Virtual Classes of Character Stacks

In this paper, we extend the Topological Quantum Field Theory developed by Gonz\'alez-Prieto, Logares and Mu\~noz for computing virtual classes of representation varieties of closed orientable surfaces in the Grothendieck ring of varieties to the setting of the character stacks. To this aim, we define a suitable Grothendieck ring of representable stacks, over which this Topological Quantum Field Theory is defined. We apply this theory to the case of the affine linear group of rank 1, providing an explicit expression for the virtual class of the character stack of closed orientable surfaces of arbitrary genus. This virtual class remembers the natural adjoint action, and in particular from this we can derive the virtual class of the character variety.