Electroweak baryogenesis and dark matter from a singlet Higgs (1210.4196v3)

Abstract: If the Higgs boson H couples to a singlet scalar S via lambda_m |H|^2 S^2, a strong electroweak phase transition can be induced through a large potential barrier that exists already at zero temperature. In this case properties of the phase transition can be computed analytically. We show that electroweak baryogenesis can be achieved using CP violation from a dimension-6 operator that couples S to the top-quark mass, suppressed by a new physics scale that can be well above 1 TeV. Moreover the singlet is a dark matter candidate whose relic density is < 3% of the total dark matter density, but which nevertheless interacts strongly enough with nuclei (through Higgs exchange) to be just below the current XENON100 limits. The DM mass is predicted to be in the range 80-160 GeV.

• The paper demonstrates that coupling a singlet scalar with the Higgs facilitates a strong first-order electroweak phase transition critical for baryogenesis.
• It explains how a dimension-6 CP-violating operator alters top quark interactions, creating the necessary asymmetry during the phase transition.
• Furthermore, the model proposes the singlet scalar as a dark matter candidate that interacts via the Higgs portal, consistent with current direct detection constraints.

Electroweak Baryogenesis and Dark Matter from a Singlet Higgs

The paper by James M. Cline and Kimmo Kainulainen explores a compelling framework that contemplates the dual role of a singlet scalar field coupled to the Higgs boson: as a potential candidate for both electroweak baryogenesis (EWBG) and dark matter (DM). At the heart of this model is an additional real singlet scalar $S$ interacting with the Higgs doublet $H$. This interaction is parameterized by a coupling $\lambda_m |H|^2 S^2$, which significantly alters the dynamics of the electroweak phase transition (EWPT).

Central Propositions of the Model

1. Electroweak Phase Transition: The inclusion of a singlet scalar allows the model to facilitate a strong first-order electroweak phase transition. Unlike in the Standard Model, where such conditions are elusive, the large potential barrier resulting from the singlet-Higgs coupling allows this EWPT to be achieved relatively easily.
2. Mechanism of CP Violation: The paper investigates a dimension-6 operator that induces CP violation necessary for baryogenesis. This operator, $\eta/\Lambda^2\, S^2$, modifies the top quark mass in the vicinity of the bubble wall, addressing the need for the CP asymmetry that can bias sphaleron interactions in the early universe for the production of baryons over antibaryons.
3. Dark Matter Candidate: The singlet $S$ is purported to be a dark matter candidate. Although its contribution to the overall dark matter relic density is less than 3%, it interacts strongly with nuclei via the Higgs portal, closely adjacent to limits discerned by direct detection experiments like XENON100.
4. CP-Violating Phase and New Physics Scale: Alongside, the role of CP-violation from the dimension-6 interaction involving the singlet scalar is quantified, necessitating a new physics scale ($\Lambda$) potentially above 1 TeV, reflecting significant implications for high-energy physics beyond the Standard Model.

Methodology and Results

• Analytical and Numerical Techniques: The authors employ a mix of analytical calculations for the effective potential and Monte Carlo simulations to scan the parameter space favoring strong EWPT and viable DM phenomenology. The analytical approach to the tree-level potential artfully simplifies the assessment of transitions and the associated dynamics of freeze-out in dark matter relic density calculations.
• Constraints Considered: The paper navigates through constraints such as invisible Higgs decay, relic density limits, and direct detection cross-sections, effectively winnowing out less viable parameter configurations.
• Calculated Baryon Asymmetry: By modeling the transport equations for producing baryon asymmetries, the research demonstrates that with assumed CP-violation, achieving observed baryon asymmetry during the EWPT becomes feasible. The relationship between baryon chemical potential and sphaleron interactions is systematically explored.

Implications and Future Directions

This model substantiates that the interplay of singlet scalars with the Higgs can simultaneously address open questions about baryogenesis and dark matter, contingent on the dimension-6 operators with specified CP-violating phases. The ramifications are substantial, as they portend potential new physics at scales beyond those directly probed by the LHC.

The research underscores the significance of future experiments directed at both Higgs-related phenomena and dark matter direct searches. The paper suggests that further refinements in collider reach or DM search sensitivity could substantively test and possibly falsify key aspects of this model.

Overall, the intricate coupling of Higgs sector physics with dark matter and baryogenesis milestones underscores a fruitful but challenging landscape, straddling theoretical innovation and experimental exploration. The results anticipate a continued and possibly enhanced focus on singlet-extended models in ongoing and future theoretical and observational studies in particle physics and cosmology.