Electroweak Doublet Dark Matter for a Galactic Halo Gamma-Ray Excess

Abstract: Weakly interacting massive particles provide a well-motivated framework for dark matter, naturally reproducing the observed relic abundance through thermal freeze-out. A recent claim of an indirect-detection signal from the Galactic halo, consistent with dark matter annihilation in the mass range 400--800 GeV, motivates a reexamination of minimal models that can account for such a signal while remaining consistent with existing constraints. In this paper, we analyze the simplest extensions of the Standard Model capable of explaining the signal. We show that electroweak doublet dark matter with Higgs-portal interactions provides a natural and economical explanation. The model predicts annihilation predominantly into longitudinal gauge bosons with characteristic branching fractions and allows for inelastic dark matter with a mass splitting of order 100 keV, intriguingly consistent with a recent direct-detection anomaly. Possible enhancements of the present-day annihilation rate relative to the thermal value are also discussed, including a simple extension with a light scalar field, whose mass can be chosen such that the enhancement is suppressed in dwarf galaxies.

• The paper introduces the inert doublet dark matter model as a minimal, technically natural framework that explains the Galactic halo gamma-ray excess.
• It employs gauge-mediated annihilation with inelastic scattering to reconcile direct-detection limits with the observed gamma-ray signals.
• The model predicts dark matter masses of 400–800 GeV with a ~150 keV mass splitting, offering precise testable signals in collider and astrophysical experiments.

Electroweak Doublet Dark Matter and the Galactic Halo Gamma-Ray Excess

Introduction

This work systematically investigates the viability of minimal electroweak doublet dark matter (DM), with Higgs-portal interactions, as an explanation for the gamma-ray excess in the Galactic halo peaking at $\sim 20$ GeV, compatible with DM annihilation masses in the $400$–$800$ GeV range. The proposed scenario satisfies the requirements of a thermal relic, renormalizable field content, and consistency with both direct and indirect experimental constraints, while remaining minimal and highly predictive.

Phenomenological Motivation: Fits to Heavy Final States

The recently observed Galactic halo gamma-ray excess can be fit by DM annihilation into not only $b\bar b$ and $W^+W^-$, but also heavier SM states such as $t\bar t$, $hh$, and $ZZ$ within DM masses $400$–$800$ GeV. The best-fit annihilation cross section hovers near the canonical thermal relic value, although mildly larger values (by one or two orders of magnitude) are permitted if significant astrophysical boost factors are invoked.

Figure 1: Representative fits to the Galactic halo excess for different SM final states highlight the flexibility and commonality across $400$0, $400$1, $400$2, and longitudinal electroweak gauge boson channels.

This observation imposes constraints on the nature of viable DM candidates. Models in which annihilation into a single SM channel dominates, such as simple Higgs-portal scenarios, generally cannot achieve the observed signal without overproducing direct-detection signals. Conversely, scenarios predicting a specific mixture of heavy SM final states—driven by either gauge or Higgs interactions—are favored.

Minimal gauge extensions with an $400$3 triplet ("wino") or doublet ("higgsino") DM typically exhibit thermal masses near $400$4–$400$5 TeV, exceeding the favored mass window for the observed excess. These scenarios are therefore disfavored without additional ingredients.

Singlet Scalar Higgs-Portal Dark Matter

A minimal Higgs-portal scenario involving a real scalar singlet $400$6 is excluded in the relevant mass range, as the required Higgs-mediated coupling to achieve the desired relic abundance produces elastic scattering rates with nuclei far above direct-detection bounds. The situation is unchanged for a complex scalar singlet. The branching ratios in the heavy-mass regime correctly predict dominant annihilation into $400$7, $400$8, and $400$9 with ratios $800$0, but these scenarios are experimentally untenable due to direct-detection constraints.

Electroweak Doublet (‘Inert Doublet’) Dark Matter

An SU(2)$800$1 doublet scalar, stabilized by a $800$2 symmetry, provides an economical and viable alternative. The DM candidate is the lightest neutral component of the doublet $800$3. Annihilation to $800$4 and $800$5 proceeds with relative strengths $800$6, predominantly set by gauge interactions. Relic density is naturally achieved for $800$7–$800$8 GeV doublet masses, comfortably within the signal region.

Critically, this scenario predicts the DM to be inelastic, with two nearly degenerate neutral states split by $800$9 set by the $b\bar b$0 parameter in the scalar potential. Direct-detection limits are evaded for $b\bar b$1 keV, for which elastic $b\bar b$2-mediated scattering is kinematically forbidden. This technical naturalness for small $b\bar b$3 is protected by an enhanced global symmetry for $b\bar b$4.

Due to the interplay of suppressed Higgs-portal couplings (as required by direct-detection null results) and dominant gauge-mediated annihilation, the inert doublet model in this regime is theoretically minimal, highly predictive, and consistent with current bounds. The inferred values for $b\bar b$5, mass, and the required small $b\bar b$6 are aligned with recent direct-detection anomalies indicating an inelastic signature with a splitting $b\bar b$7 keV and $b\bar b$8 GeV.

Indirect Detection Cross Section and Sommerfeld Enhancement

The best-fit indirect-detection cross section is $b\bar b$9 cm$W^+W^-$0/s, exceeding the naïve thermal value by $W^+W^-$1. This motivates consideration of nontrivial present-day enhancements, such as Sommerfeld enhancement via the exchange of a new scalar $W^+W^-$2. The requisite enhancement is achieved for a coupling $W^+W^-$3 and light mediator masses $W^+W^-$4 MeV, such that enhancement is efficient in the Galactic halo (velocity $W^+W^-$5), but can be suppressed in dSphs (smaller $W^+W^-$6 and larger $W^+W^-$7), thereby avoiding tension with their gamma-ray limits.

The scalar $W^+W^-$8 may be incorporated with negligible impact on direct detection, relic abundance, or collider signatures, provided its mixing with the SM Higgs is sufficiently small. Alternative mechanisms for late-time enhancement (e.g., near-resonant s-channel or $W^+W^-$9-wave Sommerfeld) are also mentioned, with associated model-building challenges (e.g., collisionless constraints, direct-detection bounds).

Theoretical and Experimental Implications

The inert doublet scenario situates thermal DM at the confluence of indirect-detection (gamma-ray observations), direct-detection (inelastic signals), and collider searches. Present LHC and LEP limits are weak for inert doublet masses $t\bar t$0 with small mass splittings. Predicted signals at future lepton colliders running at $t\bar t$1 provide a sharp test of the model via direct production.

The theoretically minimal, renormalizable, and technically natural construction—together with correlated signals in multiple experimental fronts—places electroweak doublet DM in a privileged position for falsification as data improves.

Conclusion

The electroweak inert doublet model with suppressed Higgs-portal coupling and inelastic neutral states robustly explains the Galactic halo gamma-ray excess for DM masses $t\bar t$2–$t\bar t$3 GeV. It simultaneously accounts for inelastic direct-detection anomalies and avoids experimental bounds. Sommerfeld enhancement via a light scalar can bridge discrepancies in annihilation cross sections, with model parameters set to simultaneously evade dSph constraints.

This framework maintains consistency with all current data and provides concrete, testable predictions for future indirect, direct, and collider probes. Ongoing and planned experiments will either discover or robustly exclude this minimal WIMP scenario.