# Cosmological constraints on the velocity-dependent baryon-dark matter coupling

## TL;DR

This paper constrains baryon-dark matter interaction cross sections using Planck CMB and SDSS Lyman-alpha data, revealing mass-dependent bounds for dark matter below 1 MeV, contrasting previous mass-independent claims.

## Contribution

It introduces a mass-dependent analysis of baryon-dark matter coupling constraints for dark matter masses below 1 MeV, incorporating free-streaming effects similar to warm dark matter.

## Key findings

- Mass-dependent bounds on baryon-dark matter cross section for m_χ down to 5 keV.
- Constraints show mass dependence, unlike previous mass-independent results.
- Provides new limits on dark matter interactions in the sub-MeV mass range.

## Abstract

We present the cosmological constraints on the cross section of baryon-dark matter interactions for the dark matter mass below the MeV scale from the Planck CMB (cosmic microwave background) and SDSS (Sloan Digital Sky Survey) Lyman-$\alpha$ forest data. To explore the dark matter mass $m_{\chi}\lesssim 1$ MeV for which the dark matter's free-streaming effect can suppress the observable small scale density fluctuations, in addition to the acoustic oscillation damping in existence of the baryon-dark matter coupling, we apply the approximated treatment of dark matter free-streaming analogous to that of the conventional warm dark matter. We also demonstrate the mass dependence of the baryon-dark matter cross section bounds (for the dark matter mass down to $m_{\chi} \sim 5~{\rm keV}$), in contrast to the dark matter mass independence of the cross section constraints for the light dark matter below the MeV scale claimed in the previous literature.