On the strong coupling limit of Yang-Mills matrix models
We study the strong coupling limit of mass deformed Yang--Mills matrix models, with the aim of understanding when the matrices become effectively commuting. The Yang--Mills interaction classically drives the matrices toward mutually commuting valleys, where the matrices can potentially be interpreted as coordinates of an emergent space. However, taking into consideration the integration measure, commutativity is not automatic since the commuting locus is entropically suppressed, and in the bosonic models with $D\geq3$ the strong coupling limit remains non-commuting. We find that fermions change this competition in a sharp way. As the number of fermionic degrees of freedom is increased, there is a critical value $\mathcal N_c=2(D-2)$, realized by the supersymmetric Yang--Mills matrix models, at which the matrices commute at strong coupling. The same critical models also exhibit universality under deformations by $O(N^2)$, or huge, operators: the normalized eigenvalue densities are insensitive to the microscopic details of the huge operators. Increasing the number of fermions beyond the critical point still gives commuting matrices, but the huge-operator universality is lost. Thus commutativity and universality are related but distinct: the matrix models with supersymmetric field content sit at the critical boundary where we have both.