@unpublished{sneppen2024helium,
  author = {{Sneppen}, Albert and {Just}, Oliver and {Bauswein}, Andreas and {Damgaard}, Rasmus and {Watson}, Darach and {Shingles}, Luke J. and {Collins}, Christine E. and {Sim}, Stuart A. and {Xiong}, Zewei and {Martinez-Pinedo}, Gabriel and {Soultanis}, Theodoros and {Vijayan}, Vimal},
  title = {{Helium as an Indicator of the Neutron-Star Merger Remnant Lifetime and its Potential for Equation of State Constraints}},
  year = {2024},
  month = nov,
  eprint = {2411.03427}
}

The time until black hole formation in a binary neutron-star (NS) merger contains invaluable information about the nuclear equation of state (EoS) but has thus far been difficult to measure. We propose a new way to constrain the merger remnant’s NS lifetime, which is based on the tendency of the NS remnant neutrino-driven winds to enrich the ejected material with helium. Based on the He I λ1083.3 nm line, we show that the feature around 800-1200 nm in AT2017gfo at 4.4 days seems inconsistent with a helium mass fraction of X_\mathrmHe ≳0.05 in the polar ejecta. Recent neutrino-hydrodynamic simulations of merger remnants are only compatible with this limit if the NS remnant collapses within 20-30 ms. Such a short lifetime implies that the total binary mass of GW170817, M_\mathrm\rm tot, lay close to the threshold binary mass for direct gravitational collapse, M_\mathrmthres, for which we estimate M_\mathrmthres≲2.93 M_⊙. This upper bound on M_\mathrmthres yields upper limits on the radii and maximum mass of cold, non-rotating NSs, which rule out simultaneously large values for both quantities. In combination with causality arguments, this result implies a maximum NS mass of M_\mathrmmax\lesssim2.3 M_⊙. The combination of all limits constrains the radii of 1.6 M_⊙NSs to about 12\pm1 km for M_\mathrmmax = 2.0 M_⊙and 11.5\pm1 km for M_\mathrmmax = 2.15 M_⊙. This \sim2 km allowable range then tightens significantly for M_\mathrmmax above \approx2.15 M_⊙. This rules out a significant number of current EoS models. The short NS lifetime also implies that a black-hole torus, not a highly magnetized NS, was the central engine powering the relativistic jet of GRB170817A. Our work motivates future developments... [abridged]

@unpublished{george2024evolution,
  author = {{George}, Manu and {Xiong}, Zewei and {Wu}, Meng-Ru and {Lin}, Chun-Yu},
  title = {{Evolution and the quasistationary state of collective fast neutrino flavor conversion in three dimensions without axisymmetry}},
  year = {2024},
  month = sep,
  eprint = {2409.08833}
}

We investigate in this work the evolution of the collective fast neutrino flavor conversion (FFC) in a three dimensional (3D) cubic box with periodic boundary condition for three different neutrino angular distributions that are axially asymmetric. We find that the system evolves toward a quasistationary state where the angular distribution of the spatially averaged neutrino electron-minus-muon lepton number (ELN) does not contain any crossings. In the quasistationary state, near flavor equilibration is achieved in one angular domain enclosed by the initial ELN angular crossing contour, similar to the conclusion derived based on simplified one dimensional (1D) system with axially symmetric neutrino angular distributions. We have also performed additional simulations in coordinates where the initial first ELN angular moment has only one nonvanishing spatial component by using the original axially asymmetric ELN angular distributions as well as the corresponding axisymmetric ELN distributions, and find interesting similarity between these two sets. Finally, we propose three different analytical prescriptions generalized from earlier 1D models to 3D models, and evaluate their performances in predicting the post-FFC moments. Our findings suggest that further development of effective classical transport model in multidimensions to capture the effect of FFC is promising.

@unpublished{xiong2024robust,
  author = {{Xiong}, Zewei and {Wu}, Meng-Ru and {George}, Manu and {Lin}, Chun-Yu},
  title = {{Robust integration of fast flavor conversions in classical neutrino transport}},
  year = {2024},
  month = mar,
  eprint = {2403.17269}
}

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. In this Letter, we incorporate the analytical prescriptions into global simulations that solve the classical neutrino transport equation including collisions and advection under spherical symmetry. We demonstrate excellent agreement between results obtained using this approach and those directly from the corresponding global quantum kinetic simulations. In particular, this effective method can also precisely capture the collisional feedback effect for cases where the FFC happens inside the neutrinosphere. Our work highlights that a robust integration of FFCs in classical neutrino transport used in astrophysical simulation can be feasible.

