First-Order Electroweak Phase Transition and Baryogenesis from a Naturally Light Singlet Scalar

TL;DR

This paper explores a minimal singlet-scalar extension of the Standard Model that enables a strong first-order electroweak phase transition, potentially explaining baryogenesis while remaining consistent with current experimental constraints.

Contribution

It introduces a naturally light singlet scalar with an approximate shift symmetry that facilitates electroweak baryogenesis without requiring new UV physics.

Findings

The gravitational-wave signal from the phase transition is too weak for near-future detection.

The electroweak vacuum remains meta-stable but with a lifetime exceeding the universe's age.

The model's parameter space can be tested through rare Kaon decays and CMB observations.

Abstract

We investigate a minimal singlet-scalar extension to the Standard Model that achieves a strong first-order electroweak phase transition. The singlet can be naturally light because of an approximate shift symmetry and no extra hierarchy problem beyond that of the Standard Model Higgs is introduced. We discuss the two-field dynamics of the phase transition in detail and find that the gravitational-wave signal is too weak to be detected by near-future observations. We also discuss the meta-stability of the zero-temperature scalar potential. Despite the apparent instability just above the electroweak scale, we show that the lifetime of the electroweak vacuum is much longer than the age of the universe and hence the setup does not require UV completion near the electroweak scale. The baryon asymmetry of the universe may be explained by local electroweak baryogenesis arising from a coupling between the singlet and weak gauge boson. The predicted electron electric dipole moment is much below the current bound. The viable parameter space can be probed by the observations of rare Kaon decay and the cosmic microwave background.