# Cooling in a Dissipative Dark Sector

## TL;DR

This paper develops analytic formulas for energy dissipation and ionization processes in a simple dissipative dark sector, enabling future hydrodynamical simulations to explore dark disk formation from dark matter self-interactions.

## Contribution

It provides the first detailed analytic prescriptions for energy loss and ionization rates in a minimal dissipative dark sector, facilitating numerical simulation efforts.

## Key findings

- Derived approximate formulas for energy loss rates due to multiple processes.
- Included rates of change for dark ion and atom densities.
- Presented heating rate from photoionization.

## Abstract

The possibility of a subdominant component of dark matter dissipating energy could lead to dramatic new phenomenology such as the formation of a dark disk. One rigorous way to assess this possibility and settle the debate on its feasibility is to include the dissipative dark component in a numerical hydrodynamical simulation. A necessary input to such a simulation is a prescription including energy dissipation rates of different processes and rates of processes that change the number densities of dark ions and atoms. In this article, we study the simplest dissipative dark sector which consists of a dark electron and proton, both charged under a dark gauged U(1). We present approximate analytic formulas for energy loss rates due to Compton scattering, bremsstrahlung, recombination, collisional ionization and collisional excitation as well as the rates of number density change. We also include the heating rate due to photoionization. The work serves as the first step to realize a numerical simulation including a dissipative dark sector, which hopefully can shed more light on the formation and properties of a dark disk originating from dark matter self-interactions.