Is the concept of the non-Hermitian effective Hamiltonian relevant in the case of potential scattering?

TL;DR

This paper investigates the relevance and properties of the non-Hermitian effective Hamiltonian in potential resonance scattering, providing a self-consistent formalism and analyzing its application to a chain of delta barriers.

Contribution

It introduces a self-adjoint formulation of potential scattering, clarifies the role of the non-Hermitian operator H, and constructs the effective Hamiltonian for specific boundary conditions.

Findings

The physical scattering amplitude is unique despite different boundary condition choices.

The effective Hamiltonian H_{eff} accurately describes resonance energies and widths.

Formation of doorway states and long-lived trapped states is demonstrated.

Abstract

We examine the notion and properties of the non-Hermitian effective Hamiltonian of an unstable system using as an example potential resonance scattering with a fixed angular momentum. We present a consistent self-adjoint formulation of the problem of scattering on a finite-range potential, which is based on separation of the configuration space on two, internal and external, segments. The scattering amplitude is expressed in terms of the resolvent of a non-Hermitian operator H. The explicit form of this operator depends both on the radius of separation and the boundary conditions at this place which can be chosen in many different ways. We discuss this freedom and show explicitly that the physical scattering amplitude is, nevertheless, unique though not all choices are equally adequate from the physical point of view. The energy-dependent operator H should not be confused with the non-Hermitian effective Hamiltonian H_{eff} exploited usually to describe interference of overlapping resonances. We apply the developed formalism to a chain of L delta-barriers whose solution is also found independently in a closed form. For a fixed band of L overlapping resonances, the smooth energy dependence of H can be ignored so that complex eigenvalues of the LxL submatrix H_{eff} define the energies and widths of the resonances.We construct H_{eff} for the two commonly considered types of the boundary conditions (Neumann and Dirichlet) for the internal motion. Formation in the outer well of a short-lived doorway state is explicitly demonstrated together with the appearance of L-1 long-lived states trapped in the inner part of the chain.