Derivative expansion in a two-scalar field theory

The derivative expansion of the effective action is considered in the model with two interacting real scalar fields in curved spacetime. Using the functional approach and local momentum representation, the coefficient of the derivative term is calculated up to the first order in curvature in the one-scalar theory. The two-scalar problem is solved by extracting normal modes and consequent reduction to the single-scalar case. The method can be applied to a larger number of scalars. In the theory with strong hierarchy of masses, the renormalized effective potential and the coefficients of the second-order derivative terms demonstrate the quantum decoupling in the low-energy limit.