Light WIMPs And Equivalent Neutrinos

Very light WIMPs (chi), thermal relics that annihilate late in the early Universe, change the energy and entropy densities at BBN and at recombination. BBN, in combination with the CMB, can remove some of the degeneracies among light WIMPs and equivalent neutrinos, constraining the existence and properties of each. Depending on the nature of the light WIMP (Majorana or Dirac fermion, real or complex scalar) the joint BBN + CMB analyses set lower bounds to m(chi) in the range 0.5 - 5MeV (m(chi)/m(e) greater than or similar to 1 - 10), and they identify best fit values for m(chi) in the range 5 - 10 MeV. The joint BBN + CMB analysis finds a best fit value for the number of equivalent neutrinos, Delta N-nu approximate to 0.65, nearly independent of the nature of the WIMP. In the absence of a light WIMP (m(chi) greater than or similar to 20 MeV), N-eff = 3.05(1 + Delta N-nu/3). In this case, there is excellent agreement between BBN and the CMB, but the joint fit reveals Delta N-nu = 0.40 +/- 0.17, disfavoring standard big bang nucleosynthesis (SBBN) (Delta N-nu = 0) at similar to 2.4 sigma, as well as a sterile neutrino (Delta N-nu = 1) at similar to 3.5 sigma. The best BBN + CMB joint fit disfavors the absence of dark radiation (Delta N-nu = 0 at similar to 95% confidence), while allowing for the presence of a sterile neutrino (Delta N-nu = 1 at less than or similar to 1 sigma). For all cases considered here, the lithium problem persists. These results, presented at the TAUP 2013 Conference, are based on Nollett & Steigman [14]. (C) 2015 The Authors. Published by Elsevier B.V.