Singlet Higgs Phenomenology and the Electroweak Phase Transition

Abstract: We study the phenomenology of gauge singlet extensions of the Standard Model scalar sector and their implications for the electroweak phase transition. We determine the conditions on the scalar potential parameters that lead to a strong first order phase transition as needed to produce the observed baryon asymmetry of the universe. We analyze the constraints on the potential parameters derived from Higgs boson searches at LEP and electroweak precision observables. For models that satisfy these constraints and that produce a strong first order phase transition, we discuss the prospective signatures in future Higgs studies at the Large Hadron Collider and a Linear Collider. We argue that such studies will provide powerful probes of phase transition dynamics in models with an extended scalar sector.

• The paper determines key parameter ranges of the scalar potential that enable a strong first order electroweak phase transition.
• It analyzes singlet and SM Higgs mixing effects that alter Higgs decays and production rates observable at colliders.
• The study connects extended Higgs sectors with baryogenesis, offering testable predictions for electroweak precision measurements.

Overview of Singlet Higgs Phenomenology and the Electroweak Phase Transition

The research presented in "Singlet Higgs Phenomenology and the Electroweak Phase Transition" by Stefano Profumo, Michael J. Ramsey-Musolf, and Gabe Shaughnessy examines the influence of introducing gauge singlet scalars to the Standard Model (SM) on the characteristics of the electroweak phase transition (EWPT). This study addresses the potential to generate a strong first order phase transition, a necessary condition for successful electroweak baryogenesis (EWB), which explains the observed baryon asymmetry of the universe (BAU).

The paper assesses the parameters of the scalar potential that allow a strong first order EWPT and analyzes the constraints placed by LEP Higgs boson searches and electroweak precision observables (EWPO). The resulting models that meet these constraints could lead to distinct signatures of Higgs production and decay at the Large Hadron Collider (LHC) and future linear colliders.

Conditions for a Strong First Order EWPT

The authors identify the conditions under which a gauge singlet scalar can enhance the strength of the EWPT. These conditions involve the parameters of the scalar potential, particularly focusing on the coupling terms between the singlet and the SM Higgs doublet. They develop an expression to determine when these interactions result in a sufficiently large barrier between the symmetric and broken phases at the critical temperature, ensuring a strong first order transition. This analysis covers:

1. Cubic and Quartic Terms: Including new tree-level cubic terms in the potential and understanding their impact on enhancing the transition strength.
2. Minimizing Scalar Mixing: Exploring mixing between the singlet and the SM Higgs to maintain sufficient coupling to gauge bosons and the mass scale indicated by the Higgs observed at the LHC.
3. Tuning Potential Parameters: Identifying parameter ranges in models such as the Next-to-Minimal Supersymmetric Standard Model (NMSSM), which lead to observable Higgs sector phenomenology.

Collider Implications

Significantly, the paper stresses that examining the scalar potentials facilitating a strong EWPT can also inform Higgs physics at colliders. For example:

• Higgs Decays: Potential new decay channels for the Higgs boson could be identifiable at the LHC or future linear colliders, such as decay to singlet-dominated states.
• Higgsstrahlung and Branching Ratios: Deviations in the Higgs couplings, resulting from singlet and SM Higgs mixing, could alter the expected Higgs production rates or branching ratios, serving as direct probes of these extended models.
• Electroweak Precision Tests: Constraints from EWPO such as deviations in S, T, and U parameters are crucial and indicate viable parameter space regions that stress the EWPT without conflicting with observed precision measurements.

Implications and Future Directions

The findings highlight the prospective influence of singlet scalar extensions on both cosmological events and particle phenomenology. Enhanced mechanisms for a strong first order EWPT through singlet scalars might provide roles for additional particle states beyond the SM, such as dark matter candidates—offering another layer to understanding the universe's composition beyond baryonic matter.

The exploration of Higgs decays and production modes probed by colliders will further elucidate or constrain these theoretical models. As the LHC continues its search and a linear collider becomes a possibility, these studies may verify the presence of singlet scalars, reconciling particle physics observations with cosmological phenomena like baryogenesis—the interface illustrating the synergy between microphysical processes and macrocosmic influences.

This work exemplifies how theoretical explorations inform empirical searches, outlining criteria for the threshold between theory and observation in our quest to comprehend the universe's fundamental structures. Future attention to the interplay between collider mechanisms and cosmological processes could guide further development in both natural and theoretical explorations of physics beyond the Standard Model.