Collective neutrino flavor transformation in supernovae

Abstract

We examine coherent active-active channel neutrino flavor evolution in environments where neutrino-neutrino forward scattering can engender large-scale collective flavor transformation. We introduce the concept of neutrino flavor isospin which treats neutrinos and antineutrinos on an equal footing, and which facilitates the analysis of neutrino systems in terms of the spin precession analogy. We point out a key quantity, the "total effective energy", which is conserved in several important regimes. Using this concept, we analyze collective neutrino and antineutrino flavor oscillation in the "synchronized" mode and what we term the "bi-polar" mode. We thereby are able to explain why large collective flavor mixing can develop on short timescales even when vacuum mixing angles are small in, e.g., a dense gas of initially pure νe andνe with an inverted neutrino mass hierarchy (an example of bi-polar oscillation). In the context of the spin precession analogy, we find that the co-rotating frame provides insights into more general systems, where either the synchronized or bipolar mode could arise. For example, we use the co-rotating frame to demonstrate how large flavor mixing in the bi-polar mode can occur in the presence of a large and dominant matter background. We use the adiabatic condition to derive a simple criterion for determining whether the synchronized or bi-polar mode will occur. Based on this criterion we predict that neutrinos and antineutrinos emitted from a proto-neutron star in a core-collapse supernova event can experience synchronized and bi-polar flavor transformations in sequence before conventional Mikhyev-Smirnov-Wolfenstein flavor evolution takes over. This certainly will affect the analyses of future supernova neutrino signals, and might affect the treatment of shock re-heating rates and nucleosynthesis depending on the depth at which collective transformation arises.

Symmetries in collective neutrino oscillations

“…This phenomenon is different from the conventional Mikheyev-Smirnov-Wolfenstein (MSW) effect [14,15] in that the flavour evolution histories of neutrinos in collective oscillations are coupled together and must be solved simultaneously. The possibility and consequences of collective neutrino oscillations in supernovae were not well appreciated until the discovery that the ordinary matter can be "ignored" in such phenomena [16] and the first numerical demonstrations of "stepwise spectral swapping" (or "spectral split") [17,18] which is the imprint left by the collective flavour transformation on neutrino energy spectra.…”

Resonances driven by a neutrino gyroscope and collective neutrino oscillations in supernovae

“…The dominance of neutrino-neutrino over neutrino-electron forward scattering couples flavor evolution of neutrinos that have different energy and travel on different trajectories, thereby giving rise to new phenomena. Assuming no CP-violation in the neutrino sector, [8][9][10] first showed that collective oscillations of supernova neutrinos can occur for the vacuum-mass-squared difference δm 2 13 relevant for atmospheric neutrino oscillations. This collective flavor transformation can be treated effectively as 2 × 2 mixing between ν e and ν x (ν e andν x ), where ν x (ν x ) is a superposition of ν µ and ν τ (ν µ andν τ ), because ν µ and ν τ (ν µ andν τ ) have identical emission characteristics and interactions in supernovae.…”

“…Subsequent to the study in [10], a number of authors have investigated collective oscillations of supernova neutrinos (see [4] for a review). The most prominent feature of collective oscillations is the stepwise swap of the ν e and ν x energy spectra at some characteristic "split" energy [11,12].…”

“…In accordance with the experimental limits on θ 13 , we take θ v ≪ 1 (NH) orθ v ≡ π/2 − θ v ≪ 1 (IH). Following the definition of [10], we represent each neutrino by a neutrino flavor isospin (NFIS) s of magnitude 1/2 in a three-dimensional flavor space. In particular,…”

Decoherence in supernova neutrino transformations suppressed by deleptonization

“…In the previous literature it was recognized that a constant matter profile essentially reduces the effective mixing angle so that matter should have the same influence as a small vacuum mixing angle [4,7]. We illustrate this point in Fig.…”