Fast flavor conversions (FFCs) of neutrinos, driven by the fast flavor instability (FFI), can reshape the neutrino flavor content in dense astrophysical environments such as core-collapse supernovae and neutron star mergers. Most studies of FFCs adopt a two-step approach, in which a flavor-unstable state containing deep electron-minus-heavy-flavor lepton number (E-XLN) angular crossings is first constructed and subsequently evolved. Because realistic crossings should instead develop gradually through neutrino transport, the validity of such setups has been called into question. We investigate this issue by solving the neutrino quantum kinetic equations with self-consistent collisional rates in a spherically symmetric supernova background whose electron fraction evolves in time through a sequence of stages, starting from a configuration free of E-XLN crossings. We find that the evolution proceeds through three characteristic episodes. In the shallow-crossing episode, FFCs develop from marginally unstable, shallow crossings, carrying small-scale structures consistent with linear stability analysis. In the near-crossing-elimination episode, the balance between collisions and FFCs keeps the system in a near-quasistationary state in which the emerging crossings are continuously eliminated, so that the strongly unstable regime is never reached. In the swapping episodes, the E-XLN reverses sign and a dynamically propagating flavor-swap E-XLN zero surface forms. We find that the evolved flavor content at the end of different time stages broadly agrees with the quasistationary solutions obtained in the corresponding two-step models adopting fixed matter backgrounds. In addition, we investigate the robustness of the effective classical transport (ECT) framework that adopts subgrid flavor redistribution using different parametrized prescriptions. Notably, except during the swapping [abridged]

The nuclear gamma-ray ( γ -ray) lines in the MeV range of the electromagnetic spectrum hold a vast variety of astrophysical, particle-physical, and fundamental physical information that is otherwise extreme difficult to access. MeV γ -ray line observations provide the most direct evidence for ongoing nucleosynthesis in galaxies by measuring freshly produced radioactive isotopes from massive stars, supernovae (SNe), classical novae (CNe), or binary neutron star mergers (BNSMs). Their flux ratios can determine the low-energy cosmic ray (LECR) spectrum in different objects and of the Milky Way as a whole. Different phases of the interstellar medium (ISM) are traced by hot nucleosynthesis ejecta, cooling positrons, or cosmic ray (CR) interactions with molecular clouds. Positron annihilation itself can be considered as an astrophysical messenger as their production and destruction in typical space environments is inevitable. Finally, as-of-yet unknown signatures from beyond Standard Model (BSM) physics might have their elusive imprints in γ -ray lines. This article gives an overview of historical γ -ray line measurements, newest results, and open questions that may only be solved by a new generation of MeV telescopes.

We propose a novel strategy to probe feebly-interacting particles (FIPs) by exploiting the dense, confined circumstellar medium (CSM) surrounding core-collapse supernovae (CCSNe). FIPs produced in the proto-neutron star can deposit substantial visible energy into the CSM via decay prior to the shock breakout from the progenitor star. This energy injection heats and ionizes the CSM, establishing a FIP-induced photosphere that generates distinctive precursor blackbody emission. Using early-time observations of SN 2023ixf, we translate the non-detection of excessive precursor luminosity into stringent new constraints on MeV-scale dark photons as an exemplary model. Our results significantly extend existing CCSN bounds and exclude previously unexplored regions of parameter space. We further demonstrate that the FIP-induced dust sublimation offers robust diagnostics for future Galactic SNe, opening a new avenue to explore the dark sector.

Neutrinos in dense astrophysical environments such as core-collapse supernovae (CCSNe) and neutron star mergers (NSMs) can undergo FFCs, which could develop on extremely small scales. A necessary condition for the occurrence of FFCs is the presence of a zero crossing in the electron lepton number (ELN) angular distribution of neutrinos. In this work, we explore machine learning (ML) approaches to detect non-axisymmetric ELN crossings in these environments, based on input features of the ν_e and _e zeroth and first angular moments. Overall, the ML models demonstrate relatively good generalizability for most of the unseen test datasets generated by various methods that do not assume the same underlying angular distributions as used in the training set. Interestingly, while the model's performance is mediocre for an axisymmetric distribution dataset derived by solving the discretized Boltzmann transport equation under 1D CCSN background, imposing an artificial non-axisymmetry substantially improves the performance. We also find that for the flavor-equilibrated angular distributions, although our ML model trained based solely on ELN inputs performs poorly when the true crossings depend on the post-equilibrated angular distributions of heavy lepton neutrinos and antineutrinos, which become different, it delivers strong performance in detecting ELN crossings when the heavy-lepton neutrino and antineutrino distributions are artificially removed. This highlights the need for additional input features to further improve the model. This is a crucial step toward successfully integrating FFCs into large-scale CCSN and NSM simulations.

