutrinos do not come in different colours, as they are always uncoloured, ie without colour charge. The antiparticle always have the opposite sign on both charges, electric and colour.

The photon, being the force-carrying particle for the electromagnetic force, couples to any particle with electric charge, such as electrons or quark (but not the uncharged neutrino). The gluon, being the force carrying particle of the strong force, couples to any particle with colour charge, such as the quarks (but not the colourless electron or neutrino).

A hadron is bound by the strong force, and is composed of particles with colour charge (ie, quarks). As the strong force is so much stronger than the electromagnetic force, the electromagnetic forces between the (electromagnetically charged) quarks are so small that they can be neglected in most calculations. So while there is a repulsive electromagnetic force between the two up-quarks in a proton for example, the strong force between the three quarks is so much stronger that they still stay together.

I hope that cleared up the issue with free quarks a bit. There is ofc a lot more to know, but I would end up repeating a textbook on QCD, and I'm sure a textbook would explain it better. :P I guess all this will come in the course you are following, or the next one. Good luck, cheers.

Quarks are confined in hadrons in color singlets until the temperature is below a critical temperature of deconfinement, or, said in different words, if the energy scale is below a critical energy. If I remember correctly, it occurs at about T = 200 MeV and has been observed at LHC in heavy ions collisions (and also at RHIC).

QGP is the state where quarks (and gluons) deconfine and form a "soup" of coloured particles. It is supposed to be existed few microseconds after the BB, when T was high enough; when T dropped below 200 MeV there was a phase transition from QGP to hadrons. It also may exist inside compact objects as neutron stars.

There were no free quarks seen in any collider or accelerator experiment. However, free quarks were claimed to be found in Niobium superconducting spheres experiment at SLAC in cca. 1982. People do not cite these results, but there were approx 2 Phys.Rev.Lett. published by the SLAC group claiming to observe fractional charges for years. If you look on Wikipedia for "Color confinement" there is Ref.[8] there indicating that point-like source of color charge can be screened fully by gluonic field. If that Ref [8] in Wikipedia is OK, free quarks might exist even below QGP phase transition, It seems however much more energy is needed to create free quark surrounded by gluonic shielding , than is required to create new hadrons out of (q---q') string. What do you think ?

Can one quarks be bound to more than one anti-quark? Is it correct to say that this binding to many anti-quarks would require the quark to emit several gluons?

The observed hadrons are all mesons (q+qbar) and baryons (q+q+q) and antibaryons (qbar+qbar+qbar). People have been looking (and some maybe still are looking) for pentaquarks (q+q+q+q+qbar), which essentially is a bound state of a baryon and a meson, but no conclusive experimental signal has been seen (as I understand).

The bound quarks in any hadron is constantly exchanging a lot of gluons between each other, in the same way that a bound electron is constantly exchanging photons with the nucleus of an atom.

Where does the binding between the baryon and meson occur for a pentaquark?

If such meson-baryon binding exists (scientists are not sure) then it comes probably from the same mechanism as binding of nucleons in nuclei = nuclear force. The origin of the nuclear force between protons and neutrons (baryon-baryon binding) is not very clear. Usually it is explained as Van-der-Waals type of force between baryons. So meson-baryon binding can be of similar nuclear-Van-der-Waals type.

Peter and Christoffer,

It seems that gluonic bonds can explain the holding force between protons and neutrons and mesons just as well as a nuclear force. Essentially, all nucleons are made of quarks that can emit gluons that can bind with other quarks? It makes just as much sense to posit that bonding between baryons are a result of multi gluonic forces between quarks creating these baryonic quark structures? Why haven't physicists just gone with this theory instead of postulating a nuclear force for which there is unclear evidence?

Quoting the wikipedia entry for "nuclear force":

"The nuclear force is now understood as a residual effect of the even more powerful strong force, or strong interaction, which is the attractive force that binds particles called quarks together, to form the nucleons themselves. This more powerful force is mediated by particles called gluons. Gluons hold quarks together with a force like that of electric charge, but of far greater power."

While everyone today agrees that the force between hadrons at short range (the nuclear force) is the strong force, the complicated nature of QCD calculations at low energies (as is the case in nuclear physics) makes it very hard to actually get useful results in nuclear physics. So in practice I think (it's not really my field, please correct me if I got this wrong) most calculations in nuclear physics are still done with phenomenological approaches rather than direct calculations from QCD.

