Vacuum structure and θ states of adjoint QCD in two dimensions Original Research Article

We address the issue of topological angles in the context of two-dimensional SU(N) Yang-Mills theory coupled to massive fermions in the adjoint representation. Classification of the resulting multiplicity of vacua is carried out in terms of asymptotic fundamental Wilson loops, or equivalently, charges at the boundary of the world. We explicitly demonstrate that the multiplicity of vacuum states is equal to N for the SU(N) gauge group. Different worlds of the theory are classified by the integer number k = 0, 1,…, N − 1 (superselection rules) which plays a role analogous to the θ parameter in QCD. We study the physical properties of these unconnected worlds as a function of k. We achieve this by using two completely independent approaches: First, we apply the well-known machinery of the loop calculus in order to calculate the effective string tensions in the theory as function of k. The second way of doing the same physics is the standard particle/field theoretic calculation for the binding potential of a pair of infinitely massive fermions. We also calculate the vacuum energy as function of k.