Complex Scalar Singlet Model: Electroweak Phase Transition and Gravitational Waves

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.