This, coupled with the fact the nuclear physics often is introduced well before QCD in most undergraduate programs, has made the term "nuclear force" stay around, rather than being replaced by "the strong force" or something similar.

Also Hyperons = baryons like proton and neutron (just containing s-quark) are subject to nuclear forces and hyper-nuclei exist containing hyperons, protons and neutrons. At present, scientists describe nuclear force as interaction generated by exchange of pions, omega and rho mesons (also other mesons, like sigma meson). This is our description, which is probably rather close to "reality", but in fact we do not know exactly, what is going on, I think. At least, not many people would put their hand into fire for the exactness of such a description. Origin of "nuclear" forces is not very "clear"...

It would make just as much sense to me if a meson was created between the binding of a quark from a baryon such as a proton and an anti-quark from an anti-baryon such as an anti-proton rather than between a free quark and a free anti-quark. The total opposite colour and electric charge emitted from each of the baryon and anti-baryon should hold the two subatomic particles of matter and anti-matter together. Perhaps this is what the pentaquark really is; just that the fourth quark and the anti-quark have been measured twice - once as part of a baryon and anti-baryon respectively giving total colour charges of 1 and -1 ;and again as an anti-quark/quark pair giving colour charge of 0, thus giving this perception of a chain of four quarks and a single quark.

Has anyone thought that is might be the case?

The experimental search for pentaquarks is not made by counting quarks. They look for resonances in collisions, which essentially measure the mass, time of decay and to the extent it is possible, the decay products.

As you can read on http://en.wikipedia.org/wiki/Pentaquark, a (statistically weak singal for a) resonance was seen in the mid 2000's, close to the mass that a pentaquark was predicted to have. Later experiments with more statistics however showed that this resonance was not there.

To come back to the question. Would it be correct to say that free quarks may exist but only at high enough energies such that the strong coupling is no-longer strong enough to cause confinement, i.e. asymptotic freedom.

In microscopic viewpoint of quantum chromodynamics (QCD), not only ordinary qqbar mesons and 3q baryons, but also exotic hadrons, such as multiquark hadrons (tetraquark mesons, pentaquark baryons and dibaryons), hybrid mesons (qqbar g, · · · ), hybrid baryons (qqqg, · · · ) and glueballs (gg, ggg, · · · ) are expected to appear. The inter-quark force is one of the elementary quantities for the study of the multi-quark systems and is regarded as an important quantity to connect “the quark world” to “the hadron world”. As Christoffer mentioned, it is the resonance that sets the signature for a system, may that be an ordinary baryon/meson or exotic hadron. Using Lattice QCD, it is possible to explore the nature of the observed resonance, for example, analysing the static multiquark potential, if you like. In one of our recent studies on hexa-quark system (ppbar and h-dibaryon), we observed that the system is well described by the potential derived in the strong coupling expansion (which has a confining part proportional to the minimal length flux tube joining the quarks) and the static energy of the system is less than the sum of two baryonic potentials. This indicates that hexaquark system behaves as multiquark bound state rather than breaking into mesons and baryons.

Re Jonathan: If by high enough energy you mean high temperature (Hot QCD, if you like), then answer is yes in terms of Quark Gluon Plasma, where hadrons are simply a hot soup of free quarks and gluons. Quark gluon plasma is the central issues in the discussion of experimental signals that can emerge from high-energy heavy-ion collision data from three new experiments running on ALICE, ATLAS and CMS of Large Hadron Collider.Guess we have to wait and see what comes next.

Thanks Mushtaq. I was originally thinking about the post-inflationary period of the early universe which is filled with a quark-gluon plasma where quarks are free as you described.

Hi and thanks Jonathan. Hope you are keeping well.

Back to your query >>

To exploring the observed modifications of hadrons in presence of thermal medium and examine the possibility if observed modifications are indeed related to the disappearance of meson bound state, would require a detailed study of the structure of spectral function and pole masses of hadrons above the critical temperature. The vector mesons, and their pole mass which is closely related to the QGP creation experiments, play an important role as they couple directly to virtual photons, which can decay into dileptons. This process could be helpful in carrying the information about possible in-medium changes of the vector mesons to the detectors. Experimental signatures at high temperature like dilepton production rates are directly related to meson correlation function, the spectral analysis of which became recently accessible. Our recent analysis of temporal and spatial correlation functions in the vicinity of the critical temperature gives evidence of substantial changes in the properties of hadronic states between the confined phase and hot plasma phase of QCD.

