[Paper Review] A light scalar WIMP through the Higgs portal?

Michel H. G. Tytgat|arXiv (Cornell University)|Dec 2, 2010

Dark Matter and Cosmic Phenomena37 references4 citations

TL;DR

This paper investigates a light scalar dark matter candidate (mass ~6–10 GeV) via the Higgs portal in a real singlet scalar model, showing that its predicted annihilation cross-section into Standard Model particles—particularly τ⁺τ⁻ and bb̄—is strongly constrained by Fermi-LAT data on Milky Way dwarf spheroidal galaxies and the isotropic diffuse gamma-ray background. The study finds that such light WIMPs are excluded at 95% C.L. for masses below ~8 GeV when annihilating into τ⁺τ⁻, especially under conservative astrophysical assumptions.

ABSTRACT

In these proceedings, I report on the status of a simple singlet scalar dark matter model in the light of recent results from both direct detection experiments, in particular DAMA, CoGeNT, CDMS-II and Xenon10/100, and indirect searches, in particular Fermi-LAT. Specifically, I confront the light scalar WIMP candidates, M_DM ~ few GeV, that are consistent with CoGeNT and/or DAMA, to constraints that may be set using the recent Fermi-LAT data on Milky Way dwarf spheroidal galaxies (dSphs) and the isotropic diffuse gamma-ray emission. I show that the latter observations set relevant exclusion limits on the lightest WIMP candidates.

Motivation & Objective

• To assess the viability of a light scalar dark matter candidate (M_DM ~ few GeV) in the Higgs portal model, consistent with CoGeNT and DAMA direct detection hints.
• To evaluate indirect detection constraints from Fermi-LAT observations of Milky Way dwarf spheroidal galaxies (dSphs) and the isotropic diffuse gamma-ray background.
• To determine whether the relic abundance and spin-independent scattering cross-section required by direct detection experiments are compatible with indirect detection limits.
• To explore the implications for the Higgs boson decay branching ratio into scalar pairs, given the required coupling strength for light dark matter.
• To assess the sensitivity of indirect detection constraints to astrophysical uncertainties, particularly dark matter halo profiles and early kinetic decoupling.

Proposed method

• Uses a real scalar singlet model with a Z₂ symmetry, where the scalar DM interacts with the Standard Model Higgs boson via a quartic coupling λ_S.
• Relies on the Higgs portal interaction to relate direct detection (nuclear recoil cross-section) and indirect detection (annihilation cross-section) processes through the effective operator O_S ~ (λ_S / M_h²) S² f̄f.
• Computes the relic abundance via thermal freeze-out, requiring ⟨σv⟩ ≈ 3×10⁻²⁶ cm³/s to match WMAP data, which fixes the model's parameter space.
• Applies Fermi-LAT gamma-ray flux limits from dSphs (Ursa Minor, Draco) using median J-factor values to constrain the annihilation cross-section.
• Analyzes isotropic diffuse gamma-ray emission data from Fermi-LAT, incorporating redshift-dependent dark matter halo mass functions and NFW profiles.
• Varying assumptions on early dark matter halo formation and kinetic decoupling temperature (T ~ 150 MeV for this model) to assess robustness of constraints.

Experimental results

Research questions

• RQ1 Can a light scalar WIMP (M_DM ~ 6–10 GeV) consistent with CoGeNT and DAMA direct detection signals survive indirect detection constraints from Fermi-LAT?
• RQ2 What is the impact of astrophysical uncertainties—particularly dark matter halo profiles and early kinetic decoupling—on indirect detection limits for light WIMPs?
• RQ3 How does the Higgs boson branching ratio into scalar pairs (SS) depend on the scalar DM mass and coupling, and what are the implications for LHC searches?
• RQ4 To what extent do constraints from the isotropic diffuse gamma-ray background extend the exclusion reach beyond direct detection experiments for light dark matter?
• RQ5 Can the annihilation into τ⁺τ⁻ or bb̄ final states be probed by current Fermi-LAT data, and what are the resulting 95% C.L. upper limits on the annihilation cross-section?

Key findings

• The 8 GeV scalar WIMP candidate annihilating into τ⁺τ⁻ is excluded at 95% C.L. by Fermi-LAT data on dSphs, with upper limits on ⟨σv⟩ < 2.4×10⁻²⁶ cm³/s for Draco and < 2.5×10⁻²⁶ cm³/s for Ursa Minor.
• For a 6 GeV candidate with 20% branching ratio into τ⁺τ⁻, the 95% C.L. upper limit on ⟨σv⟩ is < 2×10⁻²⁶ cm³/s, indicating strong exclusion potential.
• Constraints from the isotropic diffuse gamma-ray emission extend to lighter masses and exclude candidates below ~8 GeV under conservative astrophysical assumptions.
• The model predicts a significant branching ratio for Higgs decay into SS pairs (up to ~100%) for light DM, making it a potential target for LHC searches.
• The kinetic decoupling temperature (~150 MeV) for the scalar DM implies early formation of dense dark matter halos, leading to stronger indirect detection constraints than for typical WIMPs.
• The combination of dSph and diffuse gamma-ray constraints provides a robust, model-independent exclusion of light scalar WIMPs in the 6–8 GeV range, especially in the τ⁺τ⁻ channel.