BOUNDARY ENERGY AND BOUNDARY STATES IN INTEGRABLE QUANTUM-FIELD THEORIES

We study the ground-state energy of integrable 1 + 1 quantum field the ories with boundaries (the genuine Casimir effect). In the scalar case , this is done by introducing a new ''R-channel TBA'', where the bound ary is represented by a boundary state, and the thermodynamics involve s evaluating scalar products of boundary states with all the states of the theory, In the non-scalar, sine-Gordon case, this is done by gene ralizing the method of Destri and De Vega. The two approaches are comp ared. Miscellaneous other results are obtained, in particular formulas for the overall normalization and scalar products of boundary states, exact partition functions for the critical Ising model in a boundary magnetic field, and also results for the energy, excited states and bo undary S-matrix of O(n) and minimal models.