Simple method for determining asymptotic states of fast neutrino-flavor conversion

Neutrino-neutrino forward scatterings potentially induce collective neutrino oscillation in dense neutrino gases in astrophysical sites such as core-collapse supernovae (CCSN) and binary neutron star mergers (BNSM). In this paper, we present a detailed study of fast neutrino-flavor conversion (FFC), paying special attention to asymptotic states, by linear stability analysis and local simulations with a periodic boundary condition. We find that asymptotic states can be characterized by two key properties of FFC: (1) the conservation of lepton number for each flavor of neutrinos and (2) the disappearance of ELN(electron neutrino-lepton number)-XLN(heavy-leptonic one) angular crossings in the spatial- or time-averaged distributions. The system which initially has the positive (negative) ELN-XLN density reaches a flavor equipartition in the negative (positive) ELN-XLN angular directions, and the other part compensates it to preserve the conservation laws. These properties of FFCs offer an approximate scheme determining the survival probability of neutrinos in asymptotic states without solving quantum kinetic equations. We also demonstrate that the total amount of flavor conversion can vary with species-dependent neutrino distributions for identical ELN-XLN ones. Our results suggest that even shallow or narrow ELN angular crossings have the ability to drive large flavor conversion, exhibiting the need for including the effects of FFCs in the modeling of CCSN and BNSM.Comment: 11 pages, 6 figures, Accepted to PR

Connecting small-scale to large-scale structures of fast neutrino-flavor conversion

We present a systematic study of fast neutrino-flavor conversion (FFC) with both small-scale and large-scale numerical simulations in spherical symmetry. We find that FFCs can, in general, reach a quasi-steady state, and these features in the non-linear phase are not characterized by the growth rate of FFC instability but rather angular structures of electron neutrino lepton number (ELN) and heavy one (XLN). Our result suggests that neutrinos can almost reach a flavor equipartition even in cases with low growth rate of instability (e.g., shallow ELN crossing) and narrow angular regions (in momentum space) where flavor conversions occur vigorously. This exhibits that ELN and XLN angular distributions can not provide a sufficient information to determine total amount of flavor conversion in neutrinos and antineutrinos of all flavors. Based on the results of our numerical simulations, we provide a new approximate scheme of FFC that is designed so that one can easily incorporate effects of FFCs in existing classical neutrino transport codes for the study of core-collapse supernova (CCSN) and binary neutron star merger (BNSM). The scheme has an ability to capture key features of quasi-steady state of FFCs without solving quantum kinetic neutrino transport, which will serve to facilitate access to FFCs for CCSN and BNSM theorists.Comment: Corrected typos, 22 pages, 18 figure

Characterizing quasi-steady states of fast neutrino-flavor conversion by stability and conservation laws

The question of what ingredients characterize the quasi-steady state of fast neutrino-flavor conversion (FFC) is one of the long-standing riddles in neutrino oscillation. Addressing this issue is necessary for accurate modeling of neutrino transport in core-collapse supernova and binary neutron star merger. Recent numerical simulations of FFC have shown, however, that the quasi-steady state is sensitively dependent on boundary conditions in space, and the physical reason for the dependence is not clear at present. In this study, we provide a physical interpretation of this issue based on arguments with stability and conservation laws. The stability can be determined by the disappearance of ELN(electron neutrino-lepton number)-XLN(heavy-leptonic one) angular crossings, and we also highlight two conserved quantities characterizing the quasi-steady state of FFC: (1) lepton number conservation along each neutrino trajectory and (2) conservation law associated with angular moments, depending on boundary conditions, for each flavor of neutrinos. We demonstrate that neutrino distributions in quasi-steady states can be determined in an analytic way regardless of boundary conditions, which are in good agreement with numerical simulations. This study represents a major step forward a unified picture determining asymptotic states of FFCs.Comment: 10 pages, 5 figures, submitted to PR

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.Comment: 22 pages, 11 figure

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 ($\nu_e$) and heavy-leptonic neutrinos ($\nu_x$). As a result, the number density of $\nu_x$ 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 $\nu_x$ in the high-energy region convert into $\nu_e$ via flavor conversions and then they are absorbed through charged current reactions, exhibiting the possibility of new path of heating matters.Comment: 18 pages, 17 figures, submitted to PR

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).Comment: Accepted in PR

Reexamination of purely physical separation of the phase enriched in noble gases from the Allende meteorite

We have prepared a new sample of the "floating fraction" of the Allende meteorite and have analyzed the elemental and isotopic compositions of the noble gases to examine the properties of the separation technique. The "floating fraction" is the black fine material, which floats on the water during freeze-thaw disaggregation. The floating fraction separated in this study was enriched in noble gases, and its isotopic compositions were very similar to those of chemical residues. We thus confirmed that the physical separation is an effective method to obtain a noble-gas-rich fraction than the chemical treatment. The only major difference between our floating fraction and those previously prepared was that the former had small excesses in ^Ar and ^Xe which were supposed to be due to the contamination from small amounts of silicate like sodalite. There were some variations in the Xe-Q/Xe-HL ratios among floating fractions, suggesting that Q-and HL-components might be separable by a physical process

Imaging the Schwarzschild-radius-scale Structure of M87 with the Event Horizon Telescope Using Sparse Modeling

We propose a new imaging technique for radio and optical/infrared interferometry. The proposed technique reconstructs the image from the visibility amplitude and closure phase, which are standard data products of short-millimeter very long baseline interferometers such as the Event Horizon Telescope (EHT) and optical/infrared interferometers, by utilizing two regularization functions: the ℓ_1-norm and total variation (TV) of the brightness distribution. In the proposed method, optimal regularization parameters, which represent the sparseness and effective spatial resolution of the image, are derived from data themselves using cross-validation (CV). As an application of this technique, we present simulated observations of M87 with the EHT based on four physically motivated models. We confirm that ℓ_1 + TV regularization can achieve an optimal resolution of ~20%–30% of the diffraction limit λ/D_(max), which is the nominal spatial resolution of a radio interferometer. With the proposed technique, the EHT can robustly and reasonably achieve super-resolution sufficient to clearly resolve the black hole shadow. These results make it promising for the EHT to provide an unprecedented view of the event-horizon-scale structure in the vicinity of the supermassive black hole in M87 and also the Galactic center Sgr A*