Synchronization vs. decoherence of neutrino oscillations at intermediate densities

TL;DR

This paper investigates how neutrino oscillations transition between synchronized and decoherent states at various densities, revealing phase transitions and resonance effects that influence coherence.

Contribution

It introduces a comprehensive analysis of neutrino collective oscillations at intermediate densities, identifying phase transitions and deriving an analytic relation for late-time coherence.

Findings

Existence of a phase transition to partial synchronization above a density threshold.

Parametric resonance can induce decoherence at small mixing angles.

Derived an analytic formula to predict late-time coherence from the spectrum.

Abstract

We study collective oscillations of a two-flavor neutrino system with arbitrary but fixed density. In the vacuum limit, modes with different energies quickly de-phase (kinematical decoherence), whereas in the limit of infinite density they lock to each other (synchronization). For intermediate densities, we find different classes of solutions. There is always a phase transition in the sense of partial synchronization occurring only above a density threshold. For small mixing angles, partial or complete decoherence can be induced by a parametric resonance, introducing a new time scale to the problem, the final outcome depending on the spectrum and mixing angle. We derive an analytic relation that allows us to calculate the late-time degree of coherence based on the spectrum alone.