Neutron star mergers produce r-process elements, with yields that are sensitive to the kinematic and thermodynamic properties of the ejecta. These ejecta properties are potentially affected by dynamically important feedback from r-process heating, which is usually not coupled to the hydrodynamics in postmerger simulations modeling the ejecta launching and expansion. The multimessenger detection of GW170817 showed the importance of producing reliable ejecta predictions to maximize the diagnostic potential of future events. In this paper, we develop a prescription for including r-process heating as a source term in the hydrodynamic equations. This prescription depends on local fluid properties and the Y-e history as recorded by dedicated tracer particles, which exchange information with the grid using the cloud-in-cell method. The method is implemented in long-term viscous hydrodynamic simulations of accretion disk outflows to investigate its feedback on ejecta properties. We find that r-process heating can increase the unbound disk ejecta mass by similar to 10% relative to a baseline case that only considers alpha particle recombination. Nuclear heating also enhances the radial velocity of the ejecta with Y-e < 0.25 by up to a factor of 2 while concurrently suppressing marginally bound convective ejecta.

In dense neutrino gas, pairing correlations between neutrinos and antineutrinos with opposite momenta can be nonzero in generalized neutrino quantum kinetic equations at the mean-field level. In this Letter, we investigate for the first time the condition under which collective neutrino-antineutrino (ν) pairing instabilities can occur, using simplified toy models consisting of discretized ν pairs in a homogeneous neutrino gas. We find that, in ansiotropic systems, ν pairing instabilities generally emerge when the phase space distribution of the excessive pair-occupation number, defined as the sum of the neutrino and antineutrino occupation numbers of a pair minus 1, changes signs. The associated instability growth rate is set by the forward scattering potential and is comparable to that of collective fast neutrino flavor instabilities. The instabilities can result in pair conversions of ν occupation numbers between different momentum modes. Our results motivate further studies to assess the relevance of ν pairing effects in realistic astrophysical and cosmological environments.

We study collective slow flavor conversion (SFC) of supernova neutrinos with multi-energy, multi-angle simulations for three representative neutrino spectra in the early accretion, late accretion, and cooling phases, in which multiple crossings between the initial electron- and heavy-lepton-flavor spectra are present. By numerically solving the neutrino quantum kinetic equations in a local periodic box, we find that SFC triggered predominantly by the spatially inhomogeneous instabilities drives the system toward a spatially coarse-grained, quasi-stationary state, whose flavor conversion probability depends strongly on energy, angle, and the neutrino mass ordering. While we find that not all of the initial spectral crossings are completely erased in the final state, a simple, box-like analytical prescription inspired by studies of fast flavor conversions, which eliminates the spectral crossings, can reasonably approximate the post-SFC spectra. Using the initial and post-SFC spectra, we also evaluate the changes of the corresponding ν_e and _e heating rates as well as the absorption equilibrium electron fraction (Y_e). Within the considered scenarios, we find that the heating rates are generally enhanced by up to ∼ 80% due to the net conversion of ν_x to ν_e (and _x to _e) above their crossing energy, provided that the energy spectra above the crossing energy differ substantially. For the absorption equilibrium Y_e, spectra changes due to SFC increase it by ∼ 0.03 due to the relatively more enhanced ν_e absorption rate than _e, which potentially drives supernova materials to be more proton-rich. These results highlight the importance of energy-dependent treatments of SFC for supernova neutrinos.

