Neutrino flavor transformation in core-collapse supernovae

Ever since a collection of 19 flickers of Cherenkov radiation in neutrino detectors around the world have been linked to SN 1987a, the physics community has been certain that neutrinos are the pivotal actors in core-collapse supernovae. SN1987a confirmed Jim Wilson and Sterling Colgate's basic picture of the collapsed core of a massive star shining with neutrinos of all kinds, radiating with a luminosity of 10⁵³ erg s⁻¹. At a stroke neutrinos provided a mechanism by which nature could explode an object as massive as a star and produce the heavy elements that we find all around us. Like all great discoveries, however, the neutrino burst of SN 1987a raised more questions than it answered. Neutrinos are known to change their lepton flavor as a purely quantum mechanical process in vacuum and in the sun and the earth, and our experimental knowledge of this process leads us to an inexorable conclusion: neutrinos will change their flavor states in supernovae, as well. One of the most deucedly difficult problems in neutrino astrophysics for the last few decades has been the question of how neutrino flavor transforms in the supernova environment. The intense flux of neutrinos from the core of a supernova is so numerous that the interactions of neutrinos with one another is strong enough to create a quantum mechanical coupling of their flavor evolution histories, so that the flavor states of all neutrinos are non-linearly related to one another. This non-linear coupling of neutrino flavor states can have fascinating consequences, leading to collective flavor transformation phenomena, where all neutrinos emerging from the core begin to change flavor simultaneously. The dynamics of these non-linear effects can provide great insight into as yet unconstrained sectors of neutrino physics. Likewise, should the details of neutrino flavor mixing be worked out before the next galactic supernova, the understanding of neutrino flavor transformation might act as a neutrino telescope, allowing an observer to probe the depths of an exploding star. However, supernovae are environments that test the limits of our physical understanding. A cherished paradigm within the supernova neutrino community, the "neutrino bulb" model, has been found to be critically flawed early in the explosion. The collapsed core is not the only source of neutrinos which are important for flavor transformation. The early epochs of supernovae posses a "halo" of neutrinos, which have scattered inwards from the outer reaches of the explosion envelope. These halo neutrinos carry with them quantum information about the flavor transformation that has transpired elsewhere in the star, and can dominate the flavor evolution of neutrinos emerging from the core. This alters the fundamental nature of the problem of neutrino flavor transformation in supernovae, and will require all new approaches to addres