Abstract
Nonequilibrium quantum field theory is often used to derive an approximation for the evolution of number densities and asymmetries in astroparticle models when a more precise treatment of quantum thermal effects is required. This work presents an alternative framework using the zero-temperature quantum field theory, S-matrix unitarity, and classical Boltzmann equation as starting points leading to a set of rules for calculations of thermal corrections to reaction rates. Statistical factors due to on-shell intermediate states are obtained from the cuts of forward diagrams with multiple spectator lines. It turns out that it is equivalent to cutting closed diagrams on a cylindrical surface.
Highlights
Exploring particle physics models’ implications in cosmology, we often focus on their predictions for dark matter relic density or C P asymmetries in the standard model sector
We show that constructing forward diagrams with each propagator wound on a cylindrical surface any number of times is a helpful heuristic tool and correct statistical factors for the on-shell intermediate states are obtained1
A diagrammatic concept connecting the classical Boltzmann equation and quantum kinetic theory has been introduced in this work
Summary
Exploring particle physics models’ implications in cosmology, we often focus on their predictions for dark matter relic density or C P asymmetries in the standard model sector. The particles’ interactions are computed in terms of the zero-temperature quantum theory, while the kinetic description of the thermal medium is entirely classical. This work deals with finite-temperature effects in an alternative way, based on an in–out formalism, the S-matrix unitarity, and C P T symmetry. We would like to emphasize that the present work is not about leptogenesis in particular. Instead, it aims to introduce a novel diagrammatic approach that, once the zerotemperature asymmetries are known, gives the expressions for thermal corrections for free, making a huge difference compared to the original works [14,15] based on the in-in formalism. The approach is general and within the approximations mentioned above can be used to include thermal corrections to any processes, as shown in Sect.
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