Cosmological limits on neutrino unknowns versus low redshift priors

Recent cosmic microwave background (CMB) temperature and polarization anisotropy measurementsfrom the Planck mission have significantly improved previous constraints on the neutrino masses as well asthe bounds on extended models with massless or massive sterile neutrino states. However, due to parameterdegeneracies, additional low redshift priors are mandatory in order to sharpen the CMB neutrino bounds.We explore here the role of different priors on low redshift quantities, such as the Hubble constant, thecluster mass bias, and the reionization optical depthτ. Concerning current priors on the Hubble constantand the cluster mass bias, the bounds on the neutrino parameters may differ appreciably depending on thechoices adopted in the analyses. With regard to future improvements in the priors on the reionization opticaldepth, a value ofτ¼0.050.01, motivated by astrophysical estimates of the reionization redshift, wouldlead toPmν<0.0926eV at 90% C.L., when combining the fullPlanckmeasurements, baryon acousticoscillation, and Planck clusters data, thereby opening the window to unravel the neutrino mass hierarchywith existing cosmological probes.