Dark radiation constraints on mixed Axion/Neutralino dark matter
Recent analyses of CMB data combined with the measurement of BAO and H0 show that dark radiation — parametrized by the apparent number of additional neutrinos ΔNeff contributing to the cosmic expansion — is bounded from above by about ΔNeff1.6 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(a) 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 ΔNeff is easily satisfied if saxions are mainly thermally produced and mLSP < mams. 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.