Abstract
Any neutral boson such as a dark photon or dark Higgs that is part of a non-standard sector of particles can mix with its standard model counterpart. When very weakly mixed with the Standard Model, these particles are produced in the early Universe via the freeze-in mechanism and subsequently decay back to standard model particles. In this work, we place constraints on such mediator decays by considering bounds from Big Bang nucleosynthesis and the cosmic microwave background radiation. We find both nucleosynthesis and CMB can constrain dark photons with a kinetic mixing parameter between log ϵ ∼ −10 to −17 for masses between 1 MeV and 100 GeV . Similarly, the dark Higgs mixing angle ϵ with the Standard Model Higgs is constrained between log ϵ ∼ −6 to −15. Dramatic improvement on the bounds from CMB spectral distortions can be achieved with proposed experiments such as PIXIE.
Highlights
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The neutral gauge boson particles are referred to as dark photons [1,2,3,4,5], while the neutral scalar boson particles are referred to as dark Higgs [6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21]
In terms of models with renormalizable interactions, the only two bosonic possibilities are a dark photon that kinetically mixes with the standard model photon/Z and a dark Higgs that mixes with the standard model Higgs
Summary
An additional scalar boson Φ charged under a new U (1)D gauge group is introduced. The scalar boson gets a vacuum expectation value that breaks U (1)D, giving mass to the dark Higgs. The λ3 coupling mixes the visible and dark Higgs sectors. In the limit where λ3uv ≪ λ1v2, λ2u2, which is the limit considered in this work, the mixing angle can be written as tan 2ǫ. Given equation (II.4), the SM fermion coupling to the mass eigenstate dark Higgs is a rescaling of the SM coupling, yf f ρ ig mf 2 mW sin ǫ. There are enough free parameters to treat the dark Higgs mass and mixing parameters as independent parameters and we do so in this work. We adopt such a limit, which is the most conservative option: deviation from this limit would enhance dark Higgs production, while keeping the lifetime fixed.
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