@unpublished{gross2023data,
  author = {{Gross}, Axel and {Xiong}, Zewei and {Qian}, Yong-Zhong},
  title = {{A Data-Driven Model for Abundances in Metal-poor Stars and Implications for Nucleosynthetic Sources}},
  year = {2023},
  month = sep,
  eprint = {2309.09385}
}

We present a data-driven model for abundances of Fe, Sr, Ba, and Eu in metal-poor (MP) stars. The production patterns for core-collapse supernovae (CCSNe) and binary neutron star mergers (BNSMs) are derived from the data of Holmbeck et al. (arXiv:2007.00749) on [Sr/Fe], [Ba/Fe], and [Eu/Fe] for 195 stars. Nearly all the data can be accounted for by mixtures of contributions from these two sources. We find that on average, the Sr contribution to an MP star from BNSMs is ≈3 times that from CCSNe. Our model is also consistent with the solar inventory of Fe, Sr, Ba, and Eu. We carry out a parametric  r-process study to explore the conditions that can give rise to our inferred production patterns and find that such conditions are largely consistent with those from simulations of CCSNe and BNSMs. Our model can be greatly enhanced by accurate abundances of many r-process elements in a large number of MP stars, and future results from this approach can be used to probe the conditions in CCSNe and BNSMs in much more detail.

@article{fernandez2024viscous,
  author = {Fern{\'{a}}ndez, Rodrigo and Just, Oliver and Xiong, Zewei and Mart{\'{i}}nez-Pinedo, Gabriel},
  title = {{Viscous hydrodynamic evolution of neutron star merger accretion disks: a code comparison}},
  journal = {Phys. Rev. D},
  volume = {110},
  number = {2},
  pages = {023001},
  year = {2024},
  month = jul,
  doi = {10.1103/PhysRevD.110.023001},
  eprint = {2307.02554},
  file = y
}

The accretion disk formed after a neutron star merger is an important contributor to the total ejecta from the merger, and hence to the kilonova and the r-process yields of each event. Axisymmetric viscous hydrodynamic simulations of these disks can capture thermal mass ejection due to neutrino absorption and in the advective phase – after neutrino cooling has subsided – and are thus likely to provide a lower-limit to the total disk ejecta relative to MHD evolution. Here we present a comparison between two viscous hydrodynamic codes that have been used extensively on this problem over the past decade: ALCAR and FLASH. We choose a representative setup with a black hole at the center, and vary the treatment of viscosity and neutrino transport. We find good overall agreement (∼10% level) in most quantities. The average outflow velocity is sensitive to the treatment of the nuclear binding energy of heavy nuclei, showing a larger variation than other quantities. We post-process trajectories from both codes with the same nuclear network, and explore the effects of code differences on nucleosynthesis yields, heating rates, and kilonova light curves. For the latter, we also assess the effect of varying the number of tracer particles in reconstructing the spatial abundance distribution for kilonova light curve production.

@article{xiong2024fast,
  author = {{Xiong}, Zewei and {Wu}, Meng-Ru and {George}, Manu and {Lin}, Chun-Yu and {Khosravi Largani}, Noshad and {Fischer}, Tobias and {Mart{\'\i}nez-Pinedo}, Gabriel},
  title = {{Fast neutrino flavor conversions in a supernova: emergence, evolution, and effects}},
  journal = {Phys. Rev. D},
  volume = {109},
  number = {12},
  pages = {123008},
  year = {2024},
  month = jun,
  doi = {10.1103/PhysRevD.109.123008},
  eprint = {2402.19252},
  file = y
}

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 neutrino-matter 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.

