arXiv subjects

Chinami Kato

Publications and source records attributed to Chinami Kato.

15 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.

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arXiv2026

Comprehensive neutrino light curves and spectra: from pre-supernova evolution to early supernova phase

We present the first systematic study of neutrino emissions from massive stars, continuously tracking the late evolutionary stages through the early core-collapse supernova phase. Using progenitor and supernova models, we analyze the neutrino luminosities and spectra for progenitors with initial masses of 10--40~$M_\odot$. Our systematic analysis reveals that the compactness parameter ($\xi_{2.5}$) and carbon-oxygen core mass ($M_{\text{CO}}$) exhibit strong correlations with neutrino emission. In the pre-supernova phase, the time-integrated number of neutrinos correlates with $\xi_{2.5}$ when integrated over the final day and with $M_{\text{CO}}$ for longer durations. For the early supernova phase ($<200$ ms post-bounce), the neutrino properties are relatively insensitive to the specific stellar evolution code used, allowing for a reliable extraction of physical correlations. We confirm that the neutrino emission features, including the electron neutrino burst properties and accretion-powered luminosity of other species, reflect the progenitor's compactness. An evaluation of the observational feasibility for a nearby progenitor using a False Alarm Rate approach suggests that these correlations can persist even under practical detection conditions. Such a joint analysis of both phases provides complementary constraints on the internal structure. All calculated time-series data will be made publicly available.

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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.

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arXiv2024

Collisional flavor swap with neutrino self-interactions

Neutrinos play pivotal roles in determining fluid dynamics, nucleosynthesis, and their observables in core-collapse supernova (CCSN) and binary neutron star merger (BNSM). In this paper, we present a novel phenomenon, collisional flavor swap, in which neutrino-matter interactions trigger the complete interchange of neutrino spectra between two different flavors, aided by neutrino self-interactions. We find that a necessary condition to trigger the collisional swap is occurrences of resonance-like collisional flavor instability. In cases where neutrino self-interactions substantially dominate over the collision rate, the collisional swap occurs in the entire neutrino energy spectrum, while intriguing energy dependent features can emerge after the completion of flavor swap. Since flavor swaps correspond to the most extreme case in flavor conversions, they have a great potential to affect CCSN and BNSM phenomena.

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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.

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arXiv2022

Effects of energy-dependent scatterings on fast neutrino flavor conversions

Neutrino self-interactions in a dense neutrino gas can induce collective neutrino flavor conversions. Fast neutrino flavor conversions (FFCs), one of the collective neutrino conversion modes, potentially change the dynamics and observables in core-collapse supernovae and binary neutron star mergers. In cases without neutrino-matter interactions (or collisions), FFCs are essentially energy-independent, and therefore the single energy treatment has been used in previous studies. However, neutrino-matter collisions in general depend on neutrino energy, suggesting that energy-dependent features may emerge in FFCs with collisions. In this paper, we perform dynamical simulations of FFCs with iso-energetic scatterings (emulating nucleon scatterings) under multi-energy treatment. We find that cancellation between in- and out-scatterings happens in high energy region, which effectively reduces the number of collisions and then affects the FFC dynamics. In fact, the lifetime of FFCs is extended compared to the single-energy case, leading to large flavor conversions. Our result suggests that the multi-energy treatment is mandatory to gauge the sensitivity of FFCs to collisions. We also provide a useful quantity to measure the importance of multi-energy effects of collisions on FFCs.

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arXiv2021

Neutrino transport with Monte Carlo method: II. Quantum Kinetic Equations

Neutrinos have an unique quantum feature as flavor conversions. Recent studies suggested that collective neutrino oscillations play important roles in high-energy astrophysical phenomena. Quantum kinetic equation (QKE) is capable of describing the neutrino flavor conversion, transport and matter collision self-consistently. However, we have experienced many technical difficulties in their numerical implementation. In this paper, we present a new QKE solver based on Monte Carlo (MC) approach. This is an upgraded version of our classical MC neutrino transport solver; in essence, a flavor degree of freedom including mixing state is added into each MC particle. This extension requires updating numerical treatments of collision terms, in particular for scattering processes. We deal with the technical problem by generating a new MC particle at each scattering event. To reduce statistical noise inherent in MC methods, we develop the effective mean free path method. This suppresses a sudden change of flavor state due to collisions without increasing the number of MC particles. We present a suite of code tests to validate these new modules with comparing to the results reported in previous studies. Our QKE-MC solver is developed with fundamentally different philosophy and design from other deterministic- and mesh methods, suggesting that it will be complementary to others, and potentially provide new insights into physical processes of neutrino dynamics.

