Electroweak Phase Transition and LHC Signatures in the Singlet Majoron Model

TL;DR

The paper investigates whether the singlet Majoron extension of the Standard Model can produce a strongly first-order electroweak phase transition at a TeV-scale singlet vev, a condition favorable for electroweak baryogenesis. It develops a robust finite-temperature effective potential incorporating one-loop and ring-improved corrections, preserves the zero-temperature minimum via a specific renormalization prescription, and performs a comprehensive parameter scan under collider and precision constraints. The study finds that strong EWPTs arise in sizable regions, especially with large λ_hs and substantial Majorana Yukawa couplings y_i, and predicts distinctive Higgs-sector signatures, including nonstandard Higgs masses in the ~113–200 GeV range with reduced production due to mixing and possible H → SS decays altering Higgs branching ratios. These results imply testable LHC signatures for the singlet sector and provide a viable framework for electroweak baryogenesis within current experimental bounds.

Abstract

We reconsider the strength of the electroweak phase transition in the singlet Majoron extension of the Standard Model, with a low (~TeV) scale of the singlet VEV. A strongly first order phase transition, of interest for electroweak baryogenesis, is found in sizeable regions of the parameter space, especially when the cross-coupling lambda_{hs}|S|^2|H|^2 between the singlet and the doublet Higgs is significant. Large Majorana Yukawa couplings of the singlet neutrinos, y_i S nu_i^c nu_i, are also important for strengthening the transition. We incorporate the LEP and Tevatron constraints on the Higgs masses, and electroweak precision constraints, in our search for allowed parameters; successful examples include singlet masses ranging from 5 GeV to several TeV. Models with a strong phase transition typically predict a nonstandard Higgs with mass in the range 113 GeV < m_H < 200 GeV and production cross sections reduced by mixing with the singlet, with cos^2(theta) significantly less than 1. We also find examples where the singlet is light and the decay H -> SS can modify the Higgs branching ratios relative to Standard Model expectations.