Quark anti-quark pair creation. Do we need a gluon?

What is the difference between these diagrams?

Is the presence of the gluon taken for granted in the second one?

and

EDIT: I just zoomed into the picture, it is not a single quark state. There's a quark anti-quark pair at the beginning, and I just zoomed in on one leg.

I mean, can a $q$$\overline{q} pair be created out of the vacuum like in the second diagram? Or do we need an interaction of some sort, like a photon or a gluon? • the first diagram describes an interaction. the second does not. you do not give a context, I cannot think what it could describe, as there are no free quarks. The straight on should be matched with either two quarks or an antiquark to get a physically observed paraticle. – anna v Feb 15 '15 at 15:59 2 Answers Well, quark anti-quark pair can be created by a photon in a process like$$ \mu^- + \mu^+ \to \gamma \to q + \bar{q} \,,$$which is just the time reversal of a typical Drell-Yan process, with the intermediate photon shown explicitly. Not sure if this really answers your question since you seem to be assuming a single quark initial state. • I mean, can a q$$\overline{q}$ pair be created out of the vacuum like in the second diagram? Or do we need an interaction of some sort, like a photon or a gluon? – SuperCiocia Feb 15 '15 at 16:31
• If you want to have an on-shell result (i.e. not just a vacuum fluxuation, then the energy has to come from somewhere. – dmckee --- ex-moderator kitten Feb 15 '15 at 16:39
• In the LHC energies (specifically LHCb) which process is most probable to happen? Gluon leg or something else? – SuperCiocia Mar 2 '15 at 18:06
• Gluons. Drell-Yan is controlled by the electromagnetic coupling constant and is lower probability that strong interactions up to near the grand unification scale (whatever that is...). – dmckee --- ex-moderator kitten Mar 2 '15 at 19:34

In your second diagram, there is implicitly a gauge boson source of your $q\bar{q}$ pair production. It could be a gluon, a photon or a $Z^0$. This gauge boson has to be attached to something, reasonably your single quark leg as in the first diagram.

• In the LHC energies (specifically LHCb) which process is most probable to happen? – SuperCiocia Mar 2 '15 at 18:06
• Starting from a quark (as suggested by your diagram), a gluon is easily emitted (due to color factor and $\alpha_s$). The gluon doesn't need to carry lots of energy to produce a pair of light quarks, so even soft gluons produce these pairs. – Paganini Mar 2 '15 at 21:21