Multiverse Dark Matter: SUSY or Axions

TL;DR

This paper explores how anthropic measures can explain the observed dark matter abundance, favoring low PQ scales or superpartner masses, and discusses implications for SUSY and axion models, including experimental prospects.

Contribution

It introduces a framework using anthropic measures to understand dark matter composition and predicts the likelihood of single-component dominance in multicomponent scenarios.

Findings

High probability for dark matter to be dominated by a single component.

TeV-scale LSP dark matter and axion solutions are unlikely to coexist.

Axion decay constants below current experimental reach are favored, motivating new detection techniques.

Abstract

The observed values of the cosmological constant {\it and} the abundance of Dark Matter (DM) can be successfully understood, using certain measures, by imposing the anthropic requirement that density perturbations go non-linear and virialize to form halos. This requires a probability distribution favoring low amounts of DM, i.e. low values of the PQ scale $f$ for the QCD axion and low values of the superpartner mass scale $\tilde{m}$ for LSP thermal relics. In theories with independent scanning of multiple DM components, there is a high probability for DM to be dominated by a single component. For example, with independent scanning of $f$ and $\tilde{m}$, TeV-scale LSP DM and an axion solution to the strong CP problem are unlikely to coexist. With thermal LSP DM, the scheme allows an understanding of a Little SUSY Hierarchy with multi-TeV superpartners. Alternatively, with axion DM, PQ breaking before (after) inflation leads to $f$ typically below (below) the projected range of the current ADMX experiment of $f = (3 - 30) \times 10^{11}$ GeV, providing strong motivation to develop experimental techniques for probing lower $f$.