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

## TL;DR

This paper explores how a simple fermionic extension of the Standard Model with TeV-scale vector-like leptons can induce multistep strongly first order electroweak phase transitions, producing detectable gravitational wave signals and potential collider signatures.

## Contribution

It introduces a minimal fermionic model that generates multistep first order phase transitions near the electroweak scale, linking particle physics with gravitational wave phenomenology.

## Key findings

- Multistep strongly first order phase transitions occur at TeV-scale temperatures.
- Distinct gravitational wave signatures are predicted for future space-based detectors.
- Potential LHC signatures of vector-like leptons are discussed.

## Abstract

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 Vector-Like Leptons, strongly coupled to the Higgs boson and with masses close to the TeV scale. We show how such a simple scenario can give rise to a non-trivial 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 Vector-Like Leptons.