  It has been known for some time that the dynamics of k coincident D-branes in
string theory is described effectively by U(k) Yang-Mills theory at low energy.
While these configurations appear as classical solutions in matrix models, it
was not clear whether it is possible to realize the k =/= 1 case as the true
vacuum. The massive Yang-Mills-Chern-Simons matrix model has classical
solutions corresponding to all the representations of the SU(2) algebra, and
provides an opportunity to address the above issue on a firm ground. We
investigate the phase structure of the model, and find in particular that there
exists a parameter region where O(N) copies of the spin-1/2 representation
appear as the true vacuum, thus realizing a nontrivial gauge group dynamically.
Such configurations are analogous to the ones that are interpreted in the BMN
matrix model as coinciding transverse 5-branes in M-theory.