Recent studies suggest that the most energetic cosmic rays, exceeding 100 EeV, may primarily consist of r-process nuclei. This highlights binary neutron star mergers and collapsars as promising sources of ultrahigh-energy cosmic rays (UHECRs). Building on these insights, we examine the conditions that facilitate the efficient production of UHE r-process nuclei during the prompt radiation (PR), extended emission (EE), and plateau emission phases of short gamma-ray bursts (sGRBs) following neutron star mergers. Our study reveals that jets associated with the PR phase, characterized by typical bulk Lorentz factors (>= 400-500), dissipation radii, and magnetic field strengths, can accelerate r-process nuclei to energies >= 100 EeV, while preserving them during propagation within the source. Additionally, we investigate the production of HE neutrinos from photomeson and hadronic interactions, as well as from the beta decay of accelerated r-process nuclei. We find that the HE neutrino fluxes from sGRBs, mainly produced via photomeson interactions, are significantly limited to preserve the accelerated heavy nuclei, leading to lower fluxes than the predictions without allowing for contributions to UHECRs. Our results suggest that sGRBs may potentially contribute to UHECRs during the PR phase and to HE neutrinos during the EE phase-a scenario that can be tested by future neutrino observatories.

Supernova-neutrino-boosted dark matter (SNv BDM) has emerged as a promising portal for probing sub-GeV dark matter. In this work, we investigate the behavior of BDM signatures originating from corecollapse supernovae within the Milky Way (MW) over the past one hundred thousand years, examining both their temporal evolution and present-day spatial distributions. We show that while the MW BDM signature is approximately diffuse in the nonrelativistic regime, it exhibits significant temporal variation and spatial localization when the BDM is relativistic. Importantly, we compare these local MW signatures with the previously proposed diffuse SNv BDM (DBDM), which arises from the accumulated flux of all past supernovae in the Universe [Y.-H. Lin and M.-R. Wu, Phys. Rev. Lett. 133, 111004 (2024)]. In the nonrelativistic limit, DBDM consistently dominates over the local diffuse MW BDM signature. Only when the MW BDM becomes ultrarelativistic and transitions into a transient, highly localized signal can it potentially surpass the DBDM background. This work thus reinforces the importance of DBDM for SNv BDM searches until the next galactic SN offers new opportunities.

Core-collapse supernovae undergoing a first-order quantum chromodynamics (QCD) phase transition experience the collapse of the central protoneutron star that leads to a second bounce. This event is accompanied by the release of a second neutrino burst. Unlike the first stellar core bounce neutrino burst, which consists exclusively of electron neutrinos, the second burst is dominated by electron antineutrinos. Such a condition makes QCD supernovae an ideal site for the occurrence of fast neutrino flavor conversion (FFC), which can lead to rapid flavor equilibration and significantly impact the related neutrino signal. In this work, we perform a detailed analysis of the conditions for fast flavor instability around and after the second neutrino burst in QCD phase-transition supernova models launched from 25M circle dot and 40M circle dot progenitor models. We evaluate the relevant instability criteria and find two major phases of FFC. The first phase is closely associated with the collapse and the rapidly expanding shock wave, which is a direct consequence of the protoneutron star collapse due to the phase transition. The second phase takes place a few milliseconds later when electron degeneracy is restored near the protoneutron star surface. We also characterize the growth rate of fast flavor instability and estimate its impact on the evolution of the neutrino flavor content. The potential observational consequences on neutrino signals are evaluated by comparing a scenario assuming complete flavor equipartition with other scenarios without FFC. Finally, we investigate how FFC may influences r-process nucleosynthesis associated with QCD phase-transition-driven supernova explosions.

The quantum kinetic evolution of neutrinos in dense environments, such as the core-collapse supernovae or the neutron star mergers, can result in fast flavor conversion (FFC), presenting a significant challenge to achieving robust astrophysical modeling of these systems. Recent works that directly simulate the quantum kinetic transport of neutrinos in localized domains have suggested that the asymptotic outcome of FFCs can be modeled by simple analytical prescriptions when coarse grained over a size much larger than the FFC length scale. In this Letter, by leveraging such a scale separation, we incorporate the analytical prescriptions into global simulations that solve the classical neutrino transport equation including collisions and advection under spherical symmetry. We demonstrate that taking this approach allows to obtain results that quantitatively agree with those directly from the corresponding global quantum kinetic simulations and precisely capture the collisional feedback effect for cases where the FFC happens inside the neutrinosphere. Notably, the effective scheme does not require resolving the FFC time and length scales, hence only adds negligible computational overhead to classical transport. Our work highlights that efficient and robust integration of FFCs in classical neutrino transport used in astrophysical simulation can be feasible.

