Gluons: Rest Mass & Experimental Evidence

[mathman]

As I understand it, gluons have no rest mass. In answering my previous question, particles consisting of gluons without quarks were described. Do these particles have any rest mass or do they go at the speed of light? What sort of experimental evidence is there for whatever happens?

 

[chroot]

Originally posted by mathman
As I understand it, gluons have no rest mass. In answering my previous question, particles consisting of gluons without quarks were described. Do these particles have any rest mass or do they go at the speed of light? What sort of experimental evidence is there for whatever happens?

Just because the particles have no rest mass themselves does not mean that they cannot have non-zero binding energy when stuck together. To the world outside the composite particle, it will appear to have mass equal to the binding energy of its components.

- Warren

 

[mormonator_rm]

Chroot is correct. In fact, glueballs tend to have greater rest masses than the other actual mesons in their quantum number multiplets. For example, the isoscalar and isovector members of the 1(3)P0 (scalar) nonet in SU(3) are f0(1370), f0(1710), and a0(1450). The scalar glueball, which will appear to be a supernumerary meson in the 1(3)P0 nonet, is predicted to have a rest mass of roughly 1611 MeV. So it will be heavier than its equivalent meson f0(1370), and will be close to the mass of f0(1710). f0(1710) has a greater mass because it should be a dominantly (s + -s) state (if it is not the scalar glueball itself), making it heavier than the others which consist of the lightest quarks, u and d. The tensor glueball, which should appear supernumerary to the mesons in the 1(3)P2 nonet, should have a rest mass of about 2231 MeV. This is gigantic compared to the isoscalar and isovector members of that nonet (f2(1270), f'2(1525), and a2(1320)). It is 700 MeV larger than f'2(1525), which is the dominantly (s + -s) state here.

The glueballs will indeed have mass, and should behave very much as other mesons do. The current candidates for these states, such as f0(1500) and f0(1710) for the scalar, do show interesting behavior that could set them apart as glueballs. For instance, f0(1710) does not couple to KKbar nearly as much as it should if it were the dominantly (s + -s) member of the scalar nonet. On the other hand, f0(1500) has a very narrow width for its mass, meaning it has an unusually long lifetime for a meson its size, and it could clearly be supernumerary to f0(1370). f0(1710) is also fairly narrow, and neither f0(1500) of f0(1710) seem to decay via two-photon mode. I think f0(1500) is the mostly gluonic one though, which favors it as the glueball rather than f0(1710). Hence, f0(1710) is considered to be an actual member of the 1(3)P0 nonet, while f0(1500) is the favored scalar glueball candidate. I am currently working on identifying the scalar glueball; it is the focus of my current research on ppbar annihilation data from Fermilab.