量子场论中的粒子产生、经典性和相关问题：II。来自场论的例子
Particle creation, classicality and related issues in quantum field theory: II. Examples from field theory
摘要：
我们采用了在论文I（arXiv：0708.1233）中发展的分析量子化含时振子演化的一般形式，解决了在含时外部背景下量子场论中的几个问题.特别地，我们从相空间演化及其与粒子产生的关系的角度研究了经典性的出现问题，并澄清了一些概念性问题。我们考虑了一个量子化标量场在恒定电场和FRW时空中的演化，这两种情况分别是晚期绝热演化和高度非绝热演化。利用本文中引入的粒子数密度、有效拉格朗日量等各种量的随时间变化的推广，我们对比了这两种极限下的演化，从而揭示了施温格效应与德西特背景下演化的关键区别。此外，我们的例子表明，古典的概念是多方面的，任何一个单一的标准可能不具有普遍适用性。例如，经典轨迹\emph{alone}上的相空间维格纳分布的峰值并不意味着向经典行为的转变。对论文I中介绍的\emph{classicality parameter}的行为的分析得出这样的结论：强粒子产生对于量子态在后期在相空间中变得高度相关是必要的。
英文摘要：
We adopt the general formalism, which was developed in Paper I (arXiv:0708.1233) to analyze the evolution of a quantized time-dependent oscillator, to address several questions in the context of quantum field theory in time dependent external backgrounds. In particular, we study the question of emergence of classicality in terms of the phase space evolution and its relation to particle production, and clarify some conceptual issues. We consider a quantized scalar field evolving in a constant electric field and in FRW spacetimes which illustrate the two extreme cases of late time adiabatic and highly non-adiabatic evolution. Using the time-dependent generalizations of various quantities like particle number density, effective Lagrangian etc. introduced in Paper I, we contrast the evolution in these two limits bringing out key differences between the Schwinger effect and evolution in the de Sitter background. Further, our examples suggest that the notion of classicality is multifaceted and any one single criterion may not have universal applicability. For example, the peaking of the phase space Wigner distribution on the classical trajectory \emph{alone} does not imply transition to classical behavior. An analysis of the behavior of the \emph{classicality parameter}, which was introduced in Paper I, leads to the conclusion that strong particle production is necessary for the quantum state to become highly correlated in phase space at late times.
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