On the construction of the Hadamard sates in two dimensions

The two dimensional analog of the Hadamard state condition is used to specify the local Hadamard states associated with a linear quantum field coupled to a two dimensional gravitational background. To characterize a local Hadamard state corresponding to a physical vacuum state, we apply a superselection rule in which the state dependent part of the two-point function is determined in terms of a dynamical scalar field. It implies a basic connection between the vacuum state and a scalar field coupled to gravity. We study the characteristics of the Hadamard vacuum state through this superselection rule using two different background metrics, the two dimensional analog of the schwarzschild and FRW metric.