15 research outputs found
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
() and heavy-leptonic neutrinos (). As a result, the number
density of 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
in the high-energy region convert into 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
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 (), electron fraction
(), and the degeneracy of electron-type neutrinos () need to
be ,
, and , 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 phenomenologicallyComment: accepted in PR
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*
Time-dependent, quasi-steady, and global features of fast neutrino-flavor conversion
Despite the theoretical indication that fast neutrino-flavor conversion (FFC)
ubiquitously occurs in core-collapse supernova and binary neutron star merger,
the lack of global simulations has been the greatest obstacle to study their
astrophysical consequences. In this Letter, we present the first global
simulation of FFC in spherical symmetry by using a novel approach, in which the
injected number of neutrinos into simulation box is systematically changed, and
then we explore general characteristics of FFC in global scale. We find that
FFC in all models achieves quasi-steady state in the non-linear regime, and its
saturation property of FFC is universal. We also find that temporal- and
spatial variations of FFC are smeared out at large radii due to phase
cancellation through neutrino self-interactions. Finally, we provide a new
diagnostic quantity, ELN-XLN angular crossing, to assess the non-linear
saturation of FFC.Comment: 6 pages, 3 figures, updates reference