arXiv subjects

Masamichi Zaizen

Publications and source records attributed to Masamichi Zaizen.

At least 19 recordsLinked to original sources

arXiv2026

Dynamic Competition of Fast and Collisional Neutrino Flavor Instabilities with Collisional Damping in Spatially Inhomogeneous Systems

Neutrino flavor evolution in dense astrophysical environments such as core-collapse supernova (CCSN) is influenced by collective effects. While the Fast Flavor Instability (FFI) and the Collisional Flavor Instability (CFI) are recognized as key drivers of rapid flavor conversion, their non-linear competition with collisional damping in spatially inhomogeneous systems remains poorly understood. Motivated by recent findings that FFI and resonance-like CFI co-occur in the post-bounce phase in CCSN, we scrutinize their dynamic competitions and asymptotic states. To this end, we perform numerical simulations of the quantum kinetic neutrino transport, incorporating both spatial advection and the collision terms. We demonstrate that the interplay between these coexisting neutrino flavor instabilities and collisions leads to rich dynamics. Rather than merely inducing simple decoherence, collisional damping can substantially alter the overall dynamics of collective flavor oscillations, driving the system through complex evolutionary pathways. In all cases where flavor instability develops, we find that the system converges to the same flavor-equilibrated asymptotic state, despite the diversity of intermediate dynamics. Our results suggest that realistic collisional effects drive the system to an asymptotic state distinct from the one predicted by the collisionless FFI picture. This highlights the importance of incorporating collisional effects when modeling the asymptotic outcome of flavor conversion in CCSN models.

astro-ph.HE↗

arXiv2026

Fast Neutrino-Flavor Conversion with Attenuation and Global Lepton Gradient

Fast neutrino-flavor conversion (FFC) can nontrivially alter neutrino radiation field in core-collapase supernovae (CCSN) and binary neutron-star merger (BNSM) remnants. However, its interplay with global geometry remains poorly understood because microscopic flavor conversion scales are much shorter than global transport scales. We perform global quantum kinetic neutrino transport simulations in spherical geometry with neutrino and matter backgrounds, using an attenuated oscillation Hamiltonian. We find that steep radial lepton gradients can suppress FFC, whereas the suppression is highly sensitive to the adopted attenuation parameter. This behavior is explained by an adiabatic condition: flavor coherence can grow sufficiently only while the flavor wave remains on the unstable branch in the local dispersion relation during propagation. Background variation shifts the unstable branch, while attenuation lengthens the growth timescale, making the flavor coherence following more difficult. We provide an approximate formula for the adiabaticity that can be used directly in CCSN and BNSM models developed by classical neutrino transport simulations. Our results show that attenuation artificially leads to an overestimation of the impact of background variation and should therefore be applied with caution in global simulations of neutrino flavor conversion.

astro-ph.HE↗

arXiv2025

Neutrino Constraints on Black Hole Formation in M31

We investigate neutrino signals associated with black hole formation resulting from the gravitational collapse of massive stars, motivated by the candidate failed supernova M31-2014-DS1 in the Andromeda Galaxy (M31). By compiling numerical simulation results for stellar collapse, we predict the expected neutrino emission and compare these predictions with observational limits from Super-Kamiokande (SK). The simulations reveal a characteristic precursor signal consisting of a short, intense burst whose average neutrino energy rises rapidly and then ceases abruptly once the black hole forms. We examine several nuclear equations of state, specifically the Lattimer \& Swesty, Shen, Togashi, and SFHo models, to evaluate how the emission depends on neutron-star properties and nuclear-physics uncertainties. Comparison of the predicted event counts with SK's non-detection of neutrinos coincident with M31-2014-DS1 already rules out part of the model space and highlights the sensitivity of current neutrino detectors to both progenitor mass and the EOS. These findings demonstrate the capability of neutrino astronomy to probe core collapse and black hole formation in failed supernova scenarios.

astro-ph.HE↗

arXiv2025

Dynamical equilibria of fast neutrino flavor conversion

Dense neutrino systems, which display collectivity mediated by the weak interaction, have deep parallels with mean-field kinetic systems governed by other fundamental forces. We identify analogues in fast flavor conversion (FFC) of some time-honored nonlinear phenomena in plasmas and self-gravitating systems. We focus in particular on nonlinear Landau damping and collisionless equilibria, which are likely important pieces of the unsolved puzzle of neutrino oscillations in core-collapse supernovae and neutron star mergers. Our analysis additionally reveals the previously unexplored phenomenon of flavor-wave synchronization.

