# Constraining the non-gravitational scattering of baryons and dark matter with early cosmic structure formation

## TL;DR

This paper constrains baryon-dark-matter scattering models using early cosmic structure formation data, showing that certain interaction parameters suppress primordial star formation, conflicting with observed 21-cm signals, and thus narrowing viable dark matter models.

## Contribution

It provides new constraints on non-gravitational baryon-dark-matter interactions based on the timing of primordial star formation and 21-cm observations, refining the parameter space of such models.

## Key findings

- Excluded regions with high cross-section and low DM mass.
- Suppressed Pop III star formation at high redshift in certain models.
- Reduced Pop III host density by a factor of 3-10 at z~15-20.

## Abstract

We derive new constraints on the non-gravitational baryon-dark-matter scattering (BDMS) by evaluating the mass thresholds of dark matter (DM) haloes in which primordial gas can cool efficiently to form Population III (Pop III) stars, based on the timing of the observed 21-cm absorption signal. We focus on the BDMS model with interaction cross-section $\sigma=\sigma_{1}[v/(1\ \mathrm{km\ s^{-1}})]^{-4}$, where $v$ is the relative velocity of the encounter. Our results rule out the region in parameter space with $\sigma_{1}\gtrsim 10^{-19}\ \mathrm{cm^{2}}$ and DM particle mass $m_{\chi}c^{2}\lesssim 3\times 10^{-2}\ \mathrm{GeV}$, where the cosmic number density of Pop III hosts at redshift $z\sim 20$ is at least three orders of magnitude smaller than in the standard Lambda cold DM ($\Lambda$CDM) case. In these BDMS models, the formation of Pop III stars is significantly suppressed for $z\gtrsim 20$, inconsistent with the timing of the observed global 21-cm absorption signal. For the fiducial BDMS model with $m_{\chi}c^{2}=0.3$ GeV and $\sigma_{1}=8\times 10^{-20}\ \mathrm{cm^{2}}$, capable of accommodating the measured absorption depth, the number density of Pop III hosts is reduced by a factor of $3-10$ at $z\sim 15-20$, when the 21-cm signal is imprinted, compared with the $\Lambda$CDM model. The confluence of future detailed cosmological simulations with improved 21-cm observations promises to probe the particle-physics nature of DM at the small-scale frontier of early structure formation.