Multistep Strongly First Order Phase Transitions from New Fermions at the TeV Scale

In spite of the vast literature on the subject of first order electroweak phase transitions (EWPTs), which can provide the necessary conditions for generating the baryon asymmetry in the Universe, fermion-induced EWPTs still remain a rather uncharted territory. In this paper, we consider a simple fermionic extension of the Standard Model involving one SU(2)(L) doublet and two SU(2)(L) singlet vectorlike leptons, strongly coupled to the Higgs scalar and with masses close to the TeV scale. We show how such a simple scenario can give rise to a nontrivial thermal history of the Universe, involving strongly first order multistep phase transitions occurring at temperatures close to the electroweak scale. Finally, we investigate the distinct gravitational wave (GW) signatures of these phase transitions at future space-based GW detectors, such as LISA, DECIGO, and BBO, and briefly discuss the possible LHC signatures of the vectorlike leptons.