Sterile neutrinos with a mass of a few keV can serve as cosmological warm dark matter. We study the production of keV sterile neutrinos in the early universe from the decay of a frozen-in scalar. Previous studies focused on heavy frozen-in scalars with masses above the Higgs mass leading to a hot spectrum for sterile neutrinos with masses below 8 − 10 keV. Motivated by the recent hints for an X-ray line at 3.55 keV, we extend the analysis to lighter frozen-in scalars, which allow for a cooler spectrum. Below the electroweak phase transition, several qualitatively new channels start contributing. The most important ones are annihilation into electroweak vector bosons, particularly W -bosons as well as Higgs decay into pairs of frozen-in scalars when kinematically allowed.

It has recently been speculated that the solar neutrino mixing angle is connected to the golden ratio ϕ. Two such proposals have been made, cot θ 12 = ϕ and cos θ 12 = ϕ/2. We compare these Ansätze and discuss a model leading to cos θ 12 = ϕ/2 based on the dihedral group D 10 . This symmetry is a natural candidate because the angle in the expression cos θ 12 = ϕ/2 is simply π/5, or 36 degrees. This is the exterior angle of a decagon and D 10 is its rotational symmetry group. We also estimate radiative corrections to the golden ratio predictions. * 1 Actually, prediction (A) would lie very slightly outside the 2σ range of Ref. [3], which is sin 2 θ 12 = 0.278 ÷ 0.352.

We consider the minimal model in which dark matter is stabilized by a non-Abelian discrete symmetry. The symmetry group is taken to be D3 ∼ = S3, which is the smallest non-Abelian finite group. The minimal model contains (nontrivial) singlet and doublet scalar representations of D3 which couple to the Standard Model fields via the Higgs portal. This construction predicts two species of dark matter over much of the parameter space. Nontrivial interactions under D3 lead to a novel thermal history of dark matter, while the multi-component nature of dark matter can be tested by future direct detection experiments.Understanding the nature of the cosmological dark matter (DM) that constitutes one quarter of the energy density of the universe is a central goal of particle physics today [1]. While there is little room left to doubt the existence of DM, its microscopic properties are virtually unknown. One of the few properties in which we can be confident is that DM should be stable on time scales greater than the age of the universe, suggesting the existence of a new "dark" symmetry. But precisely what symmetry stabilizes DM is a mystery.Many models employ a discrete Z 2 symmetry to stabilize DM. This Z 2 symmetry is often motivated by the need to suppress dangerous operators in new physics scenarios that solve the hierarchy problem. However, given that we have no experimental indication of what new physics, if any, addresses the naturalness issues in the SM, one may take a more general perspective regarding DM and the symmetries responsible for its stability. More pragmatically, the exploration of alternative stabilizing symmetries is warranted by the prospect of novel phenomena associated with DM, as such symmetries may predict new states and interactions.Indeed, there are many possibilities other than a Z 2parity that can protect DM against decay. In particular, besides the Abelian cyclic symmetry Z N [2], DM may well be stabilized by a non-Abelian discrete symmetry. Non-Abelian finite groups have received some limited attention within the context of DM. Motivated by improved gauge coupling unification, Ref.[3] considered an additional Higgs doublet in a non-Abelian discrete multiplet serving as DM. Non-Abelian discrete symmetries also lead to distinct decay patterns in decaying dark matter scenarios [4,5]. Continuous non-Abelian symmetries originating from broken or confined gauge theories can also ensure DM stability [6]. Models of DM stabilized by Abelian discrete symmetries that descend from higher non-Abelian ones have been motivated by astrophysical anomalies [7], discrete gauge symmetries [8], and neutrino physics [9]. Indeed, non-Abelian flavor symmetries are widely used to explain the neutrino oscillation data [10] (for recent reviews see Refs. [11,12]). Such non-Abelian discrete symmetries can come from the breaking of continuous flavor symmetries [13] or from orbifold compactification of extra-dimensions [14].In this paper we construct the minimal model of DM in which stability is a consequence of a non-Ab...

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