Why are some extra dimension theories known as strongly coupled and others as weakly coupled?

Question

I was looking at pdf file of the presentation of a conference talk. The speaker discusses two types of "mechanisms" for stabilizing the weak scale and calls them "weakly coupled" and "strongly coupled". The examples are:

Weakly coupled: SM with a light Higgs, SUSY, Little Higgs, Twin Higgs, Large extra dimensions, Universal extra dimensions.

Strongly Coupled: Technicolor, Topcolor, Top See Saw, Composite Higgs, Randall-Sundurm warped extra dimension models.

I need to understand what makes one such beyond standard model theory to be weakly coupled and another to be strongly coupled, in general and especially why large/universal extra dimension and Randall-Sundrum models belong to two separate groups.

Perhaps I should emphasize where my doubt is: I understand why theories like technicolour are thought to be "strongly coupled" but I do not at all understand why theories involving say Randall-Sundrum extra dimensional models are also so. When we say Randall-Sundrum warped extra dimension model, we mean a particular kind of background on which different types of interactions take place. But why should all interactions taking place in such a background will have to be strongly coupled? How does background spacetime decide whether an (or all) interaction(s) will be strongly coupled?

Or does this have anything to do with the fact that in (classical) GR, spacetime is dynamically determined and is coupled with matter and energy content of the universe and may be due to quantum gravity effects this strong/weak nature of interactions are/is manifested. But what is the way to explain that?

Answer 1

Strongly coupled theories are those whose coupling constant is strong i.e. greater than a particular number of order of one, $g\gt {\mathcal O}(1)$. It means that perturbative expansions (and the leading simplest Feynman diagrams) are not good approximations for the most elementary physical processes that may occur in these theories. One must find other methods because the influence of quantum mechanics is important.

This is obviously the case for technicolor whose key processes depend on the confinment of the technicolor and confinement always means that the coupling is getting strong (and even stronger, arbitrarily stronger). But the other strongly coupled models in your list are analogous. On the other hand, the weakly coupled theories in your list are analogous to the Standard Model at high energies, a few leading terms i.e. simple Feynman diagrams are enough to get a good enough approximation.