The propagator of a relativistic particle via the path-dependent vector potential

The proper time formalism for a particle propagator in an external electromagnetic field is combined with the path-dependent formulation of the gauge theory to simplify the quasiclassical propagator. The latter is achieved due to a specific choice of the gauge corresponding to the use of the classical path in the path-dependent formulation of the gauge theory, which leads to the cancellation of the interaction part of the action in the Feynman path integral. A simple expression for the quasiclassical propagator is obtained in all cases of the external field when the classical equation of motion in this field is integrable. As an example, new simple expressions for the propagators are derived for a spinless charged particle interacting with the following fields: an arbitrary constant and uniform electromagnetic field, an arbitrary plane wave and, finally, an arbitrary plane wave combined with an arbitrary constant and uniform electromagnetic field. In all these cases the quasiclassical propagator coincides with the exact result.

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