Dark Matter Interactions, Helium, and the CMB

TL;DR

This paper uses CMB helium fraction measurements to set new limits on tightly coupled dark matter subcomponents, improving constraints and guiding future experimental and theoretical work.

Contribution

It introduces a novel method to constrain tightly coupled dark matter using helium fraction data from the CMB, providing tighter bounds than previous approaches.

Findings

New upper limit on tightly coupled dark matter fraction: <0.006

Future CMB experiments could improve bounds to <0.001

Constraints are consistent with full analysis results

Abstract

The cosmic microwave background (CMB) places a variety of model-independent constraints on the strength interactions of the dominant component of dark matter with the Standard Model. Percent-level subcomponents of the dark matter can evade the most stringent CMB bounds by mimicking the behavior of baryons, allowing for larger couplings and novel experimental signatures. However, in this note, we will show that such tightly coupled subcomponents leave a measurable imprint on the CMB that is well approximated by a change to the helium fraction, $Y_{\rm He}$. Using the existing constraint on $Y_{\rm He}$, we derive a new upper limit on the fraction of tightly coupled dark matter, $f_{\rm TCDM}$, of $f_{\rm TCDM}<0.006$ (95\% C.I.). We show that future CMB experiments can reach $f_{\rm TCDM}<0.001$ (95\% C.I.) and confirm that the bounds derived in this way agree with the results of a complete analysis. We briefly comment on the implications for model building, including milli-charged dark matter.