Electroweak Phase Transition, Higgs Diphoton Rate, and New Heavy Fermions

TL;DR

This paper proposes that vector-like fermions can induce a strong first-order electroweak phase transition and enhance Higgs to diphoton decay, linking collider signals to early universe cosmology and baryogenesis.

Contribution

It introduces a novel mechanism where heavy fermions drive the electroweak phase transition and affect Higgs decay rates, connecting collider phenomenology with cosmological phase transitions.

Findings

Heavy fermions can make the electroweak phase transition strongly first-order.

Fermions can enhance the Higgs diphoton decay rate.

Measurements of Higgs properties can probe early universe phase transitions.

Abstract

We show that weak scale vector-like fermions with order one couplings to the Higgs can lead to a novel mechanism for a strongly first-order electroweak phase transition (EWPhT), through their tendency to drive the Higgs quartic coupling negative. These same fermions could also enhance the loop-induced branching fraction of the Higgs into two photons, as suggested by the recent discovery of a ~125 GeV Higgs-like state at the CERN Large Hadron Collider (LHC). Our results suggest that measurements of the diphoton decay rate of the Higgs and its self coupling, at the LHC or perhaps at a future lepton collider, could probe the EWPhT in the early Universe, with significant implications for the viability of electroweak baryogenesis scenarios.