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Quantum angular moment transport schemes are an important avenue toward describing neutrino flavor mixing phenomena in dense astrophysical environments such as supernovae and merging neutron stars. Successful implementation will require new closure relations that go beyond those used in classical transport. In this paper, we derive the first analytic expression for a quantum M1 closure, valid in the limit of small flavor coherence, based on the maximum entropy principle. We verify that the resulting closure relation has the appropriate limits and characteristic speeds in the diffusive and free-streaming regimes. We then use this new closure in a moment linear stability analysis to search for fast flavor instabilities in a binary neutron star merger simulation and find better results as compared with previously designed, , semiclassical closures. Published by the American Physical Society2025 

A computationally efficient method for calculating the transport of neutrino flavor in simulations is to use angular moments of the neutrino one-body reduced density matrix, i.e., “quantum moments.” As with any moment-based radiation transport method, a closure is needed if the infinite tower of moment evolution equations is truncated. We derive a general parametrization of a quantum closure and the limits the parameters must satisfy in order for the closure to be physical. We then derive from multiangle calculations the evolution of the closure parameters in two test cases which we then progressively insert into a moment evolution code and show how the parameters affect the moment results until the full multiangle results are reproduced. This parametrization paves the way to setting prescriptions for genuine quantum closures adapted to neutrino transport in a range of situations. Published by the American Physical Society2025 

Primordial neutrino-antineutrino asymmetries can be constrained through big-bang nucleosynthesis (BBN) relic abundances and cosmic microwave background (CMB) anisotropies, both observables being sensitive to neutrino properties. The latter constraint, which is due to gravitational effects from all neutrino flavors, is very minute since it is at least quadratic in the asymmetries. On the contrary, the constraints from primordial abundances presently dominate, although these abundances are almost only sensitive to the electron flavor asymmetry. It is generally assumed that neutrino asymmetries are sufficiently averaged by flavor oscillations prior to BBN, which allows one to constrain a common primordial neutrino asymmetry at the epoch of BBN. This simplified approach suffers two caveats that we deal with in this article, combining a neutrino evolution code and BBN calculation throughout the MeV era. First, flavor “equilibration” is not true in general, therefore an accurate dynamical evolution of asymmetries is needed to connect experimental observables to the asymmetries. Second, the approximate averaging of asymmetries through flavor oscillations is associated to a reheating of the primordial plasma. It is therefore crucial to correctly describe the interplay between flavor equilibration and neutrino decoupling, as an energy redistribution prior to decoupling does not significantly alter the final effective number of neutrino species’ value. Overall, we find that the space of allowed initial asymmetries is generically unbound when using currently available primordial abundances and CMB measurements. We forecast constraints using future CMB experiment capabilities, which should reverse this experimental misfortune. Published by the American Physical Society2024 

Abstract Multi-messenger astrophysics has produced a wealth of data with much more to come in the future. This enormous data set will reveal new insights into the physics of core-collapse supernovae, neutron star mergers, and many other objects where it is actually possible, if not probable, that new physics is in operation. To tease out different possibilities, we will need to analyze signals from photons, neutrinos, gravitational waves, and chemical elements. This task is made all the more difficult when it is necessary to evolve the neutrino component of the radiation field and associated quantum-mechanical property of flavor in order to model the astrophysical system of interest—a numerical challenge that has not been addressed to this day. In this work, we take a step in this direction by adopting the technique of angular-integrated moments with a truncated tower of dynamical equations and a closure, convolving the flavor-transformation with spatial transport to evolve the neutrino radiation quantum field. We show that moments capture the dynamical features of fast flavor instabilities in a variety of systems, although our technique is by no means a universal blueprint for solving fast flavor transformation. To evaluate the effectiveness of our moment results, we compare to a more precise particle-in-cell method. Based on our results, we propose areas for improvement and application to complementary techniques in the future.