General pseudo self-adjoint boundary conditions for a 1D KFG particle in a box

TL;DR

This paper derives the most general pseudo self-adjoint boundary conditions for a 1D Klein-Fock-Gordon particle in a box, revealing dependence on four parameters and clarifying boundary conditions for different wavefunction formulations.

Contribution

It introduces the first comprehensive set of boundary conditions for the 1D Klein-Fock-Gordon Hamiltonian, applicable to both one-component and two-component wavefunctions, depending on four real parameters.

Findings

The boundary conditions depend on four real parameters.

Two-component wavefunctions do not necessarily satisfy the same boundary conditions as the one-component wavefunction.

The results can be extended to particles with point interactions on a line.

Abstract

We consider a 1D Klein-Fock-Gordon particle in a finite interval, or box. We construct for the first time the most general set of pseudo self-adjoint boundary conditions for the Hamiltonian operator that is present in the first order in time 1D Klein-Fock-Gordon wave equation, or the 1D Feshbach-Villars wave equation. We show that this set depends on four real parameters and can be written in terms of the one-component wavefunction for the second order in time 1D Klein-Fock-Gordon wave equation and its spatial derivative, both evaluated at the endpoints of the box. Certainly, we write the general set of pseudo self-adjoint boundary conditions also in terms of the two-component wavefunction for the 1D Feshbach-Villars wave equation and its spatial derivative, evaluated at the ends of the box; however, the set actually depends on these two column vectors each multiplied by the singular matrix that is present in the kinetic energy term of the Hamiltonian. As a consequence, we found that the two-component wavefunction for the 1D Feshbach-Villars equation and its spatial derivative do not necessarily satisfy the same boundary condition that these quantities satisfy when multiplied by the singular matrix. In any case, given a particular boundary condition for the one-component wavefunction of the standard 1D Klein-Fock-Gordon equation and using the pair of relations that arise from the very definition of the two-component wavefunction for the 1D Feshbach-Villars equation, the respective boundary condition for the latter wavefunction and its derivative can be obtained. Our results can be extended to the problem of a 1D Klein-Fock-Gordon particle moving on a real line with a point interaction (or a hole) at one point.