Theta-vacuum systems via real action simulations

Inspired by the results of the Ising model within an imaginary external magnetic field, we introduce a transformation in quantum systems with a theta-vacuum term that amounts to a rescaling of z=cos(theta/2). Making use of this transformation we are able to determine the order parameter as a function of theta. The approach is successfully tested in models with both broken and unbroken CP symmetry at theta=pi.