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arXiv2020

Theoretical prediction of presupernova neutrinos and their detection

Almost 30 years have passed since the successful detection of supernova neutrinos from SN 1987A. In the last decades, remarkable progress has been made in neutrino detection technique, through which it may be possible to detect neutrinos from a new source, pre-supernova (pre-SN) neutrinos. They are emitted from a massive star prior to core bounce. Because neutrinos escape from the core freely, they carry information about the stellar physics directly. Pre-SN neutrinos may play an important role in verifying our understanding of stellar evolution for massive stars. Observations of pre-SN neutrinos, moreover, may serve as an alarm regarding a supernova explosion a few days in advance if the progenitor is located in our vicinity, enabling us to observe the next galactic supernova. In this review, we summarize the current status of pre-SN neutrino studies from both of the theoretical and observational points of view.

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arXiv2020

The sensitivity of presupernova neutrinos to stellar evolution models

We examine the sensitivity of neutrino emissions to stellar evolution models for a 15$M_\odot$ progenitor, paying particular attention to a phase prior to the collapse. We demonstrate that the number luminosities in both electron-type neutrinos ($ν_e$) and their anti-partners ($\barν_e$) differ by more than an order of magnitude by changing spatial resolutions and nuclear network sizes on stellar evolution models. We also develop a phenomenological model to capture the essential trend of the diversity, in which neutrino luminosities are expressed as a function of central density, temperature and electron fraction. In the analysis, we show that neutrino luminosity can be well characterized by these central quantities. This analysis also reveals that the most influential quantity to the time evolution of $ν_e$ luminosity is matter density, while it is temperature for $\barν_e$. These qualitative trends will be useful and applicable to constrain the physical state of progenitors at the final stages of stellar evolution from future neutrino observations, although more detailed systematic studies including various mass progenitors are required to assess the applicability.

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arXiv2020

Neutrino transport with Monte Carlo method: I. Towards fully consistent implementation of nucleon recoils in core-collapse supernova simulations

The small energy exchange via nucleon recoils in neutrino-nucleon scattering is now supposed to be one of the important factors for successful explosion of core-collapse supernovae (CCSNe) as they can change neutrino spectra through accumulation of a large number of scatterings. In finite-difference methods employed for neutrino transport in CCSN simulations, we normally can not afford to deploy a large enough number of energy bins needed to resolve this small energy exchange and sub-grid techniques are employed one way or another. In this paper we study quantitatively with the Monte Carlo (MC) method how well such a treatment performs. We first investigate the effects of nucleon recoils on the neutrino spectra and confirm that the average energy is reduced by $\sim$15% for heavy-lepton neutrinos and by much smaller quantities for other types of neutrinos in a typical post-bounce situation. It is also observed that the nucleon scattering dominates the electron scattering in the thermalization of neutrino spectra in all flavors. We then study possible artifacts that the coarse energy grid may produce in the finite-difference methods. In order to mimic the latter calculation, we re-distribute MC particles in each energy bin after a certain interval in a couple of ways and study how the results are affected and depend on the energy-resolution. We also discuss possible implications of our results for the finite-difference methods.

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arXiv2019

Fast-pairwise collective neutrino oscillations associated with asymmetric neutrino emissions in core-collapse supernova

We present a linear stability analysis of the fast-pairwise neutrino flavor conversion based on a result of our latest axisymmetric core-collapse supernova (CCSN) simulation with full Boltzmann neutrino transport. In the CCSN simulation, coherent asymmetric neutrino emissions of electron-type neutrinos ($ν_{\rm e}$) and their anti-particles ($\barν_{\rm e}$), in which the asymmetry of $ν_{\rm e}$ and $\barν_{\rm e}$ is anti-correlated with each other, occur at almost the same time as the onset of aspherical shock expansion. We find that the asymmetric neutrino emissions play a crucial role on occurrences of fast flavor conversions. The linear analysis shows that unstable modes appear in both pre- and post-shock flows; for the latter they appear only in the hemisphere of higher $\barν_{\rm e}$ emissions (the same hemisphere with stronger shock expansion). We analyze in depth the characteristics of electron-lepton-number (ELN) crossing by closely inspecting the angular distributions of neutrinos in momentum space. The ELN crossing happens in various ways, and the property depends on the radius: in the vicinity of neutron star, $\barν_{\rm e}$ ($ν_{\rm e}$) dominates over $ν_{\rm e}$ ($\barν_{\rm e}$) in the forward (backward) direction: at the larger radius the ELN crossing occurs in the opposite way. We also find that the non-radial ELN crossing occurs at the boundary between no ELN crossing and the radial one, which is an effect of genuine multi-D transport. Our findings indicate that the collective neutrino oscillation may occur more commonly in CCSNe and suggest that the CCSN community needs to accommodate these oscillations self-consistently in the modelling of CCSNe.

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arXiv2019

A new possibility of the fast neutrino-flavor conversion in the pre-shock region of core-collapse supernova

We make a strong case that the fast neutrino-flavor conversion, one of the collective flavor oscillation modes, commonly occurs in core-collapse supernovae (CCSNe). It is confirmed in the numerical data obtained in realistic simulations of CCSNe but the argument is much more generic and applicable universally: the coherent neutrino-nucleus scattering makes the electron lepton number (ELN) change signs at some inward direction and trigger the flavor conversion in the outward direction in the pre-shock region. Although the ELN crossing is tiny and that is why it has eluded recognition so far, it is still large enough to induce the flavor conversion. Our findings will have an important observational consequences for CCSNe neutrinos.

