Impact of First-Order Electroweak Phase Transition on Leptogenesis and QCD Axion

by Dipendu Bhandari (IIT Guwahati)

Tuesday 2 Sept 2025, 16:00 → 17:30 Asia/Seoul

Room 1423 (KIAS)

Title: Impact of First-Order Electroweak Phase Transition on Leptogenesis and QCD Axion

Speaker:  Dipendu Bhandari (IIT Guwahati)

Abstract: We present two complementary studies that highlight the role of a first-order electroweak phase transition (FOEWPT) in addressing major open questions in early Universe cosmology. In the first work, we propose a low-scale leptogenesis mechanism within the Type-I seesaw framework, where right-handed neutrinos (RHNs) with masses even below the SM Higgs can successfully generate lepton asymmetry via CP-violating decays. The FOEWPT enables sphaleron processes to remain active below their usual decoupling temperature, GeV, facilitating baryogenesis even when the reheating temperature is below 131.7 GeV. This scenario also opens up RHN mass ranges accessible to current and future collider experiments, alongside a potential gravitational wave signal (GW) from the FOEWPT. In the second work, we explore the dynamics of the QCD axion field during such a FOEWPT, where non-renormalizable Peccei–Quinn (PQ) symmetry breaking interaction, involving the axion and the sector responsible for making the EWPT of first order, causes the axion to undergo an unusual recurrent misalignment behavior. This mechanism encompasses both the conventionally under- and overabundant regions of QCD axion dark matter parameter space without requiring fine-tuning of the initial misalignment angle. Furthermore, the FOEWPT-induced GW provides an independent probe.