I How can Gluon travel at speed of light?

1. Jan 19, 2016

A AM ARYA

How can Gluon travel at speed of light when it is bound inside a proton or neutron?

2. Jan 19, 2016

Staff: Mentor

It doesn't travel for a long distance. But that view is way too classical. Gluons on hadrons do not exist as real particles, so it is meaningless to ask about their speed.

3. Jan 19, 2016

A AM ARYA

8 New Are you saying that Gluon is a virtual particle?

4. Jan 19, 2016

Staff: Mentor

5. Jan 21, 2016

Mihail Anghelici

By specifying the gluons in hadrons , does it mean there are gluons that are NOT in hadrons ..?

6. Jan 21, 2016

jerromyjon

I think he meant they "aren't anywhere specifically" but when they are in hadrons we know they are there somewhere.

7. Jan 21, 2016

Staff: Mentor

They can be in a QCD plasma, or part of hard parton showers before hadronization occurs. In both cases, their properties are not so far away from real particles.

8. Jan 25, 2016

vanhees71

Quarks and gluons have never ever been seen as asymptotic free particles. The reason is a property of QCD, called confinement, which is a non-perturbative phenomenon and thus not rigorously understood.

What's understood within perturbative QCD is asymptotic freedom, which is related to confinement but not the same. Asymptotic freedom says that in QCD the running coupling constant becomes small at large renormalization scales, i.e., for scattering process with large energy transfers, and thus perturbative QCD is only valid in that case.

At low energies the effective degrees of freedom and asymptotic states of QCD are not quarks and gluons but rather hadrons, i.e., bound states of quarks and gluons. The most convincing evidence for this picture of confinement is from lattice-QCD calculations, which are nowadays quite accurate in obtaining the mass spectrum of hadrons.

A way to study something close to free quarks and gluons is to create a socalled Quark Gluon Plasma (QGP) in ultrarelativistic heavy-ion collisions. There you smash atomic nuclei at very high energies together. Then a hot "blob" of strongly interacting matter is formed. Now one expects from the asymptotic freedom of QCD that for a hot and/or dense enough blob of matter that the hadrons melt into quarks and gluons.

However, also this picture is highly oversimplified. First of all one can perform lattice-QCD calculations also for many-body systems in thermal equilibrium, but one is restricted to the case of low net-baryon numbers (small baryo-chemical potentials), i.e., a situation reached on the largest energies at the Relativistic Heavy Ion Collider (RHIC) in Brookhaven, Upton, NY or at the Large Hadron Collider (LHC) at CERN, Geneva. Lattice-QCD predicts that the transition from a hot and dense hadron resonance gas to a QGP) (deconfinement transition) is a cross-over transition in a temperature range of about 150-160 MeV.

A lot of heavy-ion phenomenology can be understood by describing the hot and dense fireball of strongly interacting matter created in relativistic heavy-ion collisions in terms of relativistic hydrodynamics, and even with ideal relativistic hydrodynamics. This implies that the fireball comes (a) in a astonishingly short "formation time" of about 0.5-1 fm/c to local thermal equilibrium and (b) then evolves according to (nearly) ideal hydrodynamics. Comparing the hydro simulations of the hot and dense fireball with the measured particle spectra, leads to the conclusion that indeed energy densities and temperatures are reached which are well above the critical temperature range. This also implies that the plasma is still strongly coupled. In fact it is the most "sloshy" liquid ever observed on earth. The viscosity over entropy-density ratio is close to the lower bound i.e., $\eta/s \simeq 1/(4 \pi)$.

This comparison leads to the picture that the hot and dense fireball expands as a kind of liquid drop, thereby becoming colder and more and more dilute. This implies that at one point the transition from the deconfined QGP phase to a hot hadron resonance gas occurs. Analyzing the particle abundancies leads to the conclusion that the thermal freezeout, i.e., the point at which the inelastic interactions within the fireball cease, occurs close to this transition, i.e., the particle abundancies are described by a (grand-)canonical ensemble at a temperature of about 160 MeV (at RHIC and LHC). Then the fireball evolution goes on in chemical off-equilibrium but there are still elastic interactions keeping the matter in local thermal equilibrium until also these cease at a temperature of about 100 MeV, and then the hadrons run freely to the detector.

The main chalenge of research in Heavy-Ion Collisions is thus to find signatures for this picture of the evolving QCD-matter fireball and to quantitatively understand the properties of the corresponding QCD phase diagram from the final state of hadrons at thermal freeze-out. For a semipopular treatment of some aspects of this physics, see

http://fias.uni-frankfurt.de/~hees/publ/SMP07_Vanhees.pdf

9. Jan 25, 2016

MacRudi

QCD explanation is much too complicated to understand and cannot be really understood in pictures. And we don't know if this is really true.
It is much better to see quarks as branes linked with multi strings as gluons. Then you get a picture you can handle it much better.
Mathematically in QCD we have to deal with non linear functions, which makes it more complicated also to understand.
Do we have really relativistic properties in gluons? A question we cannot solve like the question if an Axion exists and if it has mass or not and how it interacts with Hadrons

10. Jan 25, 2016

vanhees71

Well, QCD has the advantage of being in quantitative accordance with all observations. I'm also not so sure that branes linked with multi strings are more "intuitive" than the QFT picture. That's a subjective issue of familiarity with the various theoretical pictures.

11. Jan 25, 2016

MacRudi

Well since some years I'm used to look first at what Stringtheory has to offer for me as picture to understand, because I have never wanted to understand the unintuitive framework of QCD or QFT at university I learned 20 years ago. I want to understand the beauty in physics and not the ugly non intuitive QT, which is not really exact as you believe. It is only a randomly playing with numbers in the end without true explanation. I'm socialised more with the GRT and not QT. So for me there is much more familiarity with stringtheory, although in origin stringtheory came from QT. But the thinking behind is much better to understand and more intuitive familiar for me. I think this is a common reason, why so many believe more in string theory. I say believe because we can only believe and have no reason to think that one framework is true and the other not. QCD and QFT is only a framework like stringtheory is. Not more.

Last edited: Jan 25, 2016
12. Jan 25, 2016

Staff: Mentor

String theory is a quantum field theory - with strings instead of "normal" particles. If you think string theory is intuitive, then QFT with the simpler particles of the standard model should be trivial.

13. Jan 25, 2016

jerromyjon

Isn't that to do with duality? The strings producing gravitation is way more complicated but the duality to SM is identical, isn't it?

14. Jan 25, 2016

vanhees71

Well, string theory has very little overlap with observable facts. Theoretical physics is all about modeling observable facts...

If you refer to AdS/CFT which is an interesting toy model to explore strongly interacting supersymmetric Yang-Mills theory, let me stress that this is NOT QCD!

15. Jan 25, 2016

jerromyjon

You could make up anything and model it with string theory, it provides no bounds to reality...

16. Jan 25, 2016

vanhees71

This is a totally misguided point of view of physics. A theory never ever provides bounds to reality but reality provides constraints to models and theories!

17. Jan 25, 2016

MacRudi

interesting aspect

now we are in the psi ontologist and psi phenomenologist debate.

18. Jan 25, 2016

jerromyjon

I'm still wondering if gluons are massless by themselves and do travel at c and who's theory has the best answer...

19. Jan 25, 2016

vanhees71

Ok, for me the thread is over...

20. Jan 25, 2016

Staff: Mentor

They are massless, in the rare cases where "speed" makes sense their speed is c, and this is true for every theory that is not in conflict with observations.