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arXiv2017

Neutrino emissions in all flavors up to the pre-bounce of massive stars and the possibility of their detections

This paper is a sequel to our previous one (Kato et al.2015), which calculated the luminosities and spectra of electron-type anti-neutrinos ($\barν_e$'s) from the progenitors of core-collapse supernovae. Expecting that a capability to detect electron-type neutrinos ($ν_e$'s) will increase dramatically with the emergence of liquid-argon detectors such as DUNE, we broaden the scope in this study to include all-flavors of neutrinos emitted from the pre-bounce phase. We pick up three progenitor models of an electron capture supernova (ECSN) and iron-core collapse supernovae (FeCCSNe). We find that the number luminosities reach $\sim10^{57} \mathrm{s^{-1}}$ and $\sim10^{53} \mathrm{s^{-1}}$ at maximum for $ν_e$ and $\barν_e$, respectively. We also estimate the numbers of detection events at terrestrial neutrino detectors including DUNE, taking flavor oscillations into account and assuming the distance to the progenitors to be 200 pc. It is demonstrated that $\barν_e$'s from the ECSN-progenitor will be undetected at almost all detectors, whereas we will be able to observe $\gtrsim$15900 $ν_e$'s at DUNE for the inverted mass hierarchy. From the FeCCSN-progenitors, the number of $\barν_e$ events will be largest for JUNO, 200-900 $\barν_e$'s, depending on the mass hierarchy whereas the number of $ν_e$ events at DUNE is $\gtrsim$2100 for the inverted mass hierarchy. These results imply that the detection of $\barν_e$'s is useful to distinguish FeCCSN- from ECSN-progenitors, while $ν_e$'s will provide us with detailed information on the collapse phase regardless of the type and mass of progenitor.

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arXiv2017

Dependence of weak interaction rates on the nuclear composition during stellar core collapse

We investigate the influences of the nuclear composition on the weak interaction rates of heavy nuclei during the core collapse of massive stars. The nuclear abundances in nuclear statistical equilibrium (NSE) are calculated by some equation of state (EOS) models including in-medium effects on nuclear masses. We systematically examine the sensitivities of electron capture and neutrino-nucleus scattering on heavy nuclei to the nuclear shell effects and the single nucleus approximation. We find that the washout of shell effects at high temperatures brings significant change to weak rates by smoothing the nuclear abundance distribution: the electron capture rate decreases by $\sim$20$\%$ in the early phase and increases by $\sim$40$\%$ in the late phase at most, while the cross section for neutrino-nucleus scattering is reduced by $\sim$15$\%$. This is because the open-shell nuclei become abundant instead of those with closed neutron shells as the shell effects disappear. We also find that the single-nucleus description based on the average values leads to underestimations of weak rates. Electron captures and neutrino coherent scattering on heavy nuclei are reduced by $\sim$80$\%$ in the early phase and by $\sim$5$\%$ in the late phase, respectively. These results indicate that NSE like EOS accounting for shell washout is indispensable for the reliable estimation of weak interaction rates in simulations of core-collapse supernovae.

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arXiv2015

Pre-supernova neutrino emissions from ONe cores in the progenitors of core-collapse supernovae: are they distinguishable from those of Fe cores?

Aiming to distinguish two types of progenitors of core collapse supernovae, i.e., one with a core composed mainly of oxygen and neon (abbreviated as ONe core) and the other with an iron core (or Fe core), we calculated the luminosities and spectra of neutrinos emitted from these cores prior to gravitational collapse, taking neutrino oscillation into account. We found that the total energies emitted as $\barν_e$ from the ONe core are $\lesssim 10^{46}\ {\rm erg}$, which is much smaller than $\sim 10^{47}\ {\rm erg}$ for Fe cores. The average energy, on the other hand, is twice as large for the ONe core as those for the Fe cores. The neutrinos produced by the plasmon decays in the ONe core are more numerous than those from the electron-positron annihilation in both cores but they have much lower average energies $\lesssim 1\ {\rm MeV}$. Although it is difficult to detect the pre-supernova neutrinos from the ONe core even if it is located within 200$\ $pc from the earth, we expect $ \sim 9 - 43$ and $\sim 7 - 61$ events for Fe cores at KamLAND and Super-Kamiokande, respectively, depending on the progenitor mass and neutrino-mass hierarchy. These numbers might be increased by an order of magnitude if we envisage next-generation detectors such as JUNO. We will hence be able to distinguish the two types of progenitors by the detection or non-detection of the pre-supernova neutrinos if they are close enough ($\lesssim1\ {\rm kpc}$).

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