Dirac field in $\mathrm{AdS}_2$ and representations of $\widetilde{\mathrm{SL}}(2,\mathbb{R})

We study the solutions to the Dirac equation for the massive spinor field in the universal covering space of two-dimensional anti-de Sitter space. For certain values of the mass parameter, we impose a suitable set of boundary conditions which make the spatial component of the Dirac operator self-adjoint. Then, we use the transformation properties of the spinor field under the isometry group of the theory, namely, the universal covering group of $\mathrm{SL}(2,\mathbb{R})$, in order to determine which self-adjoint boundary conditions are invariant under this group. We identify the corresponding solution spaces with unitary irreducible representations of this group using the classification given by Pukanzki, and determine which of these correspond to invariant positive- and negative-frequency subspaces and, hence, in a vacuum state invariant under the isometry group. Finally, we examine the cases where the self-adjoint boundary condition leads to an invariant theory with non-invariant vacuum state and determine the unitary representation to which the vacuum state belongs.