Whats The Effect Of Bulk Velocity Of Matter On The Fast Flavor Conversion Of Neutrinos? (Cosmology)

Ian and Shalgar studied the effect of the bulk velocity of matter on fast pairwise conversions and demonstrated that depending on the direction and the magnitude of the bulk velocity, neutrino flavor conversions could be significantly enhanced or suppressed. Their study recently appeared in Arxiv.

Core collapse supernovae are one of the densest astrophysical objects which copiously produce neutrinos. Three flavors of neutrinos and antineutrinos are produced within about 10 seconds which carry most of the energy produced in the supernova. The neutrinos produced within this short duration play a vital in the supernova explosion mechanism according to the delayed neutrino-driven supernova explosion mechanism. The neutrino-driven explosion mechanism is often invoked to explain the revival of the shock after it loses its energy photo-dissociating iron group nuclei in the outer parts of the core. The stalled shock is revived because of the nonuniform deposition of energy by neutrinos in the outer envelope of the supernova, which leads to convection, resulting in convection driven hydrodynamical instabilities.

The energy deposition by neutrinos is flavor dependent as the electron type neutrinos have a larger cross section with matter than non-electron type neutrinos. This flavor dependent neutrino heating can play a role in sustaining convection inside the hot and dense matter. Now, Ian and Shalgar explored the effects of the bulk velocity of matter on fast pairwise conversions.

FIG. 1. Time averaged survival probabilities (left panels) and growth rates of the flavor instability (right panels) for three selected values of the bulk velocity: v = –0.025 (top panels), v = –0.05 (middle panels) and v = –0.1 (bottom panels). Qualitatively, the radially inward case is not very sensitive to the magnitude of v since the time averaged survival probabilities and the growth rates are very similar to each other. Interestingly, a radially inward matter suppresses oscillations and resembles, at least qualitatively, the isotropic case © Ian and Shalgar

They showed that since the fast flavor evolution of neutrino depends strongly on the angle dependence of the coherent forward scattering potential, the angle dependence of the matter potential cannot be ignored. The angle dependence in the matter potential can naturally arise due to the bulk velocity of matter in astrophysical environments. In some instances, the bulk velocity of matter present in astrophysical environments can be as large as 10% of the speed of light. They found that bulk velocities much smaller than the maximum allowed velocities can substantially change the neutrino flavor conversion rate.

“There are two alternative ways of understanding the reason for the impact of the bulk velocity of matter on neutrino flavor evolution. One which we have already seen in this study, is to consider the problem in terms of the modification in the angular dependence of the potential experienced by the neutrinos due to the bulk velocity of matter. The neutrinos traveling in the direction of the bulk velocity of the matter will see a smaller flux compared to the neutrinos in the opposite direction giving rise to the angle dependence of the matter potential. Another way to think about the problem is to consider the problem in the rest frame of the matter. Due to the change in the reference frame, the angular distribution of neutrinos is modified and could be modified in a way that either enhances or suppressed the neutrino flavor conversion rates. It should be noted that the two approaches mentioned here are two different ways of think about the same phenomenon; in two different reference frames.”

— they wrote.

The demonstration of possible enhancement of neutrino flavor conversion rate due to the bulk velocity of matter also raises several important issues that are of relevance in the supernova mechanism. For instance, in the widely popular delayed neutrino driven supernova mechanism, the emphasis is on investigating the role of neutrinos in triggering convection; however, the role of the convective flow of matter in triggering neutrino flavor evolution can possibly lead to a feedback mechanism that is either positive or negative.

“Future studies with realistic velocity profiles could elucidate the possible implications on the phenomenology of core-collapse supernovae and neutron star mergers”

— they concluded.

Reference: Ian Padilla-Gay and Shashank Shalgar, “Fast flavor conversion of neutrinos in presence of matter bulk velocity”, Arxiv, 2021.

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