Dark radiation constraints on mixed Axion/Neutralino dark matter

Recent analyses of CMB data combined with the measurement of BAO and H ₀ show that dark radiation - parametrized by the apparent number of additional neutrinos ΔN_eff contributing to the cosmic expansion - is bounded from above by about ΔN_eff≲1.6 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(ã) 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 ΔN_eff is easily satisfied if saxions are mainly thermally produced and m_LSP < m_ã≲m_s. However, if the dominant mechanism of saxion production is through coherent oscillations, the CMB 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 CMB, 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 aa is the dominant saxion decay mode.