Fast flavor conversion of Supernova neutrinos : Beginning to End

Soumya Bhattacharyya^1*

¹High Energy Theory Group, Institute of Physics, Academia Sinica, Taipei, Taiwan, Taipei, Taiwan

* Presenter:Soumya Bhattacharyya, email:soumyabhattacharyya475@gmail.com

A supernova natively emits neutrinos and antineutrinos of all
flavors with different luminosities and energy spectra. These neutrinos
can undergo neutrino-neutrino forward-scattering inside the dense
anisotropic interior of the star and can lead to neutrino flavors to
oscillate collectively at a very fast rate known as "Fast Flavor
Conversion''. Although the triggering and initial growth of fast
conversions are understood, owing to its complicated nonlinear
evolution, the final impact is not yet known. Interestingly, stellar
explosions and the neutrino signal are sensitive to the processed
flavor-dependent fluxes, but the required neutrino theory prediction is
still lacking. Moreover, the fact that the different neutrino flavors
have hierarchical interaction rates can produce relative forward
excesses in the fluxes of non-electron flavored neutrinos over the electron flavored ones in the decoupling region of a supernova which can develop a
zero crossing in phase space for the neutrino angular distributions and
act as a criterion for the fast instability to occur. In the first half
of my talk I will address this crucial theoretical or phenomenological
obstacle and present the fast ever theory of "Fast Flavor Conversion''
beyond the linear regime to show how the different neutrino flavors are
brought closer to each other or in other words "Flavor Depolarizes''
due to irreversible decoherence-like processes occurring at a Tera-Hz
rate, determined by the large neutrino density. Our theory explains how
depolarization happens, when it happens, what determines the epoch and
finally describes a method to calculate the extent of depolarized
neutrino fluxes. In the second half of my talk I will present a
linearized analysis to explain the dependence of the initial growth of
fast oscillations on the nature of the neutrino angular distributions,
especially its symmetry property, the number of zero crossings, lepton
asymmetry etc. All these new results can play a crucial role for future
supernova theory and neutrino phenomenology.

Keywords: Flavor Depolarization, Supernova Neutrinos , Fast Neutrino Flavor Conversion, Neutrinos