(Mainly) axion dark matter

TL;DR

This paper discusses the theoretical motivation for axions as dark matter candidates, their connection to the strong CP problem, and prospects for their detection alongside WIMPs in current and future experiments.

Contribution

It provides a comprehensive overview of axion physics, its role in solving the strong CP problem, and the implications for dark matter detection strategies.

Findings

Axions can account for cold dark matter via their coherent oscillations.

Experiments like ADMX are beginning to probe the QCD axion parameter space.

Detection of both axions and WIMPs is expected in future experiments.

Abstract

The strong CP problem of QCD is at heart a problem of naturalness: why is the F\tilde{F} term highly suppressed in the QCD Lagrangian when it seems necessary to explain why there are three and not four light pions? The most elegant solution posits a spontaneously broken Peccei-Quinn (PQ) symmetry which requires the existence of the axion field a. The axion field settles to the minimum of its potential thus removing the offensive term but giving rise to the physical axion whose coherent oscillations can make up the cold dark matter. Only now are experiments such as ADMX beginning to explore QCD axion parameter space. Since a bonafide scalar particle-- the Higgs boson-- has been discovered, one might expect its mass to reside at the axion scale f_a~ 10^{11} GeV. The Higgs mass is elegantly stabilized by supersymmetry: in this case the axion is accompanied by its axino and saxion superpartners. Requiring naturalness also in the electroweak sector implies higgsino-like WIMPs so then we expect mixed axion-WIMP dark matter. Ultimately we would expect detection of both an axion and a WIMP while signals for light higgsinos may show up at LHC and must show up at ILC.