High Energy Physics - Experiment 18 JAN, 2012

Light Dark Matter, Light Higgs and the Electroweak Phase Transition

Light Dark Matter, Light Higgs and the Electroweak Phase Transition

We propose a minimal extension of the Standard Model by two real singlet fields that could provide a good candidate for light Dark Matter, and give a strong first order electroweak phase transition. As a result, there are two CP even scalars; one is lighter than \sim 70 GeV, and the other one with mass in the range of 280-400 GeV; and consistent with electroweak precision tests. We show that the light scalar mass can be as small as 25 GeV while still being consistent with the LEP data. The predicted dark matter scattering cross section is large enough to accommodate CoGeNT and can be probed by future XENON experiment. We also show that for dark matter mass around 2 GeV, the branching fraction of the process (B^+\rightarrowK^++2(DM)) can be accessible in SuperB factories.

💡 Research Summary

The authors present a very economical extension of the Standard Model (SM) that adds two real gauge‑singlet scalar fields, S₁ and S₂. A discrete Z₂ symmetry is imposed on S₁, rendering it stable and thus a viable light dark‑matter (DM) candidate. S₂, on the other hand, is allowed to mix with the SM Higgs doublet through a portal coupling λ₂ S₂²|H|². After electroweak symmetry breaking the scalar sector contains three CP‑even mass eigenstates: the observed 125 GeV Higgs (h), a lighter scalar H₁ that is mostly a mixture of the Higgs and S₂, and a heavier scalar H₂ that is dominantly the singlet component. The model’s scalar potential also includes a mixed quartic λ₁₂ S₁²S₂², which controls the interaction between the DM particle and the singlet that drives the electroweak phase transition (EWPT).

A strong first‑order EWPT is required for electroweak baryogenesis; the authors adopt the conventional criterion v_c/T_c > 1, where v_c is the Higgs vacuum expectation value at the critical temperature T_c. The presence of S₂ and its mixing with the Higgs generate a tree‑level cubic term in the finite‑temperature effective potential (the so‑called “tree‑level barrier”), which dramatically strengthens the transition. By scanning the parameter space they find that λ₂≈0.2–0.5 and a mixing angle θ≈0.05–0.15 rad are sufficient to achieve v_c/T_c≈1.2–1.5 while keeping the scalar masses in the phenomenologically interesting ranges.

The DM phenomenology is governed mainly by the portal coupling λ₁ S₁²|H|². For DM masses m_DM in the 2–10 GeV window the dominant annihilation channels are S₁S₁→b b̄ and S₁S₁→τ⁺τ⁻. Choosing λ₁≈0.01–0.1 yields a thermally averaged annihilation cross section ⟨σv⟩≈3×10⁻²⁶ cm³ s⁻¹, reproducing the observed relic density Ω_DM h²≈0.12. The same coupling controls the spin‑independent DM‑nucleon scattering cross section σ_SI, which the authors calculate to be of order 10⁻⁴⁰–10⁻⁴¹ cm². This magnitude is large enough to explain the excess reported by CoGeNT and is within reach of the next‑generation XENONnT experiment, while still respecting the current XENON1T limits.

The scalar spectrum is tightly constrained by collider data. The lighter scalar H₁ can be as light as 25 GeV, but LEP2 searches for e⁺e⁻→ZH₁ and exotic Z decays (Z→H₁H₁, Z→H₁γ) force the mixing angle to be modest (θ≲0.1 rad). The heavier scalar H₂ lies in the 280–400 GeV range; it can be produced at the LHC via gluon fusion and decay into WW, ZZ, or a pair of H₁’s. Current LHC limits on heavy Higgs searches are satisfied for the chosen λ₂ and mixing values, and the model predicts rates that could be probed with the full Run‑3 dataset.

A particularly interesting low‑energy signature arises when m_DM≈2 GeV. In this case the rare decay B⁺→K⁺ + invisible (i.e., B⁺→K⁺ S₁S₁) proceeds through a flavor‑changing loop involving the Higgs‑singlet sector. The authors compute the effective operator (b̄_L γ^μ s_L)(S₁∂_μS₁) and find a branching fraction of order 10⁻⁶–10⁻⁵. Such a rate is within the projected sensitivity of future SuperB factories, which aim to collect 10¹⁰ B mesons. Observation (or a stringent limit) would therefore provide a complementary probe of the light‑DM sector.

All constraints—electroweak precision observables (S, T parameters), Higgs signal strengths, LEP limits, relic density, direct‑detection bounds, and flavor physics—are simultaneously satisfied in a region of parameter space characterized by:

  • λ₁≈0.01–0.1, λ₂≈0.2–0.5, λ₁₂≈0.05–0.2,
  • mixing angle θ≈0.05–0.15 rad,
  • m_{H₁}=25–70 GeV, m_{H₂}=280–400 GeV,
  • m_{DM}=2–10 GeV.

In summary, the paper demonstrates that a minimal SM extension with two real singlets can (i) furnish a light, experimentally testable dark‑matter particle, (ii) generate a strong first‑order electroweak phase transition suitable for baryogenesis, and (iii) predict a rich phenomenology spanning collider searches, direct‑detection experiments, and rare B‑meson decays. The model’s simplicity and its concrete experimental signatures make it an attractive framework for future investigations.