非平衡QFT中细长发生的系统方法：顶点对CP破坏参数的贡献
Systematic approach to leptogenesis in nonequilibrium QFT: vertex contribution to the CP-violating parameter
摘要：
通过轻子生成的重子不对称性的产生通常是通过经典动力学方程来研究的，而经典动力学方程对早期热膨胀宇宙中的过程的适用性是值得怀疑的。最先进的描述所暗示的近似可以在基于非平衡场论技术的第一原理方法中进行测试。在这里，我们将Schwinger-Keldysh/Kadanoff-Baym形式主义应用于一个简单的leptogenesis玩具模型。我们发现，在玩具模型中，介质效应增加了顶点对CP破坏参数的贡献。在高温下，它比真空中大几倍，并且随着温度降低而渐近地达到真空值。与先前在热场论框架内得到的结果相反，修正在粒子数密度中仅是线性的。Kadanoff-Baym形式主义的一个重要特征是它没有重复计数问题，即不需要真正的中间状态减法。特别地，这意味着方程的结构自动地确保不对称性在平衡中消失。这些结果给了第一次瞥见一些新的和有趣的效果，可以在非平衡场论的框架内进行研究。
英文摘要：
The generation of a baryon asymmetry via leptogenesis is usually studied by means of classical kinetic equations whose applicability to processes in the hot and expanding early universe is questionable. The approximations implied by the state-of-the-art description can be tested in a first-principle approach based on nonequilibrium field theory techniques. Here, we apply the Schwinger-Keldysh/Kadanoff-Baym formalism to a simple toy model of leptogenesis. We find that, within the toy model, medium effects increase the vertex contribution to the CP-violating parameter. At high temperatures it is a few times larger than in vacuum and asymptotically reaches the vacuum value as the temperature decreases. Contrary to the results obtained earlier in the framework of thermal field theory, the corrections are only linear in the particle number densities. An important feature of the Kadanoff-Baym formalism is that it is free of the double-counting problem, i.e. no need for real intermediate state subtraction arises. In particular, this means that the structure of the equations automatically ensures that the asymmetry vanishes in equilibrium. These results give a first glimpse into a number of new and interesting effects that can be studied in the framework of nonequilibrium field theory.
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