Chaotic neutrino flavor outflows in neutron star mergers

In neutron star mergers (NSM), neutrinos play a pivotal role in altering the neutron-proton ratio that seeds the condition for heavy element production. There, neutrino-neutrino coherent forward scattering leads to a complex flavor oscillation phenomenon that nowadays is a big source of uncertainty in global simulations of NSM. Recent neutrino simulations suggest chaos in flavor evolution. This could cause trajectories to rapidly diverge in the flavor space when initial conditions vary slightly, potentially limiting our ability to predict neutrino flavor behavior in NSM. To address this issue, we model neutrino behavior within a narrow centimeter-scale region inside a NSM. Our aim is to examine how nearby flavor states behave when flavor instabilities are present. Our findings show that in the non-linear phase of neutrino flavor evolution, solutions with close flavor initial states exhibit exponential divergence and chaos. This results in significant uncertainty in the spatial distribution of neutrino flavors and the density matrix of computational particles. However, the domain-averaged neutrino density matrix component remains relatively stable, with chaos affecting it minimally (up to 1% uncertainty). The domain-averaged neutrino density matrix component could be used as a key variable in global neutrino simulations in NSM.