Neutrinos experience collective flavor conversion in extreme astrophysical environments such as corecollapse supernovae (CCSNe). One manifestation of collective conversion is slow flavor conversion (SFC), which has recently attracted renewed interest owing to its ubiquity across different regions of the supernova environment. In this study, we systematically examine the evolution of kinematic decoherence in a dense neutrino gas undergoing SFC, considering lepton number asymmetries as large as 30%. Our findings show that the neutrino gas asymptotically evolves toward a generic state of coarse-grained flavor equilibration which is constrained by approximate lepton number conservation. The equilibration occurs within a few factors of the inverse vacuum oscillation frequency, omega(-1), which corresponds to (anti)neutrinos reaching near flavor equipartition after a few kilometers for typical supernova neutrino energies. Notably, the quasisteady state of the neutrino number densities can be quantitatively described by the neutrinoantineutrino number density ratio n(nu e)/n(nu e) alone. Such a simple estimation opens new opportunities for incorporating SFC into CCSN simulations, particularly in regions where SFC develops on scales much shorter than those of collisions.

Accurate neutrino transport is crucial for reliably modeling explosive astrophysical events like core-collapse supernovae (CCSNe) and neutron star mergers (NSMs). However, in these extremely neutrino-dense systems, flavor oscillations exhibit challenging nonlinear effects rooted in neutrino–neutrino forward scattering. Evidence is quickly accumulating that these collective phenomena can substantially affect explosion dynamics, neutrino and gravitational-wave signals, nucleosynthesis, and kilonova light curves. We review the progress made so far on the difficult and conceptually deep question of how to correctly include this physics in simulations of CCSNe and NSMs. Our aim is to take a broad view of where the problem stands and provide a critical assessment of where it is headed.

We study how a spatially varying matter potential λ, arising from neutrino-electron forward scattering, affects the onset, evolution, and nonlinear outcome of fast neutrino flavor conversions (FFCs) triggered by the presence of zero crossings in the angular distribution of the neutrino electron lepton number (ELN). We find that increasing the spatial variation rate of λ can strongly influence FFC dynamics and even stabilize systems that are otherwise unstable. Using stability analysis based solely on initial conditions, we identify for the first time a critical variation rate above which no FFC occurs even if the flavor instability exists. Below this critical rate, a substantial λ variation delays the onset of FFCs and quickly generates small-scale, incoherent features in the nonlinear regime, which leads to a similar coarse-grained outcome that eliminates the ELN crossing as in the homogeneous case. Our findings emphasize the need to consider matter inhomogeneity in improved supernova models accounting for FFCs, and we propose simple analytical ways to incorporate this effect.

Fast flavor conversions (FFCs) of neutrinos, which can occur in core -collapse supernovae (CCSNe), are multiangle effects. They depend on the angular distribution of the neutrino 's electron lepton number (ELN). In this work, we present a comprehensive study of the FFCs by solving the multienergy and multiangle quantum kinetic equations with an extended set of collisional weak processes based on a static and spherically symmetric CCSN matter background profile. We investigate the emergence and evolution of FFCs in models featuring different ELN angular distributions, considering scenarios with two and three neutrino flavors. The spectrogram method is utilized to illustrate the small-scale spatial structure, and we show that this structure of neutrino flavor coherence and number densities in the nonlinear regime is qualitatively consistent with the dispersion relation analysis. On the coarse -grained level, we find that different asymptotic states can be achieved following the FFCs depending on the locations and shapes of the ELN distributions, despite sharing a common feature of the elimination of the ELN angular crossing. While equilibration among different neutrino flavors may be achieved immediately after the prompt FFCs, it is not a general outcome of the asymptotic state, as subsequent feedback effects from collisional neutrinomatter interactions come into play, particularly for cases where FFCs occur inside the neutrinosphere. The impacts of FFCs and the feedback effect on the net neutrino heating rates, the equilibrium electron fraction of CCSN matter, and the free -streaming neutrino energy spectra are quantitatively assessed. Other aspects including the impact of the vacuum term and the coexistence with other type of flavor instabilities are also discussed.

