Abstract
We argue that extensions of the Standard Model (SM) with a strongly first-order electroweak phase transition generically predict significant deviations of the Higgs couplings to gluons, photons, and Z bosons from their SM values. Precise experimental measurements of the Higgs couplings at the LHC and at the proposed next-generation facilities will allow for a robust test of the phase transition dynamics. To illustrate this point, in this paper we focus on the scenario in which loops of a new scalar field are responsible for the first-order phase transition, and study a selection of benchmark models with various SM gauge quantum numbers of the new scalar. We find that the current LHC measurement of the Higgs coupling to gluons already excludes the possibility of a first-order phase transition induced by a scalar in a sextet, or larger, representation of the SU(3)_c. Future LHC experiments (including HL-LHC) will be able to definitively probe the case when the new scalar is a color triplet. If the new scalar is not colored, an electron-positron Higgs factory, such as the proposed ILC or TLEP, would be required to test the nature of the phase transition. The extremely precise measurement of the Higgsstrahlung cross section possible at such machines will allow for a comprehensive and definitive probe of the possibility of a first-order electroweak phase transition in all models we considered, including the case when the new scalar is a pure gauge singlet.
Highlights
We developed a numerical code to analyze the dynamics of the electroweak phase transition in each of the benchmark models listed in table 1
We found a strong correlation between the strength of the phase transition and the deviations of the Higgs couplings from the Standard Model (SM)
This suggests that precise measurements of the Higgs couplings have a potential to definitively determine the order of the electroweak phase transition
Summary
The best known scenario where the EWPT could be strongly first-order is the Minimal Supersymmetric Standard Model (MSSM) with a light stop. Regardless of the fate of the MSSM, one can imagine other models where a first-order EWPT is possible. These fall into one of the two classes:. New physics at tree level: the tree-level Higgs potential may differ from the SM, either due to mixing with other scalars, or due to higher-dimension operators [14]. Both effects may affect the phase transition
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