Updated constraints on non-standard neutrino interactions from Planck

Abstract

We provide updated bounds on non-standard neutrino interactions based on data from the Planck satellite as well as auxiliary cosmological measurements. Two types of models are studied - A Fermi-like 4-point interaction and an interaction mediated by a light pseudoscalar - and we show that these two models are representative of models in which neutrinos either decouple or recouple in the early Universe. Current cosmological data constrain the effective 4-point coupling to be G_X <= (0.06 GeV)^-2, corresponding to G_X <= 2.5 × 10⁷ G_F. For non-standard pseudoscalar interactions we set a limit on the diagonal elements of the dimensionless coupling matrix, g_ij, of g_ii <= 1.2 × 10^-7. For the off-diagonal elements which induce neutrino decay the bound is significantly stronger, corresponding to g_ij <= 2.3 × 10^-11(m/0.05 eV)^-2, or a lifetime constraint of τ >= 1.2 × 10⁹ s (m/0.05 eV)³ . This is currently the strongest known bound on this particular type of neutrino decay. We finally note that extremely strong neutrino self-interactions which completely suppress anisotropic stress over all of cosmic history are very highly disfavored by current data Δ χ² ~ 10⁴).