BBN And The CMB Constrain Neutrino Coupled Light WIMPs

TL;DR

This paper combines Big Bang Nucleosynthesis and CMB data to constrain the properties of light WIMPs coupled to neutrinos, finding that such WIMPs are unlikely below 35 MeV and refining neutrino-related parameters.

Contribution

It provides new constraints on neutrino-coupled light WIMPs using combined BBN and CMB data, extending previous analyses to this specific interaction.

Findings

Light WIMPs coupled to neutrinos are excluded below 35 MeV.

BBN prefers Delta N_nu around 0.5, not favoring fully thermalized sterile neutrinos.

Combined data favor Delta N_nu between 0.2 and 0.5, disfavoring values above 1.

Abstract

(abridged) In the presence of a light WIMP (mass m_chi < 30 MeV), there are degeneracies among the WIMP's nature, its couplings to standard model particles, its mass, and the number of equivalent (additional) neutrinos, Delta N_nu. These degeneracies cannot be broken by the CMB constraint on the effective number of neutrinos, N_eff. However, since big bang nucleosynthesis (BBN) is also affected by a light WIMP and equivalent neutrinos, complementary BBN and CMB constraints can break some of the degeneracy. In a previous paper BBN and CMB were combined to explore allowed ranges for m_chi, Delta N_nu, and N_eff for light WIMPs that annihilate electromagnetically (EM) to photons and/or electrons/positrons. In this paper BBN predictions with a light WIMP that only couples to neutrinos are calculated. Recent observed abundances of ^2H and ^4He are used to limit m_chi, Delta N_nu, N_eff, and the present-day baryon density. Allowing for a neutrino coupled light WIMP and nonzero Delta N_nu, combined BBN and CMB data give lower limits to m_chi, with a best fit m_chi > 35 MeV, equivalent to no light WIMP at all. All masses below 4--9 MeV (depending on spin) are excluded. Without any light WIMP, BBN alone prefers Delta N_nu = 0.50 +- 0.23, favoring neither Delta N_nu = 0, nor a fully thermalized sterile neutrino (Delta N_nu = 1). This result is consistent with the CMB constraint, N_eff = 3.30 +- 0.27, limiting "new physics" between BBN and recombination. Combining BBN and CMB data gives Delta N_nu = 0.35 +- 0.16 and N_eff = 3.40 +- 0.16; while BBN and the CMB combined require Delta N_nu > 0 at ~98% confidence, they disfavor Delta N_nu > 1 at > 99% confidence. Allowing a neutrino-coupled light WIMP extends the allowed range slightly downward for Delta N_nu and slightly upward for N_eff simultaneously, leaving best-fit values unchanged.