Dark Radiation Constraints on Mixed Axion/Neutralino Dark Matter

TL;DR

This paper investigates constraints on mixed axion/neutralino dark matter models using WMAP9 data, focusing on how saxion production and decay affect dark radiation bounds and the viability of supersymmetric scenarios.

Contribution

It provides improved calculations of axion and saxion production, analyzes the impact of saxion decay modes, and constrains supersymmetric models with mixed dark matter using cosmological and experimental data.

Findings

WMAP9 bounds on dark radiation are satisfied if saxions are thermally produced with certain mass hierarchies.

Coherent oscillation production of saxions is strongly constrained by WMAP9 and other cosmological data.

Standard overabundance of neutralino dark matter is excluded in these models across all Peccei-Quinn scales.

Abstract

Recent analyses of WMAP9 data show that dark radiation-- parametrized by the apparent number of additional neutrinos \Delta N_{eff} contributing to the cosmic expansion-- is consistent with the Standard Model and bounded from above by about \Delta N_{eff} ~< 0.5 at 95% CL. We consider the mixed axion/neutralino cold dark matter scenario which arises in R-parity conserving supersymmetric (SUSY) models wherein the strong CP problem is solved by hadronic axions with a concommitant axion(a)/saxion(s)/axino(\ta) supermultiplet. Our new results include improved calculations of thermal axion and saxion production and include effects of saxion decay to axinos and axions. We show that the above bound on \Delta N_{eff} is easily satisfied if saxions are mainly thermally produced and m_{LSP} < m_{\ta} \lesssim m_s. However, if the dominant mechanism of saxion production is through coherent oscillations, the WMAP9 data provides a strong bound on saxion production followed by saxion decays to axions. Furthermore we show that scenarios with mixed neutralino/axion dark matter are highly constrained by combined WMAP9, BBN and Xe-100 constraints. In particular, supersymmetric models with a standard overabundance of neutralino dark matter are excluded for all values of the Peccei-Quinn breaking scale. Next generation WIMP direct detection experiments may be able to discover or exclude mixed axion-neutralino CDM scenarios where $s\to aa$ is the dominant saxion decay mode.