Neutrinos from dense environments: Flavor mechanisms, theoretical approaches, observations, and new directions

Neutrino masses and mixings produce vacuum oscillations, an established quantum mechanical phenomenon. In matter, the Mikheev-Smirnov-Wolfenstein effect, due to neutrino interactions with the background particles, triggers resonant flavor modification. In dense environments, such as core-collapse supernovae and compact mergers, sizable neutrino-neutrino interactions, shock waves, and turbulence impact the neutrino flavor content under a variety of phenomena. Theoretical approaches of neutrino propagation range from the mean-field approximation to the full quantum kinetic equations. Interesting connections have been uncovered between weakly interacting dense neutrino gases and other many-body systems and domains, from condensed matter and nuclear physics to quantum computing. Besides the intrinsic theoretical interest, establishing how neutrinos change flavor contributes to answering the long-standing open questions of how massive stars explode and of the $r$-process sites. It is also important for future observations of core-collapse supernova neutrinos and of the diffuse supernova neutrino background that should be discovered in the foreseeable future.