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Slowly decaying classical fields, unitarity, and gauge invariance

Dennis D. Dietrich
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Abstract

In classical external gauge fields that fall off less fast than the inverse of the evolution parameter (time) of the system the implementability of a unitary perturbative scattering operator ($S$-matrix) is not guaranteed, although the field goes to zero. The importance of this point is exposed for the counter-example of low-dimensionally expanding systems. The issues of gauge invariance and of the interpretation of the evolution at intermediate times are also intricately linked to that point.

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Dennis D. Dietrich. 2005-12-02. Slowly decaying classical fields, unitarity, and gauge invariance. https://doi.org/10.1103/physrevd.74.085003

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