Coupled Boltzmann calculation of mixed axion/neutralino cold dark matter production in the early universe

TL;DR

This paper numerically calculates the relic abundance of mixed axion/neutralino dark matter in supersymmetric models, considering various production and decay processes, and explores implications for the PQ scale and dark matter composition.

Contribution

It introduces a comprehensive numerical approach to coupled Boltzmann equations for mixed axion/neutralino dark matter, including multiple production and decay channels in SUSY models.

Findings

High neutralino abundance models remain incompatible with observations.

Underabundant neutralino models can be compatible with mixed dark matter scenarios.

Allowed PQ scale extends to 10^{14}-10^{15} GeV in certain models.

Abstract

We calculate the relic abundance of mixed axion/neutralino cold dark matter which arises in R-parity conserving supersymmetric (SUSY) models wherein the strong CP problem is solved by the Peccei-Quinn (PQ) mechanism with a concommitant axion/saxion/axino supermultiplet. By numerically solving the coupled Boltzmann equations, we include the combined effects of 1. thermal axino production with cascade decays to a neutralino LSP, 2. thermal saxion production and production via coherent oscillations along with cascade decays and entropy injection, 3. thermal neutralino production and re-annihilation after both axino and saxion decays, 4. gravitino production and decay and 5. axion production both thermally and via oscillations. For SUSY models with too high a standard neutralino thermal abundance, we find the combined effect of SUSY PQ particles is not enough to lower the neutralino abundance down to its measured value, while at the same time respecting bounds on late-decaying neutral particles from BBN. However, models with a standard neutralino underabundance can now be allowed with either neutralino or axion domination of dark matter, and furthermore, these models can allow the PQ breaking scale f_a to be pushed up into the 10^{14}-10^{15} GeV range, which is where it is typically expected to be in string theory models.