Investigation of CMB constraints for dark matter-helium scattering

We study dark matter-helium scattering in the early Universe and its impact on constraints from cosmic microwave background (CMB) anisotropy measurements. We describe possible theoretical frameworks for dark matter-nucleon interactions via a scalar, pseudoscalar, or vector mediator; such interactions give rise to hydrogen and helium scattering, with cross sections that have a power-law dependence on relative velocity. Within these frameworks, we consider three scenarios: dark matter coupling to only neutrons, to only protons, and to neutrons and protons with equal strength. For these various cases, we use Planck 2018 temperature, polarization, and lensing anisotropy data to place constraints on dark matter scattering with hydrogen and/or helium for dark matter masses between 10 keV and 1 TeV. For any model that permits both helium and hydrogen scattering with a non-negative power-law velocity dependence, we find that helium scattering dominates the constraint for dark matter masses well above the proton mass. Furthermore, we place the first CMB constraints on dark matter that scatters dominantly/exclusively with helium in the early Universe.

Figure 1

The 95% CL upper limits on the momentum-transfer cross section coefficient ${\sigma }_{0,B}$, defined in Eq. (15), for DM-hydrogen (solid) and DM-helium (dashed) scattering. We show results for cross sections with a power-law velocity dependence of $n=-4$ (upper left), $n=-2$ (upper right), $n=0$ (center left), $n=2$ (center right), and $n=4$ (bottom). For each value of $n$, we analyze Scenario A with only a DM-neutron coupling (red), Scenario B-SI with only a DM-proton coupling (light blue), Scenario C-SI with equal DM couplings to protons and neutrons (orange), and Scenario B/C-SD (dark blue) for SD scattering. For Scenarios B-SI and C-SI, the hydrogen and helium cross sections are related by the expressions given in Table 1. The cases labeled as “generic” do not arise in the models presented in Sec. 2, but we include them for completeness.