Dark Radiation Constraints on Mixed Axion/Neutralino Dark Matter

Recent analyses of WMAP9 data show that dark radiation-- parametrized by the apparent number of additional neutrinos Δ N_eff contributing to the cosmic expansion-- is consistent with the Standard Model and bounded from above by about Δ 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 Δ N_eff is easily satisfied if saxions are mainly thermally produced and m_LSP < m_\ta ≤sssim 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.