# Do Quarks Decay?

1. Nov 23, 2008

### sanman

When you see these particle collider experiments that have confirmed the existence of the various types of quarks, then these experiments are displaying those quarks briefly before they disappear. So what's actually happening to the quarks when they disappear? Are they decaying? If so, then what are they decaying into? Are they binding back together with other quarks? If so, then where did those other quarks come from?

They say that in deep space, there is still a certain density of free hydrogen available out there.
But there seems to be no similar availability of free-floating quarks, even in deep space.
If you have a quark floating freely in deep space, then what's going to interact with it, to snatch it up?

I'd read that it takes infinite energy to separate quarks from each other, but if that's the case, then how were quarks shown to exist unless they could be briefly separated for long enough to identify them?

Is it really right to call a quark a particle, if it can't independently exist?

2. Nov 23, 2008

### malawi_glenn

Yes Quarks decay.

Also look up "Asymptotic freedom", at high energies, quarks are "free".

3. Nov 23, 2008

### sanman

Yes, I understand that hadronization means quarks "decay" into regular matter by binding with other quarks.

But when you say that at high energies quarks are free -- how high are those energies?
Are there any naturally occurring situations in the universe which provide high enough energies for quarks to stay free for appreciably long periods of time?

4. Nov 23, 2008

### malawi_glenn

At particle physics colliders.

And in the early universe.

5. Nov 23, 2008

### sanman

Parting the Quark Sea

Since these latest headlines have come out about the QCD interactions making up most of mass, I am seeing increased mention of a "quark sea" which is supposed to be pervasive throughout all of space.

Is it feasible to consider "parting the quark sea", in the sense of having a zone of space where hadronization cannot take place?

Since gluons are the mediators of quark-quark interactions, then what is the range of the gluon force? Or is it irrelevant to talk about such a range, due to the omnipresence of this quark sea, which will supply new interaction partners for quarks which have been separated from former partners?

6. Nov 23, 2008

### sanman

But so then, there are currently no naturally occurring phenomena supplying conditions where quarks can remain free and unbound? Not even in the hearts of stars, etc?

7. Nov 23, 2008

### malawi_glenn

nope, 15M kelvin is pretty low temperature, no quark-gluon soup there.

The range is theoretical infinite, just as for EM, since gluons are massless. But du to hadronization and coulor confinement, there is a greatest distance which force can act before hadronization will occur.

8. Nov 23, 2008

### sanman

So then the quark sea can be like an aether or foam, in a sense.
It is everywhere, even in empty space.

But yet hadronization can't occur for free. It requires quark donations from the sea/aether/foam, and this leaves anti-quark orphans, doesn't it?

Shouldn't the orphaned anti-quarks then quickly be hadronizing into anti-matter? (ie. anti-protons, anti-neutrons, etc -- I dunno, some anti-hadron)

If we smash enough hadrons together to form enough free quarks and thus trigger enough hadronization, then shouldn't this lead to a corresponding equivalent amount of anti-hadronization?

In order for hadronization to occur, then doesn't anti-hadronization have to simultaneously occur somewhere?

9. Nov 30, 2008

### blechman

Let me jump in and try to answer some of these questions.

First of all, for QCD - the relevant scale is set by $\Lambda_{\rm QCD}\sim$200 MeV. This sets the sizes of all relevant scales of the lowest-energy questions (mass of light hadrons, range of interactions, deconfinement temperature) by multiplying this number by the relevant factors of $\hbar,c,k_B$. So that answers many of the questions above as far as the "range" of QCD and the "temperature" of QGP, etc.

Sanman says something earlier that isn't right: hadronization is NOT quarks decaying "into regular matter" (post #3). This is not the right way to think of it - rather, hadronization is not a decay, but a binding of a quark (or quarks) with other quarks created from the vacuum. This is not the same as a decay - since the quarks are still there inside the hadron.

This is a very subtle point that you ask about - it is certainly true that you cannot treat quarks as "asymptotic states" of a Fock space, and this complicates questions about how to compute scattering cross sections, etc. However, thanks to asymptotic freedom, as long as the quarks are significantly heavier than $\Lambda_{\rm QCD}$ (c,b,t) it is safe to consider them as free objects (up to perturbative QCD interactions). The formal treatment of this goes under the name "Heavy Quark Effective Theory" - you are welcome to read up on this stuff and ask more questions if you like. HOWEVER: you are correct that trying to treat the strange quark as a free quark and using that to compute kaon decay, for instance, is a BAD approximation and does not work. There you need the chiral Lagrangian, or some such treatment.

Another case where things work okay is if the light quarks have significant kinetic energy (again, larger than $\Lambda_{\rm QCD}$). In this case, you have "jets" of particles and can rely on tricks such as "parton-hadron duality" to compute things that make sense even on the quark level. Again, this is because of asymptotic freedom. This is also the reason why it is okay to consider proton collisions (at the LHC or Tevatron, for instance) at the quark level.

There are no free quarks left in nature, at least not that we have any evidence for. I remember playing a fun game with my friends in grad school trying to imagine what would happen if there were - we had a good laugh (I'll leave you to try and see why!!).

Given the above statement, your worries of the last post are unjustified. Thanks to the fact that QCD is a SYMMETRY, if you start with a color neutral object, you end with a color neutral object. Just like you cannot suddenly produce charged particles out of a bunch of netural particles - the sum of all the charges must be zero. So: by combining a bunch of protons together (which are color netural) - you cannot produce a single unpaired quark. You can only produce pairs (quark-antiquark), and they can fly apart pulling pairs of quark-antiquarks from the vacuum to netralize them. So there are always an equal number of quarks and antiquarks and life is good!

OK, that was a lot. I hope it helps. Let me know if there are any more questions.