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Thermal Effects on Particle Production during Cosmic Reheating by Mubarak MOHAMMED -
Mardi 30 juin 2026 à 14h00 - Auditoire CYCL01 - Chemin du Cyclotron, 2 - 1348 Louvain-la-Neuve -
The dynamics of cosmic reheating connect inflation to the hot Big Bang and play a crucial role in determining the thermal history of the Universe and the production of relic particles such as Dark Matter (DM). A consistent description of this epoch requires the computation of particle production and interaction rates in a thermal environment, where excitations are described by quasi-particles with medium-dependent properties. In this thesis, we study particle production during reheating using real-time non-equilibrium quantum field theory, with a focus on the Closed Time Path formalism. We show that in-medium interaction rates are governed by the spectral structure of self-energies, which encode both dispersive and dissipative effects. Using scalar field theories as a laboratory, we investigate the impact of thermal masses, quantum statistics, finite-width effects, and higher order processes, and assess the validity of commonly used approximations. In particular, we demonstrate that higher-order scattering processes can significantly modify the dynamics and remain relevant even when decay channels are kinematically suppressed.
In a separate part of this work, we investigate Dark Matter production during reheating in freeze-in scenarios. We find that finite-temperature corrections generally have a limited impact on the predicted relic abundance in regimes where perturbative calculations are reliable, while more complex scenarios can lead to sizable deviations. We further show that future Cosmic Microwave Background observations can provide constraints on the reheating history that are complementary to collider searches.
Overall, this work provides a systematic framework for particle production in thermal environments and clarifies the role of medium effects in cosmic reheating and Dark Matter generation.
Jury members :
Prof. Marco Drewes (UCLouvain) (Supervisor)
Prof. Andrea Giammanco (UCLouvain) (Chairperson)
Prof. Christophe Ringeval (UCLouvain) (Secretary)
Prof. Bjorn Garbrecht (Technische Universität München) (Member)
Prof. Marcello Musso (Universidad de Salamanca) (Member)
