BBN and the CMB constrain light, electromagnetically coupled WIMPs

Abstract

(Abridged) In the presence of a light WIMP (< 30 MeV), there are degeneracies among the nature of the WIMP, its couplings to standard model particles, its mass m_chi, and the number of equivalent neutrinos beyond the standard model, Delta N_nu. These degeneracies cannot be broken by the CMB constraint on the effective number of neutrinos, N_eff. However, big bang nucleosynthesis (BBN) is affected by a light WIMP and equivalent neutrinos, so the combination of BBN and CMB can break some of the degeneracies. Here, BBN predictions for abundances in the presence of a light WIMP and equivalent neutrinos are explored, and estimates of their observationally determined relic abundances are used to limit m_chi, Delta N_nu, and the present Universe baryon density (Omega_B h^2). These constraints are explored 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 constraints for the two cases are contrasted. Without a light WIMP but allowing for equivalent neutrinos, the combined BBN and CMB constraints favor N_eff = 3.46 +/- 0.17, Omega_B h^2 = 0.0224 +/- 0.0003, and Delta N_nu = 0.40 +/- 0.17. In this case, standard BBN (Delta N_nu = 0) is disfavored at ~98% confidence, and the presence of one sterile neutrino (Delta N_nu = 1) is disfavored at > 99% confidence. Allowing a light WIMP and Delta N_nu equivalent neutrinos together, the combined BBN and CMB data provide lower limits to the WIMP masses (> 0.5 - 5 MeV) that depend on the nature of the WIMP, favor m_chi ~8 MeV slightly over no WIMP, and loosen constraints on equivalent neutrinos, Delta N_nu = 0.65+0.46-0.35. While Delta N_nu = 0 is still disfavored at ~95% confidence with a light WIMP, Delta N_nu = 1 is now allowed.