Limits on the cosmic neutrino background

Abstract

Abstract We present the first comprehensive discussion of constraints on the cosmic neutrino background (CνB) overdensity, including theoretical, experimental and cosmological limits for a wide range of neutrino masses and temperatures. Additionally, we calculate the sensitivities of future direct and indirect relic neutrino detection experiments and compare the results with the existing constraints, extending several previous analyses by taking into account that the CνB reference frame may not be aligned with that of the Earth. The Pauli exclusion principle strongly disfavours overdensities ην ≫ 1 at small neutrino masses, but allows for overdensities  ην ≲ 125 at the KATRIN mass bound mν ≃ 0.8 eV. On the other hand, cosmology strongly favours 0.2 ≲ ην ≲ 3.5 in all scenarios. We find that direct detection proposals are capable of observing the CνB without a significant overdensity for neutrino masses mν ≳ 50 meV, but require an overdensity  ην ≳ 3 × 105 outside of this range. We also demonstrate that relic neutrino detection proposals are sensitive to the helicity composition of the CνB, whilst some may be able to distinguish between Dirac and Majorana neutrinos.