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During the stalled-shock phase of our 3D hydrodynamical core-collapsesimulations with energy-dependent, 3-flavor neutrino transport, thelepton-number flux (nue minus antinue) emerges predominantly in one hemisphere.This novel, spherical-symmetry breaking neutrino-hydrodynamical instability istermed LESA for "Lepton-number Emission Self-sustained Asymmetry." While theindividual nue and antinue fluxes show a pronounced dipole pattern, theheavy-flavor neutrino fluxes and the overall luminosity are almost sphericallysymmetric. Initially, LESA seems to develop stochastically from convectivefluctuations, it exists for hundreds of milliseconds or more, and it persistsduring violent shock sloshing associated with the standing accretion shockinstability. The nue minus antinue flux asymmetry originates mainly below theneutrinosphere in a region of pronounced proto-neutron star (PNS) convection,which is stronger in the hemisphere of enhanced lepton-number flux. On thisside of the PNS, the mass-accretion rate of lepton-rich matter is larger,amplifying the lepton-emission asymmetry, because the spherical stellar infalldeflects on a dipolar deformation of the stalled shock. The increased shockradius in the hemisphere of less mass accretion and minimal lepton-number flux(antinue flux maximum) is sustained by stronger convection on this side, whichis boosted by stronger neutrino heating because the average antinue energy ishigher than the average nue energy. Asymmetric heating thus supports the globaldeformation despite extremely nonstationary convective overturn behind theshock. While these different elements of LESA form a consistent picture, a fullunderstanding remains elusive at present. There may be important implicationsfor neutrino-flavor oscillations, the neutron-to-proton ratio in theneutrino-heated supernova ejecta, and neutron-star kicks, which remain to beexplored.

Self-sustained asymmetry of lepton-number emission: A new phenomenon during the supernova shock-accretion phase in three dimensions