Implications of mixed axion/neutralino dark matter for the Cosmic Frontier: a Snowmass whitepaper

TL;DR

This whitepaper explores the complex implications of mixed axion and neutralino dark matter within supersymmetric and Peccei-Quinn frameworks, highlighting diverse production mechanisms and their impact on cosmic relics and detection prospects.

Contribution

It provides a comprehensive analysis of mixed axion/neutralino dark matter scenarios, including production mechanisms, model dependencies, and observational implications.

Findings

Multiple production channels affect dark matter relic abundance.

PQ scale can vary from 10^9 to 10^16 GeV.

Axions may be beyond current experimental reach.

Abstract

If one simultaneously invokes the SUSY solution to the gauge hierarchy problem and the PQ solution to the strong CP problem, then one might expect mixed axion/neutralino dark matter (DM), i.e. two dark matter particles. In this case, dark matter production involves oscillation production of axions, thermal and non-thermal neutralino production and thermal and non-thermal production of axinos and saxions. The latter particles may feed additional neutralinos into the cosmic soup, or dilute all relics via additional entropy production, or feed dark radiation into the cosmos. Depending on the SUSY and PQ model, the PQ scale can easily exist from 10^9-10^{16} GeV. Axions may lie well beyond the range currently being probed by experiment. In the case of relic WIMPs, theoretical predictions must be rescaled for the possibility that WIMPs only comprise a fraction of the local DM abundance.