A return to observability near exceptional points in a schematic PT-symmetric model

Many indefinite-nietric (often called pseudo-Hermitian or PT-symmetric) quantum models H prove "physical" (i.e., Hermitian with respect to an innovated, ad hoc scalar product) inside a characteristic domain of parameters D. This means that the energies get complex (= unobservable) beyond the boundary partial derivative D) (= Kato's "exceptional points", EPs). In a solvable example we detect an enlargement of E) caused by the emergence of a new degree of freedom. We conjecture that such a beneficial mechanism of a return to the real spectrum near EPs may be generic and largely model-independent. (c) 2007 Elsevier B.V. All rights reserved.