@article{xiong2024production,
  author = {{Xiong}, Zewei and {Mart{\'\i}nez-Pinedo}, Gabriel and {Just}, Oliver and {Sieverding}, Andre},
  title = {{Production of p Nuclei from r -Process Seeds: The {\ensuremath{\nu}} r Process}},
  journal = {Phys. Rev. Lett.},
  volume = {132},
  number = {19},
  pages = {192701},
  year = {2024},
  month = may,
  doi = {10.1103/PhysRevLett.132.192701},
  eprint = {2305.11050},
  file = y
}

We present a new nucleosynthesis process that may take place on neutron-rich ejecta experiencing an intensive neutrino flux. The nucleosynthesis proceeds similarly to the standard r process, a sequence of neutron captures and beta decays with, however, charged-current neutrino absorption reactions on nuclei operating much faster than beta decays. Once neutron-capture reactions freeze out the produced r process, neutron-rich nuclei undergo a fast conversion of neutrons into protons and are pushed even beyond the β stability line, producing the neutron-deficient p nuclei. This scenario, which we denote as the νr process, provides an alternative channel for the production of p nuclei and the short-lived nucleus 92Nb. We discuss the necessary conditions posed on the astrophysical site for the νr process to be realized in nature. While these conditions are not fulfilled by current neutrino-hydrodynamic models of r-process sites, future models, including more complex physics and a larger variety of outflow conditions, may achieve the necessary conditions in some regions of the ejecta.

@article{abbar2024application,
  author = {{Abbar}, Sajad and {Wu}, Meng-Ru and {Xiong}, Zewei},
  title = {{Application of neural networks for the reconstruction of supernova neutrino energy spectra following fast neutrino flavor conversions}},
  journal = {Phys. Rev. D},
  volume = {109},
  number = {8},
  pages = {083019},
  year = {2024},
  month = apr,
  doi = {10.1103/PhysRevD.109.083019},
  eprint = {2401.17424},
  file = y
}

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 ≲6 % and ≲18 % for predicting the number of neutrinos in the electron channel and the relative absolute error in the neutrino moments, respectively.

@article{collins2024towards,
  author = {{Collins}, Christine E. and {Shingles}, Luke J. and {Bauswein}, Andreas and {Sim}, Stuart A. and {Soultanis}, Theodoros and {Vijayan}, Vimal and {Fl{\"o}rs}, Andreas and {Just}, Oliver and {Leck}, Gerrit and {Lioutas}, Georgios and {Mart{\'\i}nez-Pinedo}, Gabriel and {Sneppen}, Albert and {Watson}, Darach and {Xiong}, Zewei},
  title = {{Towards inferring the geometry of kilonovae}},
  journal = {Mon. Not. R. Astron. Soc.},
  volume = {529},
  number = {2},
  pages = {1333-1346},
  year = {2024},
  month = apr,
  doi = {10.1093/mnras/stae571},
  eprint = {2309.05579},
  file = y
}

Recent analysis of the kilonova, AT2017gfo, has indicated that this event was highly spherical. This may challenge hydrodynamics simulations of binary neutron star mergers, which usually predict a range of asymmetries, and radiative transfer simulations show a strong direction dependence. Here we investigate whether the synthetic spectra from a 3D kilonova simulation of asymmetric ejecta from a hydrodynamical merger simulation can be compatible with the observational constraints, suggesting a high degree of sphericity in AT2017gfo. Specifically, we determine whether fitting a simple P-Cygni line profile model leads to a value for the photospheric velocity that is consistent with the value obtained from the expanding photosphere method. We would infer that our kilonova simulation is highly spherical at early times, when the spectra resemble a blackbody distribution. The two independently inferred photospheric velocities can be very similar, implying a high degree of sphericity, which can be as spherical as inferred for AT2017gfo, demonstrating that the photosphere can appear spherical even for asymmetrical ejecta. The last-interaction velocities of radiation escaping the simulation show a high degree of sphericity, supporting the inferred symmetry of the photosphere. We find that when the synthetic spectra resemble a blackbody, the expanding photosphere method can be used to obtain an accurate luminosity distance (within 4-7 per cent).

@article{abbar2024physics,
  author = {{Abbar}, Sajad and {Wu}, Meng-Ru and {Xiong}, Zewei},
  title = {{Physics-informed neural networks for predicting the asymptotic outcome of fast neutrino flavor conversions}},
  journal = {Phys. Rev. D},
  volume = {109},
  number = {4},
  pages = {043024},
  year = {2024},
  month = feb,
  doi = {10.1103/PhysRevD.109.043024},
  eprint = {2311.15656},
  file = y
}

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 νe and ν¯e, our PINNs demonstrate remarkable accuracies, with an error margin of ≲3 %. 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.