astro-ph.HE↗

arXiv2025

Observing Supernova Neutrino Light Curves with Super-Kamiokande.VI. A Practical Data Analysis Technique Considering Realistic Experimental Backgrounds

Neutrinos from supernovae, especially those emitted during the late phase of core collapse, are essential for understanding the final stages of massive star evolution. We have been dedicated to developing methods for the analysis of neutrinos emitted during the late phase and observed at Super-Kamiokande (SK). Our previous studies have successfully demonstrated the potential of various analysis methods in extracting essential physical properties; however, the lack of background consideration has limited their practical application. In this study, we address this issue by incorporating a realistic treatment of the experimental signal and background events with the on-going SK experiment. We therefore optimize our analysis framework to reflect realistic observational conditions, including both signal and background events. Using this framework we study several long-time supernova models, simulating the late phase neutrino observation in SK and focusing in particular on the identification of the last observed event. We discuss the possibility of model discrimination methods using timing information from this last observed event.

astro-ph.HE↗

arXiv2025

Spectral diversity in collisional neutrino-flavor conversion: flavor equipartition or swap

Quantum kinetics of neutrinos are known to potentially change the classical neutrino radiation field in high-energy astrophysical sources such as core-collapse supernovae and binary neutron-star mergers. However, the mixing phenomena still have open issues in the nonlinear dynamics and the asymptotic states, particularly for recently discovered collision-induced flavor conversion. In this paper, we investigate linear and nonlinear dynamics of collisional neutrino-flavor conversion (CFC) with multi-energy neutrino gases through numerical simulations, demonstrating that the asymptotic states dramatically change depending on unstable modes dominating the system. In one unstable mode, high-energy neutrinos reach a flavor equipartition, but low-energy neutrinos return back to almost their initial states. In contrast, in the other one, rather low-energy neutrinos achieve a full flavor swap, but high-energy neutrinos undergo less flavor conversion. We clarify the distinct spectral behaviors in two different ways based on stability analysis and flavor pendulum. Our result suggests that CFC with flavor swap can become crucial at deeper radii with low electron fraction and requires more detailed theoretical modeling of neutrino quantum kinetics.

hep-ph↗

arXiv2025

Asymptotic states of fast neutrino-flavor conversions in the three-flavor framework

There has been growing evidence that mu and tau neutrinos are noticeably different due to the emergence of muons in core-collapse supernovae (CCSNe) and binary neutron star mergers (BNSMs). Recent theoretical studies also suggest that all flavors of neutrinos and antineutrinos inevitably experience some flavor mixing instabilities including fast neutrino flavor conversions (FFC), which corresponds to one of the collective neutrino oscillations powered by neutrino self-interactions. This represents a need for quantum kinetic treatment in the numerical modeling of neutrino dynamics, which is, however, a formidable computational challenge. In this paper, we present an approximate method to predict asymptotic states of FFC without solving a quantum kinetic equation under a three-flavor framework, in which mu and tau neutrino distributions are not necessarily identical to each other. The approximate method is developed based on a Bhatnagar-Gross-Krook (BGK) relaxation time prescription, capable of capturing essential features of mixing competitions among three different flavor-coherent states. Our proposed scheme is computationally inexpensive and easy to implement in any classical neutrino transport scheme.

astro-ph.HE↗

arXiv2025

Resolution requirements for numerical modeling of neutrino quantum kinetics

Neutrino quantum kinetics is a rapidly evolving field in computational astrophysics, with a primary focus on collective neutrino oscillations in core-collapse supernovae and post-merger phases of binary neutron star mergers. In recent years, there has been considerable debate concerning resolution dependence in numerical simulations. In this paper, we conduct a comprehensive resolution study in both angular- and spatial directions by using two independent schemes of quantum kinetic neutrino transport: finite volume and pseudospectral methods. We complement our discussion by linear stability analysis including inhomogeneous modes. Our result suggests that decreasing spatial resolutions underestimates the growth of flavor instability, and then leads to wrong asymptotic states of flavor conversions, which potentially has a critical impact on astrophysical consequences. We further delve into numerical results of low resolution simulations, that reveals the underlying mechanism responsible for numerical artifacts caused by insufficient resolutions. This study settles the debate on requirements of resolutions and serves as a guideline for numerical modeling of quantum kinetic neutrino transport.

