Dark Matter Interactions, Helium, and the Cosmic Microwave Background.

Area of Science:

• Cosmology and Astrophysics
• Particle Physics

Background:

• Cosmic Microwave Background (CMB) data provides model-independent constraints on dark matter interactions with the Standard Model.
• Subcomponents of dark matter can evade stringent CMB bounds by mimicking baryonic matter, allowing for larger interaction couplings and novel experimental signatures.

Purpose of the Study:

• To demonstrate that tightly coupled dark matter subcomponents leave a measurable imprint on the CMB.
• To derive new upper limits on the fraction of tightly coupled dark matter (fTCDM) using existing CMB constraints on the helium fraction (YHe).

Main Methods:

• The study approximates the CMB imprint of tightly coupled subcomponents as a change in the helium fraction (YHe).
• Existing CMB constraints on YHe are utilized to derive an upper limit on fTCDM.
• The potential of future CMB experiments to improve these constraints is evaluated.

Main Results:

• A new upper limit of fTCDM < 0.006 (95% C.I.) is derived for tightly coupled dark matter.
• Future CMB experiments are projected to achieve an upper limit of fTCDM < 0.001 (95% C.I.).
• The derived bounds are consistent with results from complete analyses.

Conclusions:

• CMB constraints on YHe offer a powerful, model-independent method to constrain tightly coupled dark matter, extending beyond Big Bang Nucleosynthesis studies.
• These findings have implications for dark matter model building, including scenarios like millicharged dark matter.
• The study highlights the versatility of CMB data in probing diverse dark matter properties.