@article{xiong2023collisional,
  author = {{Xiong}, Zewei and {Johns}, Lucas and {Wu}, Meng-Ru and {Duan}, Huaiyu},
  title = {{Collisional flavor instability in dense neutrino gases}},
  journal = {Phys. Rev. D},
  volume = {108},
  number = {8},
  pages = {083002},
  year = {2023},
  month = oct,
  doi = {10.1103/PhysRevD.108.083002},
  eprint = {2212.03750},
  file = y
}

We investigate the collision-induced flavor instability in homogeneous, isotropic, dense neutrino gases in the two-flavor mixing scenario with energy-dependent scattering. We find that the growth rate of such an instability, if it exists, is the negative average of the flavor-decohering collision rates of the neutrinos weighted by the electron lepton number distribution of the neutrinos. This growth rate is independent of the neutrino mass-splitting, the matter density, and the neutrino density, although the initial amplitude of the unstable oscillation mode can be suppressed by a large matter density. Our results suggest that neutrinos are likely to experience collision-induced flavor conversions deep inside a core-collapse supernova even when both the fast and slow collective flavor oscillations are suppressed. However, this phenomenon may not occur in a neutron star merger because the electron antineutrinos have a larger average energy and more abundance than the electron neutrinos in such an environment.

@article{xiong2023evaluating,
  author = {Xiong, Zewei and Wu, Meng-Ru and Abbar, Sajad and Bhattacharyya, Soumya and George, Manu and Lin, Chun-Yu},
  title = {{Evaluating approximate asymptotic distributions for fast neutrino flavor conversions in a periodic 1D box}},
  journal = {Phys. Rev. D},
  volume = {108},
  number = {6},
  pages = {063003},
  year = {2023},
  month = sep,
  doi = {10.1103/PhysRevD.108.063003},
  eprint = {2307.11129},
  file = y
}

The fast flavor conversions (FFCs) of neutrinos generally exist in core-collapse supernovae and binary neutron-star merger remnants and can significantly change the flavor composition and affect the dynamics and nucleosynthesis processes. Several analytical prescriptions were proposed recently to approximately explain or predict the asymptotic outcome of FFCs for systems with different initial or boundary conditions, with the aim for providing better understandings of FFCs and for practical implementation of FFCs in hydrodynamic modeling. In this work, we obtain the asymptotic survival probability distributions of FFCs in a survey over thousands of randomly sampled initial angular distributions by means of numerical simulations in one-dimensional boxes with the periodic boundary condition. We also propose improved prescriptions that guarantee the continuity of the angular distributions after FFCs. Detailed comparisons and evaluation of all these prescriptions with our numerical survey results are performed. The survey dataset is made publicly available to inspire the exploration and design for more effective methods applicable to realistic hydrodynamic simulations.

@article{xiong2023symmetry,
  author = {Xiong, Zewei and Wu, Meng-Ru and Qian, Yong-Zhong},
  title = {{Symmetry and bipolar motion in collective neutrino flavor oscillations}},
  journal = {Phys. Rev. D},
  volume = {108},
  number = {4},
  pages = {043007},
  year = {2023},
  month = aug,
  doi = {10.1103/PhysRevD.108.043007},
  eprint = {2303.05906},
  file = y
}

We identify a geometric symmetry on the two-flavor Bloch sphere for collective flavor oscillations of a homogeneous dense neutrino gas. Based on this symmetry, analytical solutions to the periodic bipolar flavor evolution are derived. Using numerical calculations, we show that for configurations without this symmetry, the flavor evolution displays deviations from the bipolar flavor motion or even exhibits aperiodic patterns. We also discuss the implication of our finding for more general three-flavor and inhomogeneous cases.

