# Is a lone quark possible?

1. Feb 21, 2014

### nikkkom

I get the idea of confinement, and how it is impossible to separate a lone quark from a baryon: it needs more energy than creation of two more quarks, so the latter happens first, and you end up with having created a (color-neutral) meson.

However, I don't see what prevents free quarks from appearing out of primordial quark-gluon plasma:

Whereas quark-gluon plasma is color-neutral on average, when it cools and "quenches" into baryons, the quarks group into color triplets *randomly*.

Even if a volume of cubic meter (or a cubic light year) of q-g plasma is strictly color neutral (it is possible to pair up (or is it triple-up?) all quarks into baryons with no leftovers), it is extremely unlikely quarks would manage to do that *randomly*.

Imagine that all of quarks successfully combined into baryons except three quarks (one red, one green, one blue) because there is small problem: they are on the order of 100 light days apart from each other. Why? Because quarks aren't sentient, they can't "plan" how to carefully pair up to avoid such a fk-up.

The cubic light year is still perfectly color neutral as a whole. However, it contains three quarks which for all practical purposes are lone quarks.

What am I missing?

2. Feb 21, 2014

### phinds

Hm ... that's an interesting question. I have no idea but your logic seems sound to me. On the other hand, I remember that even Joyce had them in triplets

3. Feb 21, 2014

### ViperSRT3g

oooo very curious as to the possibilities of this question. *follows thread for answers*

4. Feb 21, 2014

### PhotonicBoom

5. Feb 21, 2014

### Staff: Mentor

As long as the quarks are not paired, you still have a plasma. A local imbalance of quark colors (where does it come from?) would quickly get cancelled by color flow from other parts of the plasma.

6. Feb 21, 2014

### Bill_K

But "quickly cancelled" does not make "color imbalance" and "color flow" any less interesting!

7. Feb 21, 2014

### dauto

The thing that causes quark confinement is the fact that the attractive force between unpaired (un-tripled) quarks does not drop with the distance. That means that even though quarks are not sentient, they can find each other over extremely large distances. There will be no f--- up.

8. Feb 21, 2014

### Bill_K

Perhaps, but I remain unconvinced. We're talking about such a high-energy regime for QCD that there's no supporting evidence. I don't dispute that color differences will quickly be resolved, but on a short enough time-scale there may be some interesting things happening.

9. Feb 21, 2014

### Staff: Mentor

10. Feb 21, 2014

Staff Emeritus
The whole premise is off. Consider the following 1D argument with magnetic poles:

N (S N) (S N) (S N) (S N) (S N) (S N) (S N) (S N) S

you could also say "Look! It's leaving two monopoles unpaired far away!"

But what would actually happen is a re-paring.

[N S] [N S] [N S] [N S] [N S] [N S] [N S] [N S] [N S]

11. Feb 22, 2014

### nikkkom

Yes. But imagine that the line in your pic is very long. Such a re-pairing still cannot propagate faster than light - the particles do not magically know they need to re-pair, and how exactly they need to do that. (edit:) It is analogous to the movement of an electron and a somewhat distant hole in the semiconductor. Holes definitely don't move faster than light.

As long as it did not complete, you will have "free" quarks.

Last edited: Feb 22, 2014
12. Feb 22, 2014

### Jilang

I thought quarks were the ends of strings so couldn't exist on their own. It would be like my shoe lace only having one end! Is this not right?

13. Feb 22, 2014

### Bill_K

This question is discussed (but not resolved!) in this paper. (The author is a member of the ALICE team)

To me, the third of his possible solutions ("hadron resonance matter") sounds the most likely.

14. Feb 22, 2014

### phinds

Strings are still a mythical beast, believed by theoretical physicists to exist but never actually seen in the wild, much less in domestication.

15. Feb 22, 2014

### dauto

Jilang is not talking about the same kind of string you're thinking about. What Jilang is talking about is a filament of quark-gluon plasma that connects the quarks keeping them from becoming free quarks. You're thinking about string theory. Those are two completely different beasts.

16. Feb 22, 2014

### phinds

Ah ... I didn't realize that. Thank you.

17. Feb 22, 2014

### ChrisVer

But still, the pairing of the magnets is not so weird even if the sides are not spacelike separated. I mean the N and S parts don't connect with each other but with their neighbors... So the endpoint S doesn't look at the other endpoint N, but with its neighbouring N... In order to fill in the separation for a compact thing, you will have to fill in the distances accordingly and you will end up with N.... am I wrong?

