Light WIMPs and Equivalent Neutrinos

Very light WIMPs (χ), 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 mχ in the range 0.5 − 5 MeV (mχ/me ≲1 − 10), and they identify best fit values for mχ in the range 5 − 10 MeV. The joint BBN + CMB analysis finds a best fit value for the number of equivalent neutrinos, ΔNν ≈ 0.65, nearly independent of the nature of the WIMP. In the absence of a light WIMP (mχ ≲20 MeV), Neff = 3.05(1 + ΔNν/3). In this case, there is excellent agreement between BBN and the CMB, but the joint fit reveals ΔNν = 0.40 ± 0.17, disfavoring standard big bang nucleosynthesis (SBBN) (ΔNν = 0) at ∼ 2.4 σ, as well as a sterile neutrino (ΔNν = 1) at ∼ 3.5 σ. The best BBN + CMB joint fit disfavors the absence of dark radiation (ΔNν = 0 at ∼ 95% confidence), while allowing for the presence of a sterile neutrino (ΔNν = 1 at ≲1 σ). For all cases considered here, the lithium problem persists. These results, presented at the TAUP 2013 Conference, are based on Nollett & Steigman [14].