Higgs Couplings and Electroweak Phase Transition

TL;DR

Probing the order of the electroweak phase transition in SM extensions with a single new scalar, the paper links EWPT dynamics to Higgs couplings via loop effects of a Higgs portal scalar Φ. Using finite-temperature effective potentials and benchmark models with different SM gauge quantum numbers for Φ, it shows that colored scalars generically shift hgg and hgamma gamma, with current LHC data already excluding first-order EWPT scenarios from a color sextet; HL-LHC can definitively test color triplets. For non-colored Φ, Higgs coupling deviations in hgamma gamma may be too small to observe at the LHC, making precision e+e- Higgs factories (ILC/TLEP) essential, where the Higgsstrahlung cross section provides a robust probe across all models, including gauge singlets. The study demonstrates a robust, testable connection between EWPT dynamics and zero-temperature Higgs observables, and discusses limitations and future directions, such as multi-field scenarios and non-perturbative effects.

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.