@article{just2023end,
  author = {Just, Oliver and Vijayan, Vimal and Xiong, Zewei and Goriely, Stephane and Soultanis, T. and Bauswein, Andreas and Guilet, J{\'{e}}r{\^{o}}me and Janka, H.-Th. and Mart{\'{i}}nez-Pinedo, Gabriel},
  title = {{End-to-end Kilonova Models of Neutron Star Mergers with Delayed Black Hole Formation}},
  journal = {Astrophys. J. Lett.},
  volume = {951},
  number = {1},
  pages = {L12},
  year = {2023},
  month = jul,
  doi = {10.3847/2041-8213/acdad2},
  eprint = {2302.10928},
  file = y
}

We investigate the nucleosynthesis and kilonova properties of binary neutron star (NS) merger models that lead to intermediate remnant lifetimes of ∼0.1–1 s until black hole (BH) formation and describe all components of the material ejected during the dynamical merger phase, NS remnant evolution, and final viscous disintegration of the BH torus after gravitational collapse. To this end, we employ a combination of hydrodynamics, nucleosynthesis, and radiative transfer tools to achieve a consistent end-to-end modeling of the system and its observables. We adopt a novel version of the Shakura–Sunyaev scheme allowing the approximate turbulent viscosity inside the NS remnant to vary independently of the surrounding disk. We find that asymmetric progenitors lead to shorter remnant lifetimes and enhanced ejecta masses, although the viscosity affects the absolute values of these characteristics. The integrated production of lanthanides and heavier elements in such binary systems is subsolar, suggesting that the considered scenarios contribute in a subdominant fashion to r -process enrichment. One reason is that BH tori formed after delayed collapse exhibit less neutron-rich conditions than typically found, and often assumed in previous BH torus models, for early BH formation. The outflows in our models feature strong anisotropy as a result of the lanthanide-poor polar neutrino-driven wind pushing aside lanthanide-rich dynamical ejecta. Considering the complexity of the models, the estimated kilonova light curves show promising agreement with AT 2017gfo after times of several days, while the remaining inconsistencies at early times could possibly be overcome in binary configurations with a more dominant neutrino-driven wind relative to the dynamical ejecta.

@article{xiong2023evolution,
  author = {{Xiong}, Zewei and {Wu}, Meng-Ru and {Mart{\'i}nez-Pinedo}, Gabriel and {Fischer}, Tobias and {George}, Manu and {Lin}, Chun-Yu and {Johns}, Lucas},
  title = {{Evolution of collisional neutrino flavor instabilities in spherically symmetric supernova models}},
  journal = {Phys. Rev. D},
  volume = {107},
  number = {8},
  pages = {083016},
  year = {2023},
  month = apr,
  doi = {10.1103/PhysRevD.107.083016},
  eprint = {2210.08254},
  file = y
}

We implement a multigroup and discrete-ordinate neutrino transport model in spherical symmetry which allows to simulate collective neutrino oscillations by including realistic collisional rates in a self-consistent way. We utilize this innovative model, based on strategic parameter rescaling, to study a recently proposed collisional flavor instability caused by the asymmetry of emission and absorption rates between νe and ν¯ e for four different static backgrounds taken from different stages in a core-collapse supernova simulation. Our results confirm that collisional instabilities generally exist around the neutrinosphere during the supernova accretion and postaccretion phase, as suggested by Johns [arXiv:2104.11369.]. However, the growth and transport of flavor instabilities can only be fully captured by models with global simulations as done in this work. With minimal ingredient to trigger collisional instabilities, we find that the flavor oscillations and transport mainly affect (anti)neutrinos of heavy lepton flavors around their decoupling sphere, which then leave imprints on their energy spectra in the free-streaming regime. For electron (anti)neutrinos, their properties remain nearly intact. We also explore various effects due to the decoherence from neutrino-nucleon scattering, artificially enhanced decoherence from emission and absorption, neutrino vacuum mixing, and inhomogeneous matter profile, and discuss the implication of our work.

@article{george2023cosenu,
  author = {George, Manu and Lin, Chun-Yu and Wu, Meng-Ru and Liu, Tony G and Xiong, Zewei},
  title = {{COSEnu: A Collective Oscillation Simulation Engine for Neutrinos}},
  journal = {Comput. Phys. Commun.},
  volume = {283},
  pages = {108588},
  year = {2023},
  month = feb,
  doi = {10.1016/j.cpc.2022.108588},
  eprint = {2203.12866},
  file = y
}

We introduce the implementation details of the simulation code COSEν, which numerically solves a set of non-linear partial differential equations that govern the dynamics of neutrino collective flavor conversions. We systematically provide the details of both finite difference method supported by Kreiss-Oliger dissipation and finite volume method with seventh order weighted essentially non-oscillatory scheme. To ensure the reliability of the code, we perform comparison of the simulation results with theoretically obtainable solutions. In order to understand and characterize the error accumulation behavior of the implementations when neutrino self-interactions are switched on, we also analyze the evolution of the deviation of the conserved quantities for different values of simulation parameters. We report the performance of our code with both CPUs and GPUs. The public version of the COSEν package is available at https://github.com/COSEnu/COSEnu.

