[Paper Review] Singlet-Doublet Model: Dark matter searches and LHC constraints

Lorenzo Calibbi, Alberto Mariotti+1 more|arXiv (Cornell University)|May 14, 2015

Dark Matter and Cosmic Phenomena4 citations

TL;DR

This paper revisits the Singlet-Doublet Model (SDM) of dark matter, combining direct (spin-independent and spin-dependent), indirect, and LHC constraints to probe the 4D parameter space. It demonstrates that the combination of these searches closes the long-standing 'blind spots' in spin-independent direct detection, excluding up to ~275 GeV for non-pure singlet states and extending exclusion limits to several TeV for highly pure singlet DM, with strong LHC constraints in the low-mass regime.

ABSTRACT

The Singlet-Doublet model of dark matter is a minimal extension of the Standard Model with dark matter that is a mixture of a singlet and a non-chiral pair of electroweak doublet fermions. The stability of dark matter is ensured by the typical parity symmetry, and, similar to a "Bino-Higgsino" system, the extra matter content improves gauge coupling unification. We revisit the experimental constraints on the Singlet-Doublet dark matter model, combining the most relevant bounds from direct (spin independent and spin dependent) and indirect searches. We show that such comprehensive analysis sets strong constraints on a large part of the 4-dimensional parameter space, closing the notorious "blind-spots" of spin independent direct searches. Our results emphasise the complementarity of direct and indirect searches in probing dark matter models in diverse mass scale regimes. We also discuss the LHC bounds on such scenario, which play a relevant role in the low mass region of the dark matter candidate.

Motivation & Objective

• To re-evaluate the full parameter space of the Singlet-Doublet Model (SDM) of dark matter using combined experimental constraints.
• To address the limitations of spin-independent direct detection by incorporating spin-dependent and indirect detection bounds to close 'blind spots' where SI searches fail.
• To assess the role of LHC constraints, particularly in the low-mass dark matter regime.
• To explore the complementarity between direct, indirect, and collider searches across different mass scales.
• To evaluate the impact of electroweak precision observables and Higgs/Z-boson decay widths on the model's viability.

Proposed method

• The study employs a minimal extension of the Standard Model with a singlet and a non-chiral electroweak doublet fermion, stabilized by a Z₂ symmetry.
• It computes DM-nucleon scattering amplitudes at tree level via Higgs and Z-boson exchange, including both spin-independent and spin-dependent interactions.
• The analysis combines experimental bounds from direct detection (SI and SD), indirect detection (e.g., Fermi-LAT for W+W−, bb̄, τ+τ− final states), and LHC searches for mono-jet, mono-photon, and multi-lepton final states with missing energy.
• The model’s phenomenology is evaluated using the mixing matrix U, which determines the singlet and doublet components of the dark matter state, and the Yukawa coupling y.
• Decay rates for charged and neutral fermions are computed using analytic formulas for partial widths involving vector and scalar bosons.
• The parameter space is scanned over the DM mass, Yukawa coupling y, and mixing angle (via U₁₁²), with constraints applied from all experimental front-ends.

Experimental results

Research questions

• RQ1 How do combined direct, indirect, and LHC constraints close the 'blind spots' in spin-independent direct detection for the Singlet-Doublet Model?
• RQ2 What is the extent of exclusion for the dark matter candidate across the 4D parameter space when all experimental front-ends are considered?
• RQ3 How do electroweak precision observables and Z/Higgs decay widths constrain the low-mass regime of the SDM?
• RQ4 What are the dominant LHC signatures and their sensitivity in the low-mass dark matter region?
• RQ5 To what extent does the model remain viable for highly pure singlet dark matter states?

Key findings

• For Yukawa coupling y ≲ 0.1, Fermi-LAT limits on χχ → WW exclude a mostly doublet DM up to ~280 GeV.
• For y = 0.2, combined SI, SD, and collider constraints exclude DM with singlet component U₁₁² ≲ 0.65 for masses ≤ 230 GeV.
• For y = 1, SI direct searches set strong bounds, but 'blind spots' with vanishing DM-Higgs coupling are probed by SD and indirect searches, excluding DM up to ~275 GeV unless U₁₁² ≳ 0.8.
• The model remains viable only for highly pure singlet states (U₁₁² ≳ 0.8) or for masses extending up to several TeV when y is large and positive.
• LHC constraints are particularly relevant in the low-mass regime, with signatures dominated by 2–4 leptons plus missing energy, suggesting potential reach in the white region of figure 8.
• The inclusion of loop corrections in the DM-nucleon amplitude could further refine bounds, especially in the pure doublet limit where tree-level couplings are small.