The Minimal Model of nonbaryonic dark matter: a singlet scalar
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We propose the simplest possible renormalizable extension of the Standard
Model - the addition of just one singlet scalar field - as a minimalist model
for non-baryonic dark matter. Such a model is characterized by only three
parameters in addition to those already appearing within the Standard Model: a
dimensionless self-coupling and a mass for the new scalar, and a dimensionless
coupling, \lambda, to the Higgs field. If the singlet is the dark matter, these
parameters are related to one another by the cosmological abundance constraint,
implying that the coupling of the singlet to the Higgs field is large, \lambda
\sim O(0.1 - 1). Since this parameter also controls couplings to ordinary
matter, we obtain predictions for the elastic cross section of the singlet with
nuclei. The resulting scattering rates are close to current limits from both
direct and indirect searches. The existence of the singlet also has
implications for current Higgs searches, as it gives a large contribution to
the invisible Higgs width for much of parameter space. These scalars can be
strongly self-coupled in the cosmologically interesting sense recently proposed
by Spergel and Steinhardt, but only for very low masses (< 1 GeV), which is
possible only at the expense of some fine-tuning of parameters.