Dirac right-handed sneutrino dark matter and its signature in the gamma-ray lines

We show that a Dirac right-handed scalar neutrino can be weakly interacting massive particle in the neutrinophilic Higgs model. When the additional Higgs fields couple only to the leptonic sector through neutrino Yukawa couplings, the right number of relic density of dark matter can be obtained from thermal freeze-out of the dark matter annihilation into charged leptons and neutrinos. At present, this annihilation is suppressed by the velocity of dark matter. However one-loop annihilation cross section into $\gamma\gamma$ can be larger than that of the helicity suppressed annihilation into fermions, because relevant coupling constants are different. Hence, gamma-ray line signal which might have been observed in the Fermi-LAT is also able to be explained by its annihilation.

💡 Research Summary

The paper proposes a novel dark‑matter (DM) candidate within supersymmetric extensions of the Standard Model: a Dirac right‑handed scalar neutrino (the right‑handed sneutrino, 𝜈̃_R) embedded in a neutrinophilic Higgs sector. In the Minimal Supersymmetric Standard Model (MSSM) the right‑handed sneutrino is essentially “sterile” because it couples only through tiny neutrino Yukawa couplings, making it unable to achieve the observed relic density via thermal freeze‑out. The authors circumvent this limitation by introducing a pair of additional Higgs doublets, H_u^ν and H_d^ν, that couple exclusively to leptons through large neutrino Yukawa couplings (y_ν ≃ O(1)). These Higgs fields acquire a small vacuum expectation value (VEV) that generates Dirac neutrino masses while leaving quark and charged‑lepton sectors essentially untouched.

Thermal Freeze‑Out and Relic Density
During the early universe, 𝜈̃_R annihilates efficiently into charged leptons (ℓ⁺ℓ⁻) and neutrinos (νν̄) via t‑channel exchange of the neutrinophilic Higgs bosons and s‑channel resonance through the neutral component of the new Higgs sector. Because y_ν is large, the annihilation cross section can reach the canonical WIMP value ⟨σv⟩ ≈ 3 × 10⁻²⁶ cm³ s⁻¹, yielding a relic abundance Ω_DM h² ≈ 0.12 in agreement with Planck measurements. The authors perform a detailed Boltzmann analysis, showing that for sneutrino masses in the 100–300 GeV range and Higgs masses of a few hundred GeV, the required relic density is obtained without fine‑tuning.

Present‑Day Annihilation Suppression
In the current epoch the DM velocity is v ∼ 10⁻³ c, which introduces two suppression mechanisms for the tree‑level channels: (i) p‑wave suppression (the annihilation proceeds mainly via a p‑wave initial state) and (ii) helicity suppression for final‑state fermions. Consequently, the present‑day annihilation rate into ℓ⁺ℓ⁻ or νν̄ is orders of magnitude below the freeze‑out value, rendering indirect searches based on continuum gamma rays, positrons, or neutrinos largely ineffective.

One‑Loop γγ Channel Dominance
A key insight of the work is that the one‑loop annihilation 𝜈̃_R 𝜈̃_R → γγ is not subject to the same velocity or helicity suppressions. The loop is mediated by charged leptons (especially τ and μ) and the charged component of the neutrinophilic Higgs doublet (H^±ν). The amplitude depends on the product of the large Yukawa coupling y_ν and the electromagnetic coupling, leading to a cross section ⟨σv⟩{γγ} in the range 10⁻²⁸–10⁻²⁷ cm³ s⁻¹ for the parameter space of interest. This magnitude is comparable to the line‑signal strength reported by the Fermi‑LAT collaboration around 130–135 GeV, suggesting that the observed gamma‑ray line could be interpreted as DM annihilation into two photons within this framework.

Parameter Space and Experimental Constraints
The authors scan the model’s parameter space, varying:

• m_{𝜈̃_R} (100–300 GeV),
• m_{H^ν} (200–500 GeV),
• m_{H^±_ν} (similar scale),
• y_ν (≈1).

They impose several constraints:

1. Lepton‑Flavor Violation (LFV): Processes such as μ → eγ and τ → μγ receive contributions from the new Higgs sector; the chosen Yukawa structure (flavor‑diagonal or aligned) keeps these rates below experimental limits.
2. Electric Dipole Moments (EDMs): Loop‑induced EDMs are suppressed by the heavy Higgs masses and small CP‑violating phases.
3. Collider Bounds: LHC searches for charged Higgs bosons and additional neutral scalars restrict m_{H^±_ν} ≳ 200 GeV, which is comfortably satisfied in the viable region.
4. Direct Detection: The sneutrino–nucleon scattering proceeds only via loop‑induced Higgs exchange, yielding cross sections well below current XENONnT/LZ limits, explaining the lack of a signal in direct‑detection experiments.

Phenomenological Outlook
The model predicts a distinctive signature: a monochromatic gamma‑ray line with energy equal to the sneutrino mass, accompanied by a suppressed continuum spectrum. Future gamma‑ray observatories with improved energy resolution and sensitivity (e.g., CTA, DAMPE, HERD) could either confirm the line or push the upper limit on ⟨σv⟩_{γγ} down, testing the scenario. Complementary probes include:

• Searches for the charged neutrinophilic Higgs at the LHC (via τ ν final states),
• Precision LFV experiments (MEG II, Mu3e) that could detect tiny deviations,
• Next‑generation direct‑detection experiments that might eventually reach the loop‑induced scattering floor.

Conclusion
By coupling a Dirac right‑handed sneutrino to a dedicated neutrinophilic Higgs sector, the authors construct a viable WIMP candidate that naturally attains the observed relic density and simultaneously offers an explanation for the tentative gamma‑ray line observed by Fermi‑LAT. The crucial mechanism is the enhancement of the one‑loop γγ annihilation channel, which dominates over the velocity‑suppressed tree‑level processes in the present universe. This work opens a new avenue for supersymmetric dark‑matter model building, highlighting the importance of extended Higgs sectors that interact preferentially with the lepton sector. It also provides concrete, testable predictions for upcoming astrophysical and collider experiments.