Early Universe Particle Physics at Finite Temperature: Mysteries at the dawn of time

Abstract

The early universe provides a high energy lab for particle physics phenomenology, with messengers such as gravitational waves providing an insight into the events of that era. Mysteries at the dawn of time include the origin of the Baryon Asymmetry of the Universe (BAU), and the nature of the electroweak phase transition (EWPT). A first order EWPT could provide the out-of-equilibrium conditions necessary for baryogenesis, and is a natural consequence of many Standard Model extensions. An alternative source of out-of-equilibrium conditions are the decays of heavy right handed neutrinos in the early universe, as defines the leptogenesis scenario. However, vanilla leptogenesis requires fine tuning in the Higgs and neutrino sectors in order to generate the BAU. We present a model of 'Hot Leptogenesis', where a hot sector provides a factor ~50 enhancement in the BAU, resulting in a novel model that does not demand fine tuning. We proceed to investigate the EWPT in the Two Higgs Doublet Model (2HDM), in light of the reported 95 GeV di-tau and di-gamma excess. Using a dimensional reduction method to calculate the thermal potential at 1-loop with 2-loop matching, we find regions of parameter space with a first order EWPT. However, the strongest signal-to-noise ratios for the LISA experiment are too weak to provide a detection.