A Note on Enhanced Gauge Symmetries in M- and String Theory

Abstract

Two different mechanisms exist in non-perturbative String / M- theory for enhanced SU(N) (SO(2N)) gauge symmetries. It can appear in type IIA string theory or M-theory near an $A_{N-1}$ (D_N) type singularity where membrnes wrapped around two cycles become massless, or it can appear due to coincident D-branes (and orientifold planes) where open strings stretched between D-branes become massless. In this paper we exhibit the relationship between these two mechanisms by displaying a configuration in M-theory, which, in one limit, can be regarded as membranes wrapped around two cycles with $A_{N-1}$ (D_N) type intersection matrix, and in another limit, can be regarded as open strings stretched between N Dirichlet 6-branes (in the presence of an orientifold plane).