BBN and the CMB constrain light, electromagnetically coupled WIMPs

In the presence of a light weakly interacting massive particle (WIMP; m_χ30 MeV), there are degeneracies among the nature of the WIMP (fermion or boson), its couplings to the standard-model particles (electromagnetic or to neutrinos only), the WIMP mass m_χ, and the number of equivalent neutrinos beyond the standard model ΔN_ν (including possible sterile neutrinos). These degeneracies cannot be broken by the cosmic microwave background (CMB) constraint on the effective number of neutrinos, Neff. However, big bang nucleosynthesis (BBN) is affected by the presence of a light WIMP and equivalent neutrinos, so the combination of BBN and CMB constraints can help to break some of these degeneracies. Here, the BBN predictions for the primordial abundances of deuterium and ⁴He (along with ³He and ⁷Li) in the presence of a light WIMP and equivalent neutrinos are explored, and the most recent estimates of their observationally determined relic abundances are used to limit the light-WIMP mass, the number of equivalent neutrinos, and the present Universe baryon density (ΩBh²). These constraints are explored here for Majorana and Dirac fermion WIMPs, as well as for real and complex scalar WIMPs that couple to electrons, positrons, and photons. In a separate paper, this analysis is repeated for WIMPs that couple only to the standard-model neutrinos, and the constraints for the two cases are contrasted. In the absence of a light WIMP, but allowing for ΔN_ν equivalent neutrinos, the combined BBN and CMB constraints favor Neff=3.46±0.17, ΩBh²=0.0224±0.0003, and ΔN_ν=0.40±0.17 (all at a 68% C.L.). In this case, standard BBN (ΔN_ν=0) is disfavored at ~98% confidence, and the presence of one sterile neutrino (ΔN_ν=1) is disfavored at 99% confidence. Allowing for a light WIMP and ΔN_ν equivalent neutrinos together, the combined BBN and CMB data provide lower limits to the WIMP masses (m_χ0.5-5 MeV) that depend on the nature of the WIMP, favor m_χ~8 MeV (with small variations depending on the WIMP type) slightly over standard BBN, and loosen the constraints on the allowed number of equivalent neutrinos, ΔN_ν=0.65_-0.35+0.46. As a result, while ΔN_ν=0 is still disfavored at ~95% confidence when there is a light WIMP, ΔN_ν=1 is now allowed.