arXiv subjects

Steen Hannestad

Publications and source records attributed to Steen Hannestad.

182 records · Page 11Linked to original sources

arXiv1996

Nucleosynthesis and the mass of the tau neutrino

The primordial abundance of long-lived heavy Majorana neutrinos is calculated from the full Boltzmann equation. Inclusion of scattering reactions drastically change the predicted abundance of a heavy neutrino species. This loosens the well known mass constraint on MeV neutrinos from Big Bang nucleosynthesis, and allows for the existence of a Majorana tau neutrino with mass m > 11 MeV. Further experimental efforts are therefore needed to investigate the range 11 MeV < m < 24 MeV. Some interesting cosmological consequences of an MeV nu_tau are also pointed out.

hep-ph↗

arXiv1995

Neutrino decoupling in the early Universe

A calculation of neutrino decoupling in the early Universe, including full Fermi-Dirac statistics and electron mass dependence in the weak reaction rates, is presented. We find that after decoupling, the electron neutrinos contribute 0.83\% more to the relativistic energy density than in the standard scenario, where neutrinos are assumed not to share the heating from e$^\pm$ annihilation. The corresponding number for muon and tau neutrinos is 0.41\% . This has the consequence of modifying the primordial $^4$He abundance by $ΔY=+1.0\times 10^{-4}$, and the cosmological mass limit on light neutrinos by 0.2--0.5 eV.

astro-ph↗