Basic characteristics of neutrino flavor conversions in the postshock regions of core-collapse supernova

One of the active debates in core-collapse supernova (CCSN) theory is how significantly neutrino flavor conversions induced by neutrino-neutrino self-interactions change the conventional picture of CCSN dynamics. Recent studies have indicated that strong flavor conversions can occur inside neutrino spheres where neutrinos are tightly coupled to matter. These flavor conversions are associated with either collisional instability or fast neutrino-flavor conversion (FFC) or both. The impact of these flavor conversions on CCSN dynamics is, however, still highly uncertain due to the lack of global simulations of quantum kinetic neutrino transport with appropriate microphysical inputs. Given fluid profiles from a recent CCSN model at three different time snapshots in the early postbounce phase, we perform globalquantum kinetic simulations in spherical symmetry with an essential set of microphysics. We find that strong flavor conversions occur in optically thick regions, resulting in a substantial change of neutrino radiation field. The neutrino heating in the gain region is smaller than the case with no flavor conversions, whereas the neutrino cooling in the optically thick region is commonly enhanced. Based on the neutrino data obtained from our multiangle neutrino transport simulations, we also assess some representative classical closure relations by applying them to diagonal components of density matrix of neutrinos. We find that Eddington tensors can be well-approximated by these closure relations except for the region where flavor conversions occur vividly. We also analyze the neutrino signal by carrying out detector simulations for Super-Kamiokande, DUNE, and JUNO. We propose a useful strategy to identify the sign of flavor conversions in neutrino signal, that can be easily implemented in real data analyses of CCSN neutrinos.