Small Circle Expansion for Adjoint QCD$_2$ with Periodic Boundary Conditions

Abstract: We study $1+1$-dimensional $\text{SU}(N)$ gauge theory coupled to one adjoint multiplet of Majorana fermions on a small spatial circle of circumference $L$. Using periodic boundary conditions, we derive the effective action for the quantum mechanics of the holonomy and the fermion zero modes in perturbation theory up to order $(gL)^3$. When the adjoint fermion mass-squared is tuned to $g^2 N/(2\pi)$, the effective action is found to be an example of supersymmetric quantum mechanics with a nontrivial superpotential. We separate the states into the $\mathbb{Z}_N$ center symmetry sectors (universes) labeled by $p=0, \ldots, N-1$ and show that in one of the sectors the supersymmetry is unbroken, while in the others it is broken spontaneously. These results give us new insights into the $(1,1)$ supersymmetry of adjoint QCD$_2$, which has previously been established using light-cone quantization. When the adjoint mass is set to zero, our effective Hamiltonian does not depend on the fermions at all, so that there are $2^{N-1}$ degenerate sectors of the Hilbert space. This construction appears to provide an explicit realization of the extended symmetry of the massless model, where there are $2^{2N-2}$ operators that commute with the Hamiltonian. We also generalize our results to other gauge groups $G$, for which supersymmetry is found at the adjoint mass-squared $g^2 h^\vee/(2\pi)$, where $h^\vee$ is the dual Coxeter number of $G$.