Singlet Scalar Dark Matter: monochromatic gamma rays and metastable vacua

We calculate the pair-annihilation cross section of real scalar singlet dark matter into two mono-energetic photons. We derive constraints on the theory parameter space from the Fermi limits on gamma-ray lines, and we compare with current limits from direct dark matter detection. We show that the new limits, albeit typically relevant only when the dark matter mass is close to half the Standard Model Higgs mass, rule out regions of the theory parameter space that are otherwise not constrained by other observations or experiments. In particular, the new excluded regions partly overlap with the parameter space where real scalar singlet dark matter might explain the anomalous signals observed by CDMS. We also calculate the lifetime of unstable vacuum configurations in the scalar potential, and show that the gamma-ray limits are quite relevant in regions where the electro-weak vacuum is meta-stable with a lifetime longer than the age of the universe.

💡 Research Summary

The paper investigates the simplest scalar singlet dark‑matter model, in which a real Z₂‑odd scalar S couples to the Standard‑Model Higgs doublet H through the portal term λHS S² H†H. The authors focus on two phenomenological aspects that have received relatively little attention: (i) the annihilation of a pair of S particles into two monochromatic photons (S S → γγ) and (ii) the stability of the electroweak (EW) vacuum when the scalar potential develops deeper minima involving a non‑zero singlet vacuum expectation value.

First, the annihilation cross‑section σγγ is computed at one‑loop order. Because S couples to photons only via the Higgs, the process proceeds through an off‑shell Higgs propagator followed by the well‑known h → γγ loop involving W bosons, top quarks and the Higgs itself. The resulting σγγ scales as λHS² and contains the familiar Breit‑Wigner resonance factor 1/