We investigate the spectral property and a possible collective nature
of the standard-model neutrinos at electroweak scale temperature (T).
A novel ingredient in dynamics at finite T
is the scattering of the probe particle (neutrino) by
the thermally excited particles,
leading to well-known Landau damping and the modification
of the spectral property of the probe particle.
We show that the spectral density of the neutrino gets to have
a three-peak structure in the low-momentum region due to the
existance of both the Landau damping and the massive weak bosons.
The three-peak structure appears independently
of the gauge fixing and thus has a physical significance.
We discuss the possible implication of the neutrino spectral density
on the resonant leptogenesis where the lepton number creation
in the low-energy (electroweak) scale can be relevant.

Reference:
K. Miura, Y. Hidaka, D. Satow, and T. Kunihiro
arXiv:1306.1701 [hep-ph].