Complex Scalar Singlet Model: Electroweak Phase Transition and Gravitational Waves

TL;DR

This paper investigates an extension of the Standard Model with a complex scalar singlet, demonstrating that it can produce a strong first-order electroweak phase transition capable of generating observable gravitational waves and supporting baryogenesis.

Contribution

It introduces a comprehensive analysis of the complex scalar singlet model with a general potential, identifying parameter regions that enable strong first-order phase transitions and predicting detectable gravitational wave signals.

Findings

Viable parameter space for strong first-order phase transition identified.

Multi-stage transitions can produce gravitational waves within next-generation detector sensitivities.

The model links electroweak baryogenesis with observable gravitational wave signatures.

Abstract

The Standard Model (SM) cannot explain the observed baryon asymmetry of the Universe (BAU), thus driving the need for physics beyond the SM, which can generate electroweak baryogenesis through a strong first-order electroweak phase transition (SFOPT). We extend the SM with a complex singlet scalar (cxSM) and examine the phase transition behavior using a fully general renormalizable scalar potential that permits a complex vacuum expectation value for the singlet and coupled dynamics among multiple scalar fields. Employing the one-loop thermal effective potential with daisy resummation and appropriate counter terms, we conduct an extensive scan of the parameter space, enforcing both theoretical and experimental limits on the scalar sector. This analysis reveals viable domains yielding SFOPT. From these regions, we select representative benchmark scenarios demonstrating multi-stage transitions, producing stochastic gravitational wave signals via bubble nucleation dynamics. The resulting spectra lie within the projected sensitivity of next-generation observatories, including LISA, BBO, DECIGO, and U-DECIGO. Thus, the cxSM offers a compelling setting for electroweak baryogenesis, enriched by correlated gravitational-wave and collider phenomenology.