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Neutrino collective excitations are studied in the Standard Model at hightemperatures below the symmetry breaking scale. Two parameters determine theproperties of the collective excitations: a mass scale $m_\nu=gT/4$ whichdetermines the \emph{chirally symmetric} gaps in the spectrum and$\Delta=M^2_W(T)/2m_\nu T$. The spectrum consists of left handed negativehelicity quasiparticles, left handed positive helicity quasiholes and theirrespective antiparticles. For $\Delta < \Delta_c = 1.275...$ there are twogapped quasiparticle branches and one gapless and two gapped quasiholebranches, all but the higher gapped quasiparticle branches terminate at endpoints. For $\Delta_c < \Delta < \pi/2$ the quasiparticle spectrum features apitchfork bifurcation and for $\Delta >\pi/2$ the collective modes are gaplessquasiparticles with dispersion relation below the light cone for $k\ll m_\nu$approaching the free field limit for $k\gg m_\nu$ with a rapid crossoverbetween the soft non-perturbative to the hard perturbative regimes for $k\simm_\nu$.The \emph{decay} of the vector bosons leads to a \emph{width} of thecollective excitations of order $g^2$ which is explicitly obtained in thelimits $k =0$ and $k\gg m_\nu \Delta$. At high temperature this damping rate isshown to be competitive with or larger than the collisional damping rate oforder $G^2_F$ for a wide range of neutrino energy.Comment: 32 pages 16 figs. Discussion on screening corrections. Results  unchanged to appear in Phys. Rev.

Neutrino collective excitations in the standard model at high temperature