However, recent experimental discoveries at the RHIC indicate that, at least at temperatures within a factor of two of that at which hadrons ionize, the dynamics of quark-gluon plasma is closer to the ideal liquid limit than to the ideal gas limit.

@Jonathan

There is no asymptotic freedom in QCD due to the nature of its ground state. The force between quarks grows with their distance.

@Marc,

While there is no stable single quark, yet the CDF and DZero collaborations at Fermilab have discovered rare single top quark in 2009. ( http://www.fnal.gov/pub/presspass/press_releases/Single-Top-Quark-March2009.html ).

The secret lies in deciphering the Dark Matter that dominates our non visible world. It may well be a sea of frozen free quarks from the earliest of time for our universe. Dark matter does not interact with visible matter, except that it exerts a strange repulsive gravity towards visible matter consisting of baryon matter( bound quarks ). LHC may not provide such answer as one requires tremendously higher energies to approach the earliest of times of origin of the universe!

Free quarks do not exist because the QCD vacuum prevents it. There is large body of evidence that hadrons create bubbles in a " true QCD vacuum" in which quarks and gluons can move almost freely as long as their collective colours combine to a colour free state. Now if you try to liberate a quark and pull it out of the rest with the opposite colour, a colour field is created between them and squeezed by the pressure of the surrounding vacuum to a string. This mechanism actually generates the so called confinement - that string is like a rubber band with attractive force growing with the distance between the colour charges. At a so called critical distance the energy in the colour field potential is so high that it will spontaneously transfor into the mass of two quaks - a quark-antiquark pair will be spontaneously created from vacuum - and shield the colour field. This is energetically the lowest state. This leads to the string between the original quark and the remainder of the hadron to break, the original hadron has become a hadron and a meson.

This has been calculated long ago in detail by solving the Dirac equation in that strong field - see my publication list: Fission of bags by spontaneous quark-antiquark pair production in supercritical colour fields by D. Vasak et al. in Zeitschrift für Physik C Particles and Fields 02/1983; 21(1):119-125.

This complicated situation also explains why in nuclear physics we use phenomenology to describe the nuclear force. It is just the residual QCD based interaction shielded by the vacuum, that we describe as being mediated by effective particles, mesons (mostly pions).

David Vasak comment has not cared to examine the conjecture i made postulating single quarks to constitute dark matter. The mystery of dark matter constitution can be solved if we postulate that at the very earliest beginning there was sea of heavy quarks which could decay into lighter ones under a far stronger strong force that existed then. This force lost strength rather quickly and got more or less frozen around the present value when all the quarks got frozen as such. Only the lightest ones, namely the three we use to form baryon matter did so while the heavier ones got frozen as dark matter that could not react at all with the visible matter except through mass mass interaction, that too through 'repulsive ' gravity, unlike the gravity that exists among the visible mass constituents.

We can not do such experiment today as the conditions are not available and may require super huge LHC at unbearable cost to achieve! However, if one can go to the farthest end of the universe, we may see such a situation that prevailed in the most earliest period. Again such an experiment appears too tough to conduct in cosmology today! Yes, it is all i am conjecturing as the Physics we have built thus far was not created in this manner as no observations were available to base the same. Thus, i am not disheartened with my comment being not covered by existing Physics as it too is unable to explain the presence of dominant dark matter remaining unexplained while we work out Physics entirely based on lesser abundant visible matter !!! Eager to have reactions of far more learned Physicists friends of mine!!!

Narendra Nath

Narendra, interesting idea. However, being ignorant of your work it is difficult for me to understand what you mean by "frozen", and how that contradicts what I wrote. Would you mind to elaborate on that somewhat?

David, if you give your Internet address, i can mail some mss's to get what i mean.

Narendra Nath

Dark matter mayb just be an assemble of free quarks in a frozen matter!

At the Time of Big Bang, all such things become possible!