Light WIMPs, Equivalent Neutrinos, BBN, and the CMB

 

Gary Steigman (Ohio State University)

 

New observational determinations of the primordial abundances of deuterium and helium-4 are exploited to infer the universal baryon (nucleon) density and to limit the number of equivalent neutrinos. These results are then compared to the independent determinations of these parameters from the Planck CMB data. The excellent agreement between BBN and the CMB disfavor SBBN (no equivalent neutrinos) at ~ 95% confidence and they disfavor a sterile neutrino at ~ 99% confidence.  BBN (and the CMB) are modified in the presence of a sufficiently light WIMP. These modifications are described and used to show that current BBN and CMB data favor a light WIMP with mass ~ 8 MeV and, they allow 0 or ≥ 1 equivalent neutrinos at ≤ 95% confidence. In all cases the predicted primordial lithium abundance is too high by factors ≥ 3 or more.