astro-ph.HE↗

arXiv2024

Quasi-steady evolution of fast neutrino-flavor conversions

In astrophysical environments such as core-collapse supernovae (CCSNe) and binary neutron star mergers (BNSMs), neutrinos potentially experience substantial flavor mixing due to the refractive effects of neutrino self-interactions. Determining the survival probability of neutrinos in asymptotic states is paramount to incorporating flavor conversions' effects in the theoretical modeling of CCSN and BNSM. Some phenomenological schemes have shown good performance in approximating asymptotic states of fast neutrino-flavor conversions (FFCs), known as one of the collective neutrino oscillation modes induced by neutrino self-interactions. However, a recent study showed that they would yield qualitatively different asymptotic states of FFC if the neutrino number is forced to evolve. It is not yet fully understood why the canonical phenomenological models fail to predict asymptotic states. In this paper, we perform detailed investigations through numerical simulations and then provide an intuitive explanation with a quasi-homogeneous analysis. Based on the analysis, we propose a new phenomenological model, in which the quasi-steady evolution of FFCs is analytically determined. The model also allows us to express the convolution term of spatial wave number as a concise form, which corresponds to useful information on analyses for the non-linear feedback from small-scale flavor conversions to large-scale ones. Our model yields excellent agreement with numerical simulations, which lends support to our interpretation.

astro-ph.HE↗

arXiv2024

Inspecting neutrino flavor instabilities during proto-neutron star cooling phase in supernova: I. Spherically symmetric model

In the standard model of core-collapse supernova (CCSN), all neutrinos are assumed to be in pure flavor eigenstates in CCSN cores, but the assumption becomes invalid if neutrino distributions are unstable to flavor conversions. In this paper, we present a study of the occurrences of two representative neutrino-flavor instabilities, fast- and collisional flavor instabilities, in the cooling phase of proto-neutron star (PNS) from 1- to 50 seconds. We follow the long-term evolution of a PNS under spherically symmetric and quasi-static approximations, in which the matter profile is determined by solving the Tolman-Oppenheimer-Volkoff equation with neutrino feedback under the treatment of multi-group flux limited diffusion. For the stability analysis of neutrino flavor conversions, we recompute neutrino distributions using Monte Carlo transport in order to obtain the full angular distribution needed to compute the dispersion relations. We find no signs of flavor conversions in our models; the physical reason is thoroughly investigated. We also argue that the negative conclusion in flavor conversions could be changed qualitatively if multi-dimensional effects are included, as similar to cases in the earlier phase of CCSN.

astro-ph.HE↗

arXiv2024

Muon-induced collisional flavor instability in core-collapse supernova

Neutrinos are known to undergo flavor conversion among their three flavors. In the theoretical modeling of core-collapse supernova (CCSN), there has been a great deal of attention to recent discoveries of a new type of neutrino flavor conversions, namely collisional flavor instability (CFI), in which the instability is induced by the flavor-dependent decoherence due to the disparity of neutrino-matter interactions among flavors. In this paper, we study how the appearance of on-shell muons and associated neutrino-matter interactions can impact CFIs based on linear stability analysis of flavor conversions. Some striking results emerge from the present study. First, we analytically show that breaking beta- and pair equilibrium is a necessary condition to trigger CFIs. This also indicates that CFIs with on-shell muons could appear in $e τ$ and $μτ$ neutrino mixing sectors in very high-density region ($\gtrsim 10^{13} {\rm g/cm^{3}}$), exhibiting a possibility of large impacts of CFIs on CCSN. Second, resonance-like CFIs, having a much higher growth rate than normal CFIs, can be triggered by muons. The resonance point of CFIs is different between $e τ$ and $μτ$ sectors; the former (latter) occurs at $μ_{e (μ)} = μ_{n} - μ_{p}$, where $μ_{i}$ denotes the chemical potential of $i$ constitute ($n$ and $p$ represent neutrons and protons, respectively). Our result suggests that the non-linear evolution of CFI with on-shell muons would induce flavor conversions with the complex interplay among all three different neutrino-mixing sectors.