@article{shingles2023self,
  author = {Shingles, Luke J and Collins, Christine E and Vijayan, Vimal and Fl{\"{o}}rs, Andreas and Just, Oliver and Leck, Gerrit and Xiong, Zewei and Bauswein, Andreas and Mart{\'{i}}nez-Pinedo, Gabriel and Sim, Stuart A},
  title = {{Self-consistent 3D radiative transfer for kilonovae: directional spectra from merger simulations}},
  journal = {Astrophys. J. Lett.},
  volume = {954},
  number = {2},
  pages = {L41},
  year = {2023},
  doi = {10.3847/2041-8213/acf29a},
  eprint = {2306.17612},
  file = y
}

We present three-dimensional radiative transfer calculations for the ejecta from a neutron star merger that include line-by-line opacities for tens of millions of bound-bound transitions, composition from an r-process nuclear network, and time-dependent thermalization of decay products from individual }\backslashalpha} and }\backslashbeta\^-} decay reactions. In contrast to expansion opacities and other wavelength-binned treatments, a line-by-line treatment enables us include fluorescence effects and associate spectral features with the emitting and absorbing lines of individual elements. We find variations in the synthetic observables with both the polar and azimuthal viewing angles. The spectra exhibit blended features with strong interactions by Ce III, Sr II, Y II, and Zr II that vary with time and viewing direction. We demonstrate the importance of wavelength-calibration of atomic data using a model with calibrated Sr, Y, and Zr data, and find major differences in the resulting spectra, including a better agreement with AT2017gfo. The synthetic spectra for near-polar inclination show a feature at around 8000 A, similar to AT2017gfo. However, they evolve on a more rapid timescale, likely due to the low ejecta mass (0.005 M}_\backslashodot}) as we take into account only the early ejecta. The comparatively featureless spectra for equatorial observers gives a tentative prediction that future observations of edge-on kilonovae will appear substantially different from AT2017gfo. We also show that 1D models obtained by spherically averaging the 3D ejecta lead to dramatically different direction-integrated luminosities and spectra compared to full 3D calculations.

@article{johns2022collisional,
  author = {Johns, Lucas and Xiong, Zewei},
  title = {{Collisional instabilities of neutrinos and their interplay with fast flavor conversion in compact objects}},
  journal = {Phys. Rev. D},
  volume = {106},
  number = {10},
  pages = {103029},
  year = {2022},
  month = nov,
  doi = {10.1103/PhysRevD.106.103029},
  eprint = {2208.11059},
  file = y
}

Fast and collisional flavor instabilities possibly occur in the neutrino decoupling regions of core-collapse supernovae and neutron-star mergers. To gain a better understanding of the relevant flavor dynamics, we numerically solve for the collisionally unstable evolution in a homogeneous, anisotropic model. In these calculations collisional instability is precipitated by unequal neutrino and antineutrino scattering rates. We compare the solutions obtained using neutral-current and charged-current interactions. We then study the nonlinear development of fast instabilities subjected to asymmetric scattering rates, finding evidence that the onset of collisional instability is hastened by fast oscillations. We also discuss connections to other recent works on collision-affected fast flavor conversion.

@article{roggero2022entanglement,
  author = {Roggero, Alessandro and Rrapaj, Ermal and Xiong, Zewei},
  title = {{Entanglement and correlations in fast collective neutrino flavor oscillations}},
  journal = {Phys. Rev. D},
  volume = {106},
  number = {4},
  pages = {043022},
  year = {2022},
  month = aug,
  doi = {10.1103/PhysRevD.106.043022},
  eprint = {2203.02783},
  file = y
}

