# B Can a gluon emit a photon?

1. Nov 9, 2017

### star apple

I've been googling this and can't come up with a direct answer. Can a gluon emit a photon?

I know.
1. All Standard Model particles with mass interact via the weak force.
2. All quarks and gluons interact via the strong force.
3. All quarks, charged leptons and massive weak force bosons emit and absorb photons as part of the electro-magnetic force.
4. All Standard Model particles interact via gravity as does the hypothetical graviton.

So can gluon emit a photon?

What is the rule of thumb or one you could string into your fingers for a simple law how the 4 fundamental forces (field) behave between each other? (not amongst them and the fermions for example but between the fundamental forces/gauge fields)

2. Nov 9, 2017

### Staff: Mentor

A photon doesn't have a color charge and a gluon doesn't have an electric charge.
No. At least not at tree level, with loops it is possible of course.
Gluons only interact with gluons, photons only with charged particles, and for W and Z you have to look at the terms appearing in the Lagrangian.

3. Nov 9, 2017

### star apple

So a gluon has no weak charge nor gravity charge.. just gluon all alone?

why do graviton interact with the photon, gluon and all particles.. ? it means a photon has a graviton (or GR) charge?

In the proton radius mystery, they are proposing a fifth fundamental force (field).. is there a pattern in the existing 4 fundamental gauge field and how they interact with one another so we can interpolate the behavior of a new force (or gauge field)?

4. Nov 10, 2017

### ChrisVer

A gluon doesn't have weak charge either. It only has color charge [it's a singlet for weak interactions].
loops could potentially let gluons radiate photons as they may contain quarks [which are electromagnetically charged).

Check the Standard Model Lagrangian.

What is a graviton [which theory are you quoting]? It's not part of the SM, it exists in some exotic theories but no-one knows. However, speaking for gravity, the charge gravity sees is the energy-momentum tensor (general relativity)... as a result of this, anything that carries energy (momentum) is affected by gravity.

It depends... This is part of searches, where you know your current models' predictions well, in order to search for new particles that might violate it. For example, electroweak precision measurements quite successfully predicted where the Higgs mass would have to be.

5. Nov 12, 2017 at 4:38 AM

### vanhees71

Ad 1. If you mean quarks, leptons, gauge bosons, and the Higgs boson(s), you are right. All mass terms in the standard-model Lagrangian are due to the interaction with the Higgs field which has a non-zero vacuum expectation value. It's why the Higgs mechanism has been introduced into the Standard Model in the first place, because it provides the possibility to describe massive particles in a gauge model where the gauge group is chiral.

Note, however, that the Higgs mechanism provides only about 2% of the mass of the matter surrounding us. The rest is of dynamical origin of the strong force. It's one of the most challenging questions of contemporary physics to understand the precise "mechanism" behind this. Quantitatively it's pretty well understood by the use of high-precision lattice-QCD calculations which postdict the hadron masses at a precision level of a few percent.

A gluon carries no electric charge. So it cannot directly convert into a photon at tree level. Of course, there are processes in the standard model where a photon is created where gluons are involved, e.g., gluo-Compton scattering (gluon + quark -> quark + photon).