As for strings, that's the initial use of string theory in physics... (if someone wants to check it out, he can have a look at Prof. G. t'Hoft 's lecture notes on string/superstring theory)

18. Feb 22, 2014

### Bill_K

The magnet example is misleading. If it were just a conceptual pairing, the N at the end could easily be regarded as paired with its neighbor. But what we have is a phase transition, from quarks to hadrons. A finite amount of energy is involved in the formation of each hadron, taking a finite amount of time. To "re-pair" the quarks, you have to dissolve and reform many hadrons.

The strings of String Theory are not involved. A popular and simple model of quark confinement describes it as the formation of gluon tubes, explaining why the potential energy holding a pair of quarks together grows linearly with distance. But this idea must be understood as a model only, and it relates only to confinement. In a quark-gluon plasma, the quarks are deconfined.

19. Feb 22, 2014

Staff Emeritus
Mathematically, a quark looks more like a magnetic pole than an electric charge: because the color field is strong and charged, a quark acts more like a boundary condition than an actual generator of the charge. It's often a better way to gain insight on behavior than the "color charge" model.

Many-body physics was mentioned, and this is often used to model heavy ion collisions (sometimes in mutually incompatible ways). To take the magnetic picture I discussed, the lower line has both lower energy and higher entropy. (In one dimension) You have the same thing in QCD - re-pairing the quarks has both lower energy and higher entropy. So when a QGP freezes out into hadrons, the phase transition will not leave you with free quarks.

20. Feb 23, 2014

### nikkkom

This still does not explain how quarks in a large volume would magically pick a pairing which does not leave even a single trio of spatially separated unpaired quarks. The "correct" pairing is locally indistinquishable from "incorrect" one (apart from the locations of "leftover" quarks).

Here's an illustration in 1D monopole model. Initial state is a very long but finite line of monopoles. We are looking at microscopic part of it:

.........NSNSNSNSNSNSNSNSNSNSNSNSNSNSNSNSN......

Temperature falls to to recombination threshold, particles start to pair randomly, some left unpaired:

.........(NS)(NS)(NS) N (SN)(SN)(SN)(SN) S (NS)NS(NS)(NS)(NS)(NS) N (SN)(SN)......

This is not lowest energy state, some local reshuffling happens to eliminate unpaired ones:

.........(NS)(NS)(NS)(NS)(NS)(NS)(NS)(NS)(NS)(NS)(NS)(NS)(NS)(NS)(NS)(NS)(N......

All is good, eh? Well, not really, if the global picture is like this! -

S.........(NS)(NS)(NS)(NS)(NS)(NS)(NS)(NS)(NS)(NS)(NS)(NS)(NS)(NS)(NS)(NS)(N......N

This particular choice of "locally correct" pairing is in fact the wrong one: globally, it will leave, at a minimum, two unpaired particles.

Last edited: Feb 23, 2014
21. Feb 23, 2014

Staff Emeritus
This is the same question as "how does one atom in a forming crystal know about the position of another atom a million cell spacings away? The answer is that even locally the "right" pairing has lower energy than the wrong one.

22. Feb 24, 2014

### nikkkom

You actually confirm my point: crystals of macroscopic sizes aren't perfect, they have unfilled vacancies and interstitial atoms as analogues of what I describe.

On another note, if crystal grows slowly, there is an obvios syncronization mechanism for new atoms to take the correct locations on the growth front. If crystallization would happen quickly in a large volume, crystals will be small and randomly oriented - not a lowest energy state, clearly.

23. Feb 24, 2014

Staff Emeritus
Fine. Believe whatever the heck you want. You still don't get free quarks, for precisely the reasons I describe: when the phase transition is occurring, the right pairing is favorable.

24. Feb 24, 2014

### ChrisVer

perfect or not (crystals) we still haven't found an isolated magnetic monopole (unfortunately)

25. Feb 24, 2014

### Staff: Mentor

You're pushing V50's analogy to and beyond the breaking point. Defects in a macroscopic crystal would quickly disappear if the potential barrier between the state with the defect and the lower-energy state without the defect were lower relative to the energy difference.

The interesting question is not why quarks pair up properly; it is why crystals don't form properly.