Electroweak Phase Transition and LHC Signatures in the Singlet Majoron Model

TL;DR

This paper investigates the electroweak phase transition in the singlet Majoron model, identifying parameter regions that support a strong first-order transition relevant for baryogenesis, and explores potential collider signatures.

Contribution

It demonstrates that a strong electroweak phase transition can occur in the singlet Majoron model with specific parameter choices, including significant Higgs-singlet coupling and Majorana neutrino interactions.

Findings

Strong first-order phase transition in sizeable parameter space.

Predicted Higgs mass range of 113-200 GeV with reduced production cross sections.

Possible Higgs decay mode H -> SS affecting branching ratios.

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.