# Dark Matter Bound States from Three-Body Recombination

## TL;DR

This paper investigates how three-body recombination can produce bound states of dark matter particles in the early universe, with implications for small-scale structure problems and dark matter clustering.

## Contribution

It provides the first calculation of two-body bound cluster production via three-body recombination for resonant dark matter models with bosonic particles.

## Key findings

- The fraction of dark matter in bound clusters can increase significantly after binding energy is reached.
- The present-day fraction of dark matter in such clusters remains very small, less than 10^(-6).
- Three-body recombination can enhance bound state formation in the early universe.

## Abstract

The small-scale structure problems of the universe can be solved by self-interacting dark matter that becomes strongly interacting at low energies. A particularly predictive model is resonant short-range self-interactions, with a dark-matter mass of about 19 GeV and a large S-wave scattering length of about 17 fm. Such a model makes definite predictions for the few-body physics of weakly bound clusters of the dark-matter particles. We calculate the production of two-body bound clusters by three-body recombination in the early universe under the assumption that the dark matter particles are identical bosons, which is the most favorable case for forming larger clusters. The fraction of dark matter in the form of two-body bound clusters can increase by as much as 4 orders of magnitude when the dark-matter temperature falls below the binding energy, but its present value remains less than 10^(-6).