# Collective neutrino oscillations and neutrino wave packets

## TL;DR

This paper investigates how wave packet separation causes decoherence in collective neutrino oscillations within dense environments, analyzing the effects in vacuum, matter, and non-adiabatic regimes using density matrix formalism.

## Contribution

It provides a detailed analysis of decoherence effects on collective neutrino oscillations, including the derivation of damping terms in different regimes and models of adiabaticity violation.

## Key findings

- Decoherence introduces damping in vacuum and adiabatic matter oscillations.
- No damping term generally appears in non-adiabatic regimes.
- Estimated neutrino coherence lengths relevant for supernovae environments.

## Abstract

Effects of decoherence by wave packet separation on collective neutrino oscillations in dense neutrino gases are considered. We estimate the length of the wave packets of neutrinos produced in core collapse supernovae and the expected neutrino coherence length, and then proceed to consider the decoherence effects within the density matrix formalism of neutrino flavour transitions. First, we demonstrate that for neutrino oscillations in vacuum the decoherence effects are described by a damping term in the equation of motion of the density matrix of a neutrino as a whole (as contrasted to that of the fixed-momentum components of the neutrino density matrix). Next, we consider neutrino oscillations in ordinary matter and dense neutrino backgrounds, both in the adiabatic and non-adiabatic regimes. In the latter case we study two specific models of adiabaticity violation -- one with short-term and another with extended non-adiabaticity. It is demonstrated that, while in the adiabatic case a damping term is present in the equation of motion of the neutrino density matrix (just like in the vacuum oscillation case), no such term in general appears in the non-adiabatic regime.