Time-dependent perturbation theory with permanent effects |
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| Authors: | F. Englert J.-M. Frère |
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| Affiliation: | Faculté des Sciences, Université Libre, Bruxelles, Belgique;CERN, Geneva, Switzerland |
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| Abstract: | The presence of time-varying external fields invalidates usual perturbation theories as expressed by Feynman diagrams. We derive the correct Feynman graphs, which automatically ensure unitary and causality. The relevance of these considerations for cosmological problems is pointed out. |
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