Collective neutrino oscillations play a crucial role in transporting lepton flavor in astrophysical settings like supernovae and neutron star binary merger remnants, which are characterized by large neutrino densities. In these settings, simulations in the mean-field approximation show that neutrino-neutrino interactions can overtake vacuum oscillations and give rise to fast collective flavor evolution on timescales t∝μ-1 , with μ proportional to the local neutrino density. In this work, we study the full out-of-equilibrium flavor dynamics in simple multiangle geometries displaying fast oscillations in the mean field linear stability analysis. Focusing on simple initial conditions, we analyze the production of pair correlations and entanglement in the complete many-body-dynamics as a function of the number N of neutrinos in the system, for up to thousands of neutrinos. Similarly to simpler geometries with only two neutrino beams, we identify three regimes: stable configurations with vanishing flavor oscillations, marginally unstable configurations with evolution occurring on long timescales τ ≈μ-1√N , and unstable configurations showing flavor evolution on short timescales τ ≈μ-1log (N ). We present evidence that these fast collective modes are generated by the same dynamical phase transition which leads to the slow bipolar oscillations, establishing a connection between these two phenomena and explaining the difference in their timescales. We conclude by discussing a semiclassical approximation which reproduces the entanglement entropy at short to medium timescales and can be potentially useful in situations with more complicated geometries where classical simulation methods starts to become inefficient.

@article{richers2022code,
  author = {Richers, Sherwood and Duan, Huaiyu and Wu, Meng-Ru and Bhattacharyya, Soumya and Zaizen, Masamichi and George, Manu and Lin, Chun-Yu and Xiong, Zewei},
  title = {{Code comparison for fast flavor instability simulations}},
  journal = {Phys. Rev. D},
  volume = {106},
  number = {4},
  pages = {043011},
  year = {2022},
  month = aug,
  doi = {10.1103/PhysRevD.106.043011},
  eprint = {2205.06282},
  file = y
}

The fast flavor instability (FFI) is expected to be ubiquitous in core-collapse supernovae and neutron star mergers. It rapidly shuffles neutrino flavor in a way that could impact the explosion mechanism, neutrino signals, mass outflows, and nucleosynthesis. The variety of initial conditions and simulation methods employed in simulations of the FFI prevent an apples-to-apples comparison of the results. We simulate a standardized test problem using five independent codes and verify that they are all faithfully simulating the underlying quantum kinetic equations under the assumptions of axial symmetry and homogeneity in two directions. We quantify the amount of numerical error in each method and demonstrate that each method is superior in at least one metric of this error. We make the results publicly available to serve as a benchmark.

@article{xiong2022many,
  author = {{Xiong}, Zewei},
  title = {{Many-body effects of collective neutrino oscillations}},
  journal = {Phys. Rev. D},
  volume = {105},
  number = {10},
  eid = {103002},
  pages = {103002},
  year = {2022},
  month = may,
  doi = {10.1103/PhysRevD.105.103002},
  eprint = {2111.00437},
  file = y
}

Collective neutrino oscillations are critical to determine the neutrino flavor content, which has striking impacts on core-collapse supernovae or compact binary merger remnants. It is a challenging many-body problem that so far has been mainly studied at the mean-field approximation. We use a setup that captures the relevant physics and allows exact solution for a large number of neutrinos. We find that quantitative deviation from the mean-field evolution can exist even for a large system. The underlying mechanism due to many-body decoherence in flavor space is analyzed, and similar features have been observed in a spin-1 Bose-Einstein condensate. Our results call for more careful examinations on the possible many-body corrections to collective neutrino oscillations in astrophysical environments.

@article{wu2021collective,
  author = {{Wu}, Meng-Ru and {George}, Manu and {Lin}, Chun-Yu and {Xiong}, Zewei},
  title = {{Collective fast neutrino flavor conversions in a 1D box: Initial conditions and long-term evolution}},
  journal = {Phys. Rev. D},
  volume = {104},
  number = {10},
  pages = {103003},
  year = {2021},
  month = nov,
  doi = {10.1103/PhysRevD.104.103003},
  eprint = {2108.09886},
  file = y
}

We perform numerical simulations of fast collective neutrino flavor conversions in an one-dimensional box mimicking a system with the periodic boundary condition in one spatial direction and translation symmetry in the other two dimensions. We evolve the system over several thousands of the characteristic timescale (inverse of the interaction strength) with different initial ν¯e to νe number density ratios and different initial seed perturbations. We find that small scale structures are formed due to the interaction of the flavor waves. This results in a nearly flavor depolarization in a certain neutrino phase space, when averaged over the entire box. Specifically, systems with initially equal number of νe and ν¯e can reach full flavor depolarization for the entire neutrino electron lepton number (ν ELN ) angular spectra. For systems with initially unequal νe and ν¯e, flavor depolarization can only be reached in one side of the ν ELN spectra, dictated by the net neutrino e -x lepton number conservation. Quantitatively small differences depending on the initial perturbations are also found when different perturbation seeds are applied. Our numerical study here provides new insights for efforts aiming to include impact of fast flavor conversions in astrophysical simulations while calls for better analytical understanding accounting for the evolution of fast flavor conversions.

