  Motivated by the scenario of resonant leptogenesis in which lepton number
creation in the electroweak-scale is relevant, we investigate the spectral
properties and possible collective nature of the standard model neutrinos at
electroweak scale temperature (T). We adopt the R_xi gauge fixing, which
includes the unitary gauge as a limiting case, and allows us to study the
broken as well as the restored phases of the gauge symmetry in a unified way.
We show that the spectral density of the neutrino has a three-peak structure in
the low-momentum region due to the scattering with the thermally excited
particles (i.e., Landau damping) when T becomes comparable to the weak-boson
masses in the plasma. The three peaks are identified with a novel ultrasoft
mode, the usual quasiparticle, and antiplasmino modes. Varying the gauge-fixing
parameter, we show that the three-peak structure appears independently of the
gauge fixing and thus has a physical significance. We discuss possible
implications of the neutrino spectral density obtained in the present work on
particle cosmology, in particular in the context of resonant leptogenesis.
