Peter Matak (Comenius U): Cutting rules for early universe calculations

Sydney CPPC Seminar 20th June 2024

The presence of CP asymmetric reactions in the early universe is one of the conditions for generating baryon asymmetry. For them to occur, the Lagrangian of the theory must contain couplings with irreducible complex phases, while the kinematics of such processes contain an imaginary part. Still, even when both these conditions are met, the resulting asymmetry may vanish due to the CPT and unitarity constraints. In the first part of the seminar, we introduce the holomorphic cutting rules for tracking the asymmetry cancellations at the diagrammatic level. Then, we look more closely at the Boltzmann equation for particle densities in the expanding universe. We demonstrate that, surprisingly, thermal corrections can be represented by completing the collision term by all processes implied by on-shell cuts of the forward scattering diagrams drawn on a cylindrical surface.