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 \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.
