Electroweak phase transition and LHC signatures in the singlet Majoron model

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 λhs|S|2|H|2 between the singlet and the doublet Higgs is significant. Large Majorana Yukawa couplings of the singlet neutrinos, yiSνicνi, 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 113GeVmH200GeV and production cross sections reduced by mixing with the singlet, with cos 2θ 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.