Simultaneous constraints on the number and mass of relativistic species

Recent indications from both particle physics and cosmology suggest the possible existence of more than three neutrino species. In cosmological analyses the effects of neutrino mass and number of species can in principle be disentangled for fixed cosmological parameters. However, since we do not have perfect measurements of the standard Λ cold dark matter model parameters, some correlation remains between the neutrino mass and number of species, and both parameters should be included in the analysis. Combining the newest observations of several cosmological probes (cosmic microwave background, large-scale structure, expansion rate), we obtain Neff = 3.58 +0.15 -0.16(68% CL)+0.55 -0.53(95% CL) and ∑mν < 0.60 eV(95% CL), which are currently the strongest constraints on Neff and ∑mν from an analysis including both parameters. The preference for Neff >3 is at the 2σ level