lepton number survival in the cosmic neutrino background
The Hot Big Bang model predicts the existence of a \emph{cosmic neutrino background}. The number of particles and anti-particles in this primordial bath of neutrinos can be different -- a memory of processes that took place at very early epochs. If neutrinos were massless, this asymmetry would not change once neutrinos froze out. However, in the case of massive particles, the asymmetry is not protected by conservation laws and can get erased via helicity-flipping scatterings off matter inhomogeneities. We evaluate this helicity-flipping rate and demonstrate that if relic lepton asymmetry ever existed, it would remain largely intact in the Earth's neighborhood for realistic values of neutrino masses.