Abstract
We study a light thermal scalar dark matter (DM) model with a light scalar mediator mixed with the standard model Higgs boson, including both the theoretical bounds and the current experimental constraints. The thermal scalar DM with the mass below a few GeV is usually strongly constrained by the observation of cosmic microwave background and/or indirect detection experiments because the leading annihilation mode is S-wave. However, we find that two parameter regions remain, which are the resonant annihilation region and the forbidden annihilation region. For both cases, higher partial waves dominantly contribute to the annihilation at the freeze-out era, and the constraint from the cosmological observation is weaker. We consider typical cases of these regions quantitatively, mainly focusing on the mixing angle and the mass of the new particles. Finally, we also discuss the testability of this model in future experiments.
Highlights
The existence of dark matter (DM) is well established by many cosmological observations, and the amount of DM is accurately determined as Ωh2 1⁄4 0.120 Æ 0.001 by the observation of cosmic microwave background (CMB) at the Planck experiment [1]
We have studied the two-scalar-singlet extension of the standard model (SM), focusing on the light mass region below a few GeV
The CMB observation constrains the S-wave annihilation cross section as hσvi ≲ 10−26 cm3 s−1 for such a mass region, which conflicts with the required cross section hσvi 1⁄4 3 × 10−26 cm3 s−1 to explain the observed relic abundance
Summary
The existence of dark matter (DM) is well established by many cosmological observations, and the amount of DM is accurately determined as Ωh2 1⁄4 0.120 Æ 0.001 by the observation of cosmic microwave background (CMB) at the Planck experiment [1]. The simplest model is a singlet scalar extension of the standard model (SM) with an unbroken Z2 symmetry [5–12], and only the resonant annihilation region of the Higgs boson and the large mass region still survive. Another possibility is introducing a new mediator particle that makes a connection between DM and SM particles. They suppose that there is another Z2 symmetry called Z02 under which the mediator particle is odd and other particles are even, and the mediator field has a vacuum expectation value after the symmetry breaking We remove this Z02 symmetry to consider the possibility of the two-scalar-singlet model more generally at the small mass region. VII, we summarize the light dark matter model with two-scalar-singlet extension
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