@article{xiong2021stationary,
  author = {{Xiong}, Zewei and {Qian}, Yong-Zhong},
  title = {{Stationary solutions for fast flavor oscillations of a homogeneous dense neutrino gas}},
  journal = {Phys. Lett. B},
  volume = {820},
  pages = {136550},
  year = {2021},
  month = sep,
  doi = {10.1016/j.physletb.2021.136550},
  eprint = {2104.05618},
  file = y
}

We present a method to find the stationary solutions for fast flavor oscillations of a homogeneous dense neutrino gas. These solutions correspond to collective precession of all neutrino polarization vectors around a fixed axis in the flavor space on average, and are conveniently studied in the co-rotating frame. We show that these solutions can account for the numerical results of explicit evolution calculations, and that even with the simplest assumption of adiabatic evolution, they can provide the average survival probabilities to good approximation. We also discuss improvement of these solutions and their use as estimates of the effects of fast oscillations in astrophysical environments.

@article{xiong2020potential,
  author = {{Xiong}, Zewei and {Sieverding}, Andre and {Sen}, Manibrata and {Qian}, Yong-Zhong},
  title = {{Potential Impact of Fast Flavor Oscillations on Neutrino-driven Winds and Their Nucleosynthesis}},
  journal = {Astrophys. J.},
  volume = {900},
  number = {2},
  eid = {144},
  pages = {144},
  year = {2020},
  month = sep,
  doi = {10.3847/1538-4357/abac5e},
  eprint = {2006.11414},
  file = y
}

The wind driven by the intense neutrino emission from a protoneutron star (PNS) is an important site for producing nuclei heavier than the Fe group. Because of certain features in the neutrino angular distributions, the so-called fast flavor oscillations may occur very close to the PNS surface, effectively resetting the neutrino luminosities and energy spectra that drive the wind. Using the unoscillated neutrino emission characteristics from two core-collapse supernova simulations representative of relevant progenitors at the lower and higher mass end, we study the potential effects of fast flavor oscillations on neutrino-driven winds and their nucleosynthesis. We find that such oscillations can increase the total mass loss by factors up to ∼1.5-1.7 and lead to significantly more proton-rich conditions. The latter effect can greatly enhance the production of 64Zn and the so-called light p-nuclei 74Se, 78Kr, and 84Sr. Implications for abundances in metal-poor stars, Galactic chemical evolution in general, and isotopic anomalies in meteorites are discussed.

@article{xiong2019active,
  author = {Xiong, Zewei and Wu, Meng-Ru and Qian, Yong-Zhong},
  title = {{Active–Sterile Neutrino Oscillations in Neutrino-driven Winds: Implications for Nucleosynthesis}},
  journal = {Astrophys. J.},
  volume = {880},
  number = {2},
  pages = {81},
  year = {2019},
  month = aug,
  doi = {10.3847/1538-4357/ab2870},
  eprint = {1904.09371},
  file = y
}

A protoneutron star produced in a core-collapse supernova (CCSN) drives a wind by its intense neutrino emission. We implement active-sterile neutrino oscillations in a steady-state model of this neutrino-driven wind to study their effects on the dynamics and nucleosynthesis of the wind in a self-consistent manner. Using vacuum mixing parameters indicated by some experiments for a sterile ν s of ∼1 eV in mass, we observe interesting features of oscillations due to various feedback. For the higher ν s mass values, we find that oscillations can reduce the mass-loss rate and the wind velocity by a factor of ∼1.6-2.7 and change the electron fraction critical to nucleosynthesis by a significant to large amount. In the most dramatic cases, oscillations shift nucleosynthesis from dominant production of 45Sc to that of 86Kr and 90Zr during the early epochs of the CCSN evolution.