hep-ph↗

arXiv2024

Fast neutrino-flavor swap in high-energy astrophysical environments

We assert that non-linear features of fast neutrino-flavor conversion (FFC) can be qualitatively different between core-collapse supernovae (CCSNe) and binary neutron star mergers (BNSMs). This argument arises from recent global FFC simulations in BNSM, in which fast flavor swap (FFS) emerges in very narrow spatial regions, whereas neutrinos in CCSN tend to evolve towards flavor equipartition. In this {\it Letter}, we provide the physical mechanism of FFS based on a colliding neutrino beam model. Neutrinos/antineutrinos can undergo FFS when they propagate in ambient neutrino gas that propagates in the opposite direction and also has the opposite sign of ELN-XLN, where ELN and XLN denote electron- and heavy-leptonic neutrino number, respectively. Such environments can be naturally realized in BNSMs, whereas they are unlikely in CCSNe unless the neutrino sphere is strongly deformed aspherically. Our study exhibits the diversity of non-linear dynamics of FFC.

astro-ph.HE↗

arXiv2024

BGK subgrid model for neutrino quantum kinetics

We present a new subgrid model for neutrino quantum kinetics, which is primarily designed to incorporate effects of collective neutrino oscillations into neutrino-radiation-hydrodynamic simulations for core-collapse supernovae and mergers of compact objects. We approximate the neutrino oscillation term in quantum kinetic equation by Bhatnagar-Gross-Krook (BGK) relaxation-time prescription, and the transport equation is directly applicable for classical neutrino transport schemes. The BGK model is motivated by recent theoretical indications that non-linear phases of collective neutrino oscillations settle into quasi-steady structures. We explicitly provide basic equations of the BGK subgrid model for both multi-angle and moment-based neutrino transport to facilitate the implementation of the subgrid model in the existing neutrino transport schemes. We also show the capability of our BGK subgrid model by comparing to fully quantum kinetic simulations for fast neutrino-flavor conversion. We find that the overall properties can be well reproduced in the subgrid model; the error of angular-averaged survival probability of neutrinos is within $\sim 20 \%$. By identifying the source of error, we also discuss perspectives to improve the accuracy of the subgrid model.

astro-ph.HE↗

arXiv2024

Collisional and Fast Neutrino Flavor Instabilities in Two-dimensional Core-collapse Supernova Simulation with Boltzmann Neutrino Transport

We present a comprehensive study on the occurrences of the collisional flavor instability (CFI) and the fast flavor instability (FFI) of neutrinos based on a two-dimensional (2D) core-collapse supernova (CCSN) simulation performed with a Boltzmann radiation hydrodynamics code. We find that CFI occurs in a region with the baryon-mass density of $10^{10}\lesssim ρ\lesssim 10^{12}\,\mathrm{g}\,\mathrm{cm}^{-3}$, which is similar to the previous results in one-dimensional (1D) CCSN models. In contrast to 1D, however, the CFI region varies with time vigorously in the 2D model, whereas it had a quiescent structure in 1D. This is attributed to the fact that the turbulent flows advected from a gain region account for the temporal variations. Another noticeable difference from the 1D models is the appearance of resonance-like CFI where number densities of $ν_e$, $\barν_e$ nearly coincide each other. The CFI growth rate there is enhanced and can reach $\sim10^{-3}\,\mathrm{cm}^{-1}$. As for FFI, on the other hand, it appears in three different regions; (1) the region overlapped with the resonance-like CFI, (2) neutrino decoupling regions where $\barν_e$'s are strongly emitted, and (3) optically thin regions where neutral-current scatterings dominate over charged-current reactions. Although overall properties for FFI are consistent with previous studies, we find that the number of electron-neutrinos lepton number crossing (ELN crossing) temporary becomes multiple, which can be assessed accurately only by multi-angle treatments in neutrino transport. We find that the growth rate of FFI is always higher than CFI if both of them occur, which suggests that the former is dominant for the linear evolution.

astro-ph.HE↗

arXiv2023

Universality of the Neutrino Collisional Flavor Instability in Core Collapse Supernovae

Neutrinos are known to undergo flavor conversion processes among the three flavors. The fast flavor conversion (FFC) has been the central piece of flavor conversions taking place in core-collapse supernovae (CCSNe) due to its shorter timescale to the completion of flavor conversion compared to other types of flavor conversion. Although the ordinary collisions between neutrinos and matter were once thought to decohere neutrinos and thus damp flavor conversions, it was recently realized that they can also induce the flavor conversion. The linear analysis showed that the so-called collisional flavor instability or CFI occurs in the absence of FFC. In this paper, we investigate if CFI takes place in of the post-bounce core of CCSNe, using the results of spherically symmetric Boltzmann simulations of CCSNe for four progenitor models with different masses. We also provide a necessary (but not sufficient) condition of matter properties for the occurrence of CFI in optically thick and semi-transparent regions; baryon mass density ($ρ$), electron fraction ($Y_e$), and the degeneracy of electron-type neutrinos ($η_{ν_e}$) need to be $10^{10} {\rm g/cm^3} \lesssim ρ\lesssim 10^{12} {\rm g/cm^3}$, $Y_e\lesssim 0.4$, and $η_{ν_e} \lesssim 0.5$, respectively. This condition allows us to easily locate the place of possible CFI occurence without detailed stability analyses, which is useful for analyzing CFI in CCSN models phenomenologically