Neutrinos are known to play important roles in many astrophysical scenarios from the early period of the big bang to current stellar evolution being a unique messenger of the fusion reactions occurring in the center of our sun. In particular, neutrinos are crucial in determining the dynamics and the composition evolution in explosive events such as core-collapse supernovae and the merger of two neutron stars. In this paper, we review the current understanding of supernovae and binary neutron star mergers by focusing on the role of neutrinos therein. Several recent improvements on the theoretical modeling of neutrino interaction rates in nuclear matter as well as their impact on the heavy element nucleosynthesis in the supernova neutrino-driven wind are discussed, including the neutrino-nucleon opacity at the mean field level taking into account the relativistic kinematics of nucleons, the effect due to the nucleon-nucleon correlation, and the nucleon-nucleon bremsstrahlung. We also review the framework used to compute the neutrino-nucleus interactions and the up-to-date yield prediction for isotopes from neutrino nucleosynthesis occurring in the outer envelope of the supernova progenitor star during the explosion. Here improved predictions of energy spectra of supernova neutrinos of all flavors have had significant impact on the nucleosynthesis yields. Rapid progresses in modeling the flavor oscillations of neutrinos in these environments, including several novel mechanisms for collective neutrino oscillations and their potential impacts on various nucleosynthesis processes are summarized.

In the most extreme astrophysical environments, such as core-collapse supernovae (CCSNe) and neutron star mergers (NSMs), neutrinos can undergo fast flavor conversions (FFCs) on exceedingly short scales. Intensive simulations have demonstrated that FFCs can attain equilibrium states in certain models. In this study, we utilize physics-informed neural networks (PINNs) to predict the asymptotic outcomes of FFCs, by specifically targeting the first two moments of neutrino angular distributions. This makes our approach suitable for state-of-the-art CCSN and NSM simulations. Through effective feature engineering and the incorporation of customized loss functions that penalize discrepancies in the predicted total number of $\nu_e$ and $\bar\nu_e$, our PINNs demonstrate remarkable accuracies, with an error margin of $\lesssim3\%$. Our study represents a substantial leap forward in the potential incorporation of FFCs into simulations of CCSNe and NSMs, thereby enhancing our understanding of these extraordinary astrophysical events.

Neutrinos can undergo fast flavor conversions (FFCs) within extremely dense astrophysical environments, such as core-collapse supernovae (CCSNe) and neutron star mergers (NSMs). In this study, we explore FFCs in a multienergy neutrino gas, revealing that when the FFC growth rate significantly exceeds that of the vacuum Hamiltonian, all neutrinos (regardless of energy) share a common survival probability dictated by the energy-integrated neutrino spectrum. We then employ physics-informed neural networks (PINNs) to predict the asymptotic outcomes of FFCs within such a multienergy neutrino gas. These predictions are based on the first two moments of neutrino angular distributions for each energy bin, typically available in state -of -the -art CCSN and NSM simulations. Our PINNs achieve errors as low as less than or similar to 6% and less than or similar to 18% for predicting the number of neutrinos in the electron channel and the relative absolute error in the neutrino moments, respectively.

It has been recently proposed that the boosted dark matter (BDM) by supernova neutrinos (SNν) from SN1987a or from the next Galactic supernova (SN) can serve as a novel component to probe nonvanishing interaction between dark matter (DM) and the standard model leptons [Y.-H. Lin , and Y.-H. Lin , ]. In this Letter, we extend this concept and evaluate the present-day flux of SNν BDM originated from all galaxies at higher redshifts. We show that by considering this diffuse BDM (DBDM) component, the best sensitivity on the product of the energy-independent DM-ν and DM-electron cross sections, σχνσχe≃O(10−37) cm2 for sub-MeV DM, can be obtained with large-size neutrino experiments such as Super-Kamiokande or Hyper-Kamiokande, surpassing the estimated SNν BDM bound from SN1987a. We also examine the impact due to the presence of DM spikes around the supermassive black holes in galaxies on SNν BDM and DBDM. Our results suggest that both the DBDM and the SNν BDM probes are robust to the uncertain properties of DM spikes, unless the next Galactic SN happens to occur at a location extremely close to or right behind the Galactic Center along the SN line of sight. Published by the American Physical Society 2024

We compute the static density and spin structure factors in the long wavelength limit for pure neutron matter at subsaturation densities relevant to core-collapse supernovae within the Brueckner-Hartree-Fock (BHF) approach. The BHF results are reliable at high densities, extending beyond the validity of the virial expansion. Motivated by the similarities between the dilute neutron gas and a unitary gas, we propose a phenomenological approach to derive the static structures with finite momentum transfer as well as the dynamic ones with simple analytical expressions, based on the computed static structures in the long wavelength limit. We also compare the in-medium neutrino-neutron scattering cross sections using different structure factors. Our study emphasizes the importance of accurately computing the static structure factors theoretically and utilizing the full dynamic structure factors in core-collapse supernova simulations.