Collective neutrino flavor conversion: Recent developments (1602.02766v1)

Abstract: Neutrino flavor evolution in core-collapse supernovae, neutron-star mergers, or the early universe is dominated by neutrino-neutrino refraction, often spawning "self-induced flavor conversion", i.e., shuffling of flavor among momentum modes. This effect is driven by collective run-away modes of the coupled "flavor oscillators" and can spontaneously break the initial symmetries such as axial symmetry, homogeneity, isotropy, and even stationarity. Moreover, the growth rates of unstable modes can be of the order of the neutrino-neutrino interaction energy instead of the much smaller vacuum oscillation frequency: self-induced flavor conversion does not always require neutrino masses. We illustrate these newly found phenomena in terms of simple toy models. What happens in realistic astrophysical settings is up to speculation at present.

Collective Neutrino Flavor Conversion: Recent Developments

The paper entitled "Collective Neutrino Flavor Conversion: Recent Developments" explores the intricate phenomena of neutrino flavor evolution driven by neutrino-neutrino refraction, specifically within astrophysical environments like core-collapse supernovae, neutron-star mergers, and the early universe. The authors, Sovan Chakraborty, Rasmus Hansen, Ignacio Izaguirre, and Georg Raffelt, explore the concept of self-induced flavor conversion whereby neutrino flavor transformations occur among momentum modes, often breaking initial symmetries such as axial symmetry, homogeneity, isotropy, and stationarity.

The paper elaborates on how these collective transformations are prompted by run-away modes of flavor oscillators that can grow at rates comparable to neutrino-neutrino interaction energy, contrasting with the typical vacuum oscillation frequency that is notably smaller. Notably, self-induced flavor conversion might not necessitate neutrino masses. Using simplified toy models, the authors demonstrate these newly discovered phenomena, while acknowledging that realistic astrophysical implications remain speculative.

Key Insights and Numerical Findings

• Flavor Evolution Framework: Utilizing flavor matrices to describe the neutrino mean field, the paper addresses the feedback loop in flavor evolution, resulting in potential equilibrations among modes without altering global flavor content. An example illustrated is a gas composed equally of $\nu_e$ and $\bar\nu_e$, transforming into a mixed configuration while maintaining zero flavor lepton numbers overall.
• Symmetry Assumptions: Much of the traditional understanding of supernova neutrino flavor evolution relied heavily on symmetry assumptions such as axial symmetry, homogeneity, and stationarity. The paper highlights limitations in such assumptions, demonstrating that spontaneous symmetry breaking can lead to entirely novel solutions.
• Self-Induced Flavor Conversion: The analysis draws attention to unstable modes that are sensitive to neutrino mass ordering and can lead to spectral splits, wherein flavor swaps occur across different spectrum regions. Such phenomena offer insight into understanding mass ordering, notwithstanding the measurement of mixing angle $\Theta_{13}$, which has rendered these effects more accessible experimentally.
• Fast Flavor Conversion: Contrary to earlier findings where growth rates were tied to vacuum oscillation frequencies, the paper discusses scenarios where the conversion can occur swiftly—at levels commensurate with interaction energy $\mu$. Fast flavor conversion does not rely directly on neutrino masses, although initial disturbances might stem from ordinary oscillation phenomena.

Practical and Theoretical Implications

The research opens avenues for understanding neutrino flavor conversion as a fundamental aspect of astrophysical phenomena. The spontaneous breaking of symmetry and potential fast flavor conversions suggest a reevaluation of how neutrino interactions are framed in dense environments. The implications of flavor decoherence being a generic outcome or the establishment of conditions under which this may arise could simplify the modeling of neutrino behavior in supernova simulations.

Given the complexities introduced by space-time dependent variables, the paper emphasizes the need for advanced solution techniques possibly accounting for non-linear couplings and realistic numerical studies. While speculative at present, a refined comprehension of such fast or asymmetric flavor conversion mechanisms could deeply influence the broader understanding of neutrinos within cosmology and high-energy astrophysics.

Outlook

While the paper provides a thorough exploration of collective neutrino flavor conversion, it acknowledges considerable gaps in practical numerical approaches needed to simulate realistic astrophysical scenarios. For the time being, focusing on reductionist approaches and simplified models will be crucial in conceptualizing and understanding diverse behavior patterns exhibited by neutrino gases, potentially paving the way for innovations in neutrino studies and the advancement of theoretical models in astrophysics.