Systematic approach to leptogenesis in nonequilibrium QFT: vertex contribution to the CP-violating parameter

TL;DR

This paper uses nonequilibrium quantum field theory to analyze leptogenesis, revealing medium effects increase the CP-violating parameter and highlighting advantages of the Kadanoff-Baym formalism over traditional methods.

Contribution

It applies the Schwinger-Keldysh formalism to leptogenesis, demonstrating medium effects on CP violation and addressing issues like double-counting in previous approaches.

Findings

Medium effects increase the vertex contribution to CP violation.

At high temperatures, the CP-violating parameter is several times larger than in vacuum.

The formalism naturally ensures the asymmetry vanishes in equilibrium.

Abstract

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.