CP-violating effects on gravitational waves in a complex singlet extension of the Standard Model with degenerate scalars

TL;DR

This paper investigates how CP violation influences the electroweak phase transition and gravitational wave signals in a complex singlet scalar extension of the Standard Model, especially under conditions of high mass degeneracy.

Contribution

It provides a detailed analysis of CP-violating effects on EWPT and gravitational waves using both simplified and full numerical models in a high mass degeneracy scenario.

Findings

Stronger CP violation weakens the first-order EWPT.

Gravitational wave amplitudes decrease with increased CP violation.

Future experiments could detect CP violation effects despite collider blind spots.

Abstract

We examine CP-violating effects on electroweak phase transition (EWPT) in the standard model with a complex singlet scalar focusing particularly on a scenario where additional scalars have masses close to 125 GeV. Such a high mass degeneracy makes collider signatures in the scenario standard model like, and current experimental data cannot distinguish them from the standard model predictions. We utilize a simplified scalar potential to understand impacts of CP violation on EWPT qualitatively. Then, one-loop effective potential with a thermal resummation is employed for full numerical evaluations. As a phenomenological consequence, gravitational waves from the first-order EWPT are also evaluated. We find that the strength of the first-order EWPT would get weaker as the CP-violating effect becomes larger. As a result, gravitational wave amplitudes are diminished by the size of the CP violation. Future gravitational wave experiments may shed light on CP violation in the singlet scalar sector as well as the experimental blind spot due to the high mass degeneracy.