QFT approach to dressed particle processes in preheating and non-perturbative mechanism in kinematically-forbidden regime by Hidetoshi Taya, Yusuke Yamada

“…Our approach is based on the so-called perturbation theory in the Furry picture, in which coherent background fields (i.e., inflaton and the expanding Universe) are treated non-perturbatively whereas interactions between dressed particles are taken into account perturbatively. …”
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