Does a gluon have mass or not? I was under the impression that gluons had no mass. Then Wikipedia throws this curveball:

Mass:
0 (theoretical value)
< 0.0002 eV/c2 (experimental limit)

Which one should a poor soul go by? If the theoretical value just the one that people "thought" was right, until they found out the truth, or is the experimental limit(?) an impossibility that still has not been explained? Or something third?
 A: So far gluons appear massless, and if they have mass this would mean there is some sort of symmetry breaking or Higgs mechanism involved with QCD. So far there is no evidence of this, and theory does not make predictions of gluon masses.
The coupling parameter of QCD decreases with transverse momentum or $\sqrt{s}$. This higher energy limit means that gluons behave less bound to each other. Scattering energy at ever higher energy should be tighter lower bounds on the mass a gluon could assume. If a mass signature is found at one high energy scale that does not go away at higher energy, then we will have to change our physics accordingly.
A: The Standard Model was explicitly constructed to match experiments. As of today, we have no reason to believe gluons should be massive - and as you can see, the experimental upper bound  is very tiny - and so we write no mass term in the Standard Model Lagrangian. Or put it another way: we strongly believe gluons are massless, and therefore our theoretical model predicts them to be massless.
But we are scientists, so we must take nothing for granted. No matter how sure we are gluons are massless, we must measure their mass and check whether this hypothesis is true or not. And so we go and measure it, and we find that, if it is not zero, it must be very small. Over the coming years, we will get more precise experiments, and the upper bounds will be brought down. But if it doesn't it will mean that gluons were not, after all, massless, and we will have to change the Standard Model to cope with this.
