We investigate the properties of a stochastic gravitational wave background produced by a first-order electroweak phase transition in the regime of extreme supercooling. We study a scenario whereby the percolation temperature that signifies the completion of the transition, T p , is as low as a few MeV (nucleosynthesis temperature), while most of the true vacuum bubbles are formed much earlier at the nucleation temperature, T n ∼ 50 GeV. This implies that the gravitational wave spectrum is mainly produced by the collisions of large bubbles and characterised by a large amplitude and a peak frequency as low as f ∼ 10 −9 −10 −7 Hz. We show that such a scenario can occur in (but not limited to) a model based on a non-linear realisation of the electroweak gauge group, so that the Higgs vacuum configuration is altered by a cubic coupling. In order to carefully quantify the evolution of the phase transition of this model over such a wide temperature range we go beyond the usual fast transition approximation, taking into account the expansion of the Universe as well as the behaviour of the nucleation probability at low temperatures. Our computation shows that there exists a range of parameters for which the gravitational wave spectrum lies at the edge between the exclusion limits of current pulsar timing array experiments and the detection band of the future Square Kilometre Array observatory.

We utilise the LHC Run-1 and -2 Higgs data to constrain the CP-violating top-Higgs couplings.In order to satisfy the current full Higgs data sets at 2σ level, the CP-odd component C With the new bound, we explore the impact on the CP-violating top-Higgs couplings in the Higgs production processes pp → tth, thj, hh at 13 TeV LHC, and e + e − → hZ, hγ at future 240 GeV Higgs factories. We find that the cross sections of pp → tth, pp → thj, pp → hh, e + e − → hZ and e + e − → hγ can be enhanced up to 1.41, 1.18, 2.20, 1.001 and 1.09 times as large as the SM predictions, respectively. The future precision measurement of the process e + e − → hγ with an accuracy of 5% will be able to constrain |C p t | < 0.19 at most at a 240 GeV e + e − Higgs factory.

In this paper, we put constraints on anomalous CP-violating top-Higgs couplings using the currently available Higgs data and explore the prospect of measuring these couplings at 240 GeV TLEP. We find that the CP-violating phase ξ is currently limited in the range |ξ| < 0.6π at 95% C.L. and may be further constrained to |ξ| < 0.07π at TLEP. Under this consideration, we further investigate the observability of the scalar (ξ = 0), pseudoscalar (ξ = 0.5π) and mixed (ξ = 0.25π) top-Higgs interactions through the channel pp → t(→ ℓ + ν ℓ b)h → bb j. We find that it is most promising to observe pure pseudoscalar interactions with y t = y SM t , although this will be challenging due to a low signal to background ratio. We also find that the anomalous top-Higgs couplings can lead to sizeable differences in lepton forward-backward asymmetries and can be distinguished by measuring the lepton angular distributions from polarised top quarks at 14 TeV LHC.

Negative searches for permanent electric dipole moments (EDMs) heavily constrain models of baryogenesis utilising various higher dimensional charge and parity violating (CPV) operators. Using effective field theory, we create a model independent connection between these EDM constraints and the baryon asymmetry of the universe (BAU) produced during a strongly first order electroweak phase transition. The thermal aspects of the high scale physics driving the phase transition are paramaterised by the usual kink solution for the bubble wall profile. We find that operators involving derivatives of the Higgs field yield CPV contributions to the BAU containing derivatives of the Higgs vacuum expectation value (vev), while non-derivative operators lack such contributions. Consequently, derivative operators cannot be eliminated in terms of non-derivative operators (via the equations of motion) if one is agnostic to the new physics that leads to the phase transition. Thus, we re-classify the independent dimension six operators, restricting ourselves to third generation quarks, gauge bosons and the Higgs. Finally, we calculate the BAU (as a function of the bubble wall width and the cutoff) for a derivative and a non-derivative operator, and relate it to the EDM constraints.