Can a single quark exist in isolation ?
As far as I know you can't, but then again my knowlage in this area doesn't go very deep. This site seems to confirm what I do know though;
As I understand it, a free quark can't exist because it has a net "color charge" (red, green, blue, antired, antigreen, or antiblue). Much like systems of electric charges tend to arrange themselves to become chargeless on large scales, systems of quarks tend to arrange themselves to be "colorless" in composite particles (like mesons and baryons). A big difference between this and electromagnetic forces, however, is that the effective "strong force" increases with distance, so a quark couldn't escape the pull of its colored partners as a planet could the solar system. In fact, if you try give a quark energy to make such an escape (as in a decay or collision), the potential energy built up between quark pairs will tend to lead to the creation of more quarks -- a quark jet.
Hi ST, this, " strong force" increase with distance has me baffled, quite easy,
but what is the mechanics behind this ?
Well, ST, really told you the very essence of the story. Quarks have a net color. Every net color will interact with another colour to become colourless. So if you look at quarks in their most stable state (ie the ground state energy) , the most stable configuration is the colourless quark combination. So, this combination is most likely to occur. Now, suppose you wanna take out one quark and separate it from its original structure. While doing so, the interaction between this quark and it's "collegues" in the colourless combination will increase (this is what asymptotic freedom is all about). The increase in energy will be used for the creation of other quarks. This happens because nature is as lazy as possible : The potential energy from the interaction is converted "automatically" into new quarkpairs...
Thankyou, Marlon. seem to me nature is very busy, i will read up asysptotic freedom.
This is called "infra-red slavery". "Asymptotic freedom" is something else.
Asymptotic freedom is the weakening of the coupling between matter and the gauge field at high momenta/small distances.
Asymptotic freedom would be a bit of an odd name for the increase in the strong force with distance.
First of all, if the strong force coupling constant rises when distance rises than it must lower when distance lowers, so what is your problem ? I never claimed to give the exact definition of asymptotic freedom ("free" quarks at "very" short distances, etc etc), but i wanted to illustrate the consequence of a negative beta function. So what are you objecting against ?
Secondly, the strong force is not about the interaction between matter and gauge fields but about matter-matter interactions mediated by gauge fields. So knowing this, tell me where i went wrong. Another thing, do you even know how the strength of the strong force is expressed in terms of matter and gauge fields ?
The definition of asymptotic freedom is quite straightforeward. At least i hope so, otherwise i have to rewrite my master's thesis:uhh:
TO the OP : check here for more info on asymptotic freedom
No, because, we believe , the potential energy between quark and antiquark increases with separation [V(r)~ar]. So quarks and gluons can never escape from the colour singlet states (observable particles i.e colourless hadrons). We call this confinement or infrared slavery.
Yes, confinement(large distance behaviour) is the name .
Perhaps I'm missing something, because I don't see why the distinction is so important for what marlon said. Doesn't "infrared slavery" refer to the divergence of the strong interaction at large distance/small energy, while "asymptotic freedom" refers to the behavior at small distance/large energy? I agree that confinement (that is, the lack of free quarks) is more directly due to infrared slavery, but in the context of marlon's post, it seems either would do:
He's just talking about the interaction strength increasing with distance, which I believe is true in any limit for the strong force -- it wouldn't necessarily have to refer to confinement.
No, i did not.
Now, YOU are naming things incorrectly. The linear potential is not just assumed, it follows from the model you are using. To give you an example : the dual abelian Higgs model  will predict the mentioned linear potential without it "being assumed". The basic idea behind this model is to use the theoretical concept of magnetic dipoles. In low energy QCD the coupling constant is very big. Suppose we are able to make a transformation into a "world" where the transformed coupling constant is small, this would be very good since perturbationtheory can still be used in infra red QCD. The transformation i am talking about is the one defined by replacing the E fields by the B fields. So quarks become magnetic monopoles and gluons become dual (electrical) gauge pairs.
 The Mechanism of Quark Confinement : http://arxiv.org/abs/hep-ph/9809351
I never said any of this. Please read my posts more carefully prior to reacting in this way.
My reaction was to the fact that you stated "the linear potential is assumed..." I never referred to this particular model.
Yep indeed magnetic monopoles. I made a mistake there.
Anyhow, i still argue this business about infra red slavery versus asymptotic freedom. Up till now, you did not show me, apart from some superfluous remarks, where i went wrong. This is not about me not wanting to admit to a mistake. I just don't see where i went wrong. If it is that easy to show me, than why do you need so many lines and references to prove me wrong ?
This is not the same as saying that the linear potential itself is assumed from the beginning (which is what you stated, but i am sure you are gonna deny this).
Nope, this is NOT the basic assumption. I explained to you what that was. Read the reference, please.
Err, your point being ?
Why this summation ? Did i ever say this model was completely correct ? Again, read the paper before reacting.
Marlon, you amaze me again, with you knowledge, comprehension, and ability to talk to people about it.
Aglkasjg!!! HE WAS MOZART!
Anyway, what also amazes me is how much we know about quarks. It seems a bit silly actually. There are tiny little particles that make up what we thought were elementary, with strange triangular kind of formations, stuck together with glue, and they come in different flavors! Flavors include personalities— they can be quite charming, and even some have nice derrières. They are of many different colors, red, green, blue, and even anti-red, anti-green, and anti-blue! They enjoy spinning. What experimental proof is there of quarks? Have we "seen" them? Or is it that there is just missing energy, or what?
We sure have :)
Umm, Marlon, "they are seen as the 'elementary particles'" is not really a fair answer to MK's question, "Have we seen them ?(quarks)". We have inferred quarks from such things as scaling behavior, and I guess you could say we have "seen" partons. But seeing QUARKS?
We see jets. It is not easy to justify the jets if it is not because of subcomponents of the hadrons, and these jets are near of quarks than of partons.
You were misled, what you realy need to look up is quarks confinement not asyptotic freedom.
Nope he was not, as has been explained in several posts now from ST, me and others. The only incorrect information i read in this thread is some superfluous stuff on "assumed linear potentials". Just drop this rediculous crusade and go on, please.
Separate names with a comma.