Neutrino spectral density at electroweak scale temperature

TL;DR

This paper investigates the spectral properties of standard model neutrinos at electroweak scale temperatures, revealing a three-peak structure in their spectral density due to thermal scattering effects, with implications for leptogenesis.

Contribution

It provides a gauge-independent analysis of neutrino spectral density at electroweak temperatures, identifying a three-peak structure and its physical significance.

Findings

Neutrino spectral density exhibits a three-peak structure at high temperatures.

The three peaks correspond to ultrasoft, quasiparticle, and antiplasmino modes.

The structure is gauge-independent, indicating physical relevance.

Abstract

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.