# Gluons: Are They Massless? | A Deeper Look

It is also not known for certain that the gluon is actually massless, it is only supposed; all that is certain from measurement is that if it is not zero then its mass must be very small.
I read this from wiki, I don't trust that site a lot.
Is this true?
can someone elaborate on this?

that statement simply is saying that experimentally we are not 100% sure whether gluons are massless... a fair comment. It is like saying that although we believe that an electron is "truly a point particle" but experimentally we only have an upper bound on its radius. Hence we could say: "all that is certain from measurement is that if the radius of an electron is not zero then it must be very small."

So, nothing is wrong about the quoted statement if it was actually said in the above spirit.

Now bear with me for a minute, isn't supposed that gluon's are just like photons, but the main difference that gluon's interact with each other, and carry the force charge(i'm just talking about similarity as both of them are scalar right? or am I wrong?)?
And something else, isn't supposed since there is unbroken gauge invariance, it requires the gauge boson(the gluon's) to be massless?

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If they have mass , even a small mass wouldn't that lead to a broken gauge invariance??

I think there are two issues here. One is whether we believe in our theory and the other is whether our experiments can actually "proof" it. I've got the feeling that the wiki article is speaking in the point of view of an experimentalist. let's take a simpler example... do you believe in Conservation of Energy? Now, there is no experiments on Earth that you can do that will "proof" to you that the conservation law is actually correct, as much as there is no way you can find the true size of an electron... hence, if I want to be cautious, I may say that "It is also not known for certain that conservation of Energy actually happens, it is only supposed; all that is certain from measurement is that any deviation must be very small."

Maybe I'm been confused about something else explain this to me:
Isn't because the W,Z bosons have mass, leads to broken gauge variance, which in the end lead to a CP violation in weak interaction?
So my point is if that's true, isn't supposed that we should see in some sort of an experiment a violation of a certain symmetry in color interaction?

Edit: Ahh, I see what you maybe pointing to, there is no actually way now to determine in an experiment if there is a symmetry violation in color interaction??

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Point is, how or when do you restore gauge invariance?

I don't know

If gluons did have mass, one would have to ask which color ends up being favored with which quark flavors... I am more comfortable with the idea of gluons being massless just because it would make things less complicated, and would remain analogous to the massless photon. Not only that, but the glueballs in the lattice would look a bit different, I imagine.

Particle Data group statement on gluon mass:

Mass m=0. Theoretical value. A mass as large as a few MeV may not be precluded. [F.J. Yndurain, 95 PL B345 524]
http://www.ingentaconnect.com/content/els/03702693/1995/00000345/00000004/art01677 [Broken]
http://www.slac.stanford.edu/spires/find/hep/www?indexer=1&rawcmd=find+j+PHLTA,B345,524 [Broken]

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It is also not known for certain that the gluon is actually massless, it is only supposed; all that is certain from measurement is that if it is not zero then its mass must be very small.

well, interesting question.

There exists a gluon field (analogous to the photon field),, unlike the photon field, there are excitations that are highly self interacting,,, there exist ultra high energy excitations of the field which are much like photons - they are massless particles and do not self-interact,, however low energy excitations are very different,, they are strongly self interacting, and the concept of a single gluon is not meaningful,, it is not a stable particle - it is not an elementary excitation of the field..

in this low energy regime, theory suggest the existence of glueballs,, which are special boundstates of "gluons",, bound in such a way that the entire particle is nuetral (non-interacting via the strong force).

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well, interesting question.

There exists a gluon field (analogous to the photon field),, unlike the photon field, there are excitations that are highly self interacting,,, there exist ultra high energy excitations of the field which are much like photons - they are massless particles and do not self-interact,, however low energy excitations are very different,, they are strongly self interacting, and the concept of a single gluon is not meaningful,, it is not a stable particle - it is not an elementary excitation of the field..

in this low energy regime, theory suggest the existence of glueballs,, which are special boundstates of "gluons",, bound in such a way that the entire particle is nuetral (non-interacting via the strong force).

I have studied these "glueballs", especially the scalar one, in considerable depth. They are indeed very interesting.