Dark Radiation Constraints on Mixed Axion/Neutralino Dark Matter

Understand

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.