astro-ph.HE↗

arXiv2023

Basic characteristics of neutrino flavor conversions in the post-shock regions of core-collapse supernova

One of the active debates in core-collapse supernova (CCSN) theory is how significantly neutrino flavor conversions induced by neutrino-neutrino self-interactions change the conventional picture of CCSN dynamics. Recent studies have indicated that strong flavor conversions can occur inside neutrino spheres where neutrinos are tightly coupled to matter. These flavor conversions are associated with either collisional instability or fast neutrino-flavor conversion (FFC) or both. The impact of these flavor conversions on CCSN dynamics is, however, still highly uncertain due to the lack of global simulations of quantum kinetic neutrino transport with appropriate microphysical inputs. Given fluid profiles from a recent CCSN model at three different time snapshots in the early post-bounce phase, we perform global quantum kinetic simulations in spherical symmetry with an essential set of microphysics. We find that strong flavor conversions occur in optically thick regions, resulting in a substantial change of neutrino radiation field. The neutrino heating in the gain region is smaller than the case with no flavor conversions, whereas the neutrino cooling in the optically thick region is commonly enhanced. Based on the neutrino data obtained from our multi-angle neutrino transport simulations, we also assess some representative classical closure relations by applying them to diagonal components of density matrix of neutrinos. We find that Eddington tensors can be well approximated by these closure relations except for the region where flavor conversions occur vividly. We also analyze the neutrino signal by carrying out detector simulations for Super-Kamiokande, DUNE, and JUNO. We propose a useful strategy to identify the sign of flavor conversions in neutrino signal, that can be easily implemented in real data analyses of CCSN neutrinos.

astro-ph.HE↗

arXiv2023

A Systematic Study on the Resonance in Collisional Neutrino Flavor Instability

Investigations on the resonance in the collisional flavor instability (CFI) of neutrinos, which were reported recently, are reported. We show that the resonance occurs not only for the isotropy-preserving modes as pointed out in the previous work but also for the isotropy-breaking modes and that it enhances the growth rate of CFI by orders of magnitude. Employing the linear analysis and nonlinear numerical simulations in the two-flavor scheme and under the relaxation approximation for the collision term, we discuss the criterion for the resonance, its effect on the nonlinear evolution as well as the influences of homogeneity-breaking (k \ne 0) perturbations as well as of anisotropy in the background on the resonance. We will also touch the cohabitation of the resonance with the fast flavor conversion (FFC).

hep-ph↗

arXiv2023

Flavor conversions with energy-dependent neutrino emission and absorption

Fast neutrino flavor conversions (FFCs) and collisional flavor instabilities (CFIs) potentially affect the dynamics of core-collapse supernovae (CCSNe) and binary neutron star mergers (BNSMs). Under the assumption of homogeneous neutrinos, we investigate effects of neutrino emission and absorption (EA) by matters through both single and multi-energy numerical simulations with physically motivated setup. In our models, FFCs dominate over CFIs in the early phase, while EA secularly and significantly give impacts on flavor conversions. They facilitate angular swaps, or the full exchange between electron neutrinos ($ν_e$) and heavy-leptonic neutrinos ($ν_x$). As a result, the number density of $ν_x$ becomes more abundant than the case without EA, despite the fact that the isotropization by EA terminates the FFCs earlier. In the later phase, the system approaches new asymptotic states characterized by EA and CFIs, in which rich energy-dependent structures also emerge. Multi-energy effects sustain FFCs and the time evolution of the flavor conversion becomes energy dependent, which are essentially in line with effects of the isoenergetic scattering studied in our previous paper. We also find that $ν_x$ in the high-energy region convert into $ν_e$ via flavor conversions and then they are absorbed through charged current reactions, exhibiting the possibility of new path of heating matters.

astro-ph.HE↗