Neutrino-matter Interactions Research Articles

Robustness of neutron star merger simulations to changes in neutrino transport and neutrino-matter interactions

Robustness of neutron star merger simulations to changes in neutrino transport and neutrino-matter interactions

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, 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 eτ and μτ neutrino mixing sectors in very high-density region (≳1013 g/cm3), exhibiting a possibility of large impacts of CFIs on core-collapse supernova. Second, resonancelike CFIs, having a much higher growth rate than normal CFIs, can be triggered by muons. The resonance point of CFIs is different between eτ and μτ sectors; the former (latter) occurs at μe(μ)=μn−μp, where μi denotes the chemical potential of i constitute (n and p represent neutrons and protons, respectively). Our result suggests that the nonlinear evolution of CFI with on-shell muons would induce flavor conversions with the complex interplay among all three different neutrino-mixing sectors. Published by the American Physical Society 2024

Magnetized Outflows from Short-lived Neutron Star Merger Remnants Can Produce a Blue Kilonova

We present a 3D general-relativistic magnetohydrodynamic simulation of a short-lived neutron star remnant formed in the aftermath of a binary neutron star merger. The simulation uses an M1 neutrino transport scheme to track neutrino–matter interactions and is well suited to studying the resulting nucleosynthesis and kilonova emission. A magnetized wind is driven from the remnant and ejects neutron-rich material at a quasi-steady-state rate of 0.8 × 10−1 M ⊙s−1. We find that the ejecta in our simulations underproduce r-process abundances beyond the second r-process peak. For sufficiently long-lived remnants, these outflows alone can produce blue kilonovae, including the blue kilonova component observed for AT2017gfo.

Examining Neutrino–Matter Interactions in the Cassiopeia A Supernova

Neutrino interactions with stellar material are widely believed to be fundamental to the explosion of massive stars. However, this important process has remained difficult to confirm observationally. We propose a new method to verify it using X-ray observations of the supernova remnant Cassiopeia A. The elemental composition in its Fe-rich ejecta that could have been produced at the innermost region of the supernova, where neutrinos are expected to interact, allows us to examine the presence of neutrino interactions. Here we demonstrate that the amount of Mn produced without neutrino nucleosynthesis processes (i.e., the ν- and νp-processes) is too small to explain the Mn/Fe mass ratio we measure (0.14%–0.67%). This result supports the operation of significant neutrino interactions in the Cassiopeia A supernova. If the observed Mn/Fe mass ratio purely reflects the production at the innermost region of the supernova, this would be the first robust confirmation of neutrino–matter interactions in an individual supernova. We further show that the Mn/Fe mass ratio has the potential to constrain supernova neutrino parameters (i.e., total neutrino luminosity, neutrino temperature). Future spatially resolved, high-resolution X-ray spectroscopy will allow us to investigate the details of neutrino–supernova astrophysics through its signatures in elemental composition not only in Cassiopeia A but also in other remnants.

Roles of Fast Neutrino-Flavor Conversion on the Neutrino-Heating Mechanism of Core-Collapse Supernova.

One of the greatest uncertainties in any modeling of the inner engine of core-collapse supernova (CCSN) is neutrino-flavor conversions driven by neutrino self-interactions. We carry out large-scale numerical simulations of a multienergy, multiangle, three-flavor framework and general relativistic quantum kinetic neutrino transport in spherical symmetry with an essential set of neutrino-matter interactions under a realistic fluid profile of CCSN. Our result suggests that the neutrino heating in the gain region is reduced by ∼40% due to fast neutrino-flavor conversion (FFC). We also find that the total luminosity of neutrinos is enhanced by ∼30%, for which the substantial increase of heavy-leptonic neutrinos by FFCs are mainly responsible. This study provides evidence that FFC has a significant impact on the delayed neutrino-heating mechanism.

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 isoenergetic scatterings (emulating nucleon scatterings) under multienergy treatment. We find that cancellation between in- and outscatterings happens in the 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 multienergy treatment is mandatory to gauge the sensitivity of FFCs to collisions. We also provide a useful quantity to measure the importance of multienergy effects of collisions on FFCs.

The essential character of the neutrino mechanism of core-collapse supernova explosions

Abstract Calibrating with detailed 2D core-collapse supernova (CCSN) simulations, we derive a simple CCSN explosion condition based solely upon the terminal density profiles of state-of-the-art stellar evolution calculations of the progenitor massive stars. This condition captures the vast majority of the behaviour of the one hundred 2D state-of-the-art models we performed to gauge its usefulness. The goal is to predict, without resort to detailed simulation, the explodability of a given massive star. We find that the simple maximum fractional ram pressure jump discriminant we define works well ∼90 per cent of the time and we speculate on the origin of the few false positives and false negatives we witness. The maximum ram pressure jump generally occurs at the time of accretion of the silicon/oxygen interface, but not always. Our results depend upon the fidelity with which the current implementation of our code F ornax adheres to Nature and issues concerning the neutrino–matter interaction, the nuclear equation of state, the possible effects of neutrino oscillations, grid resolution, the possible role of rotation and magnetic fields, and the accuracy of the numerical algorithms employed remain to be resolved. Nevertheless, the explodability condition we obtain is simple to implement, shows promise that it might be further generalized while still employing data from only the unstable Chandrasekhar progenitors, and is a more credible and robust simple explosion predictor than can currently be found in the literature.

Prospects of Fast Flavor Neutrino Conversion in Rotating Core-collapse Supernovae

Abstract There is mounting evidence that neutrinos undergo fast flavor conversion (FFC) in core-collapse supernova (CCSN). In this paper, we investigate the roles of stellar rotation on the occurrence of FFC by carrying out axisymmetric CCSN simulations with full Boltzmann neutrino transport. Our result suggests that electron neutrino lepton number (ELN) angular crossings, which are the necessary and sufficient condition to trigger FFC, preferably occur in the equatorial region for rotating CCSNe. By scrutinizing the neutrino–matter interaction and neutrino radiation field, we find some pieces of evidence that the stellar rotation facilitates the occurrence of FFC. The low-electron-fraction region in the post-shock layer expands by centrifugal force, enhancing the disparity of neutrino absorption between electron-type neutrinos (ν e) and their anti-particles ( ν ¯ e ). This has a significant impact on the angular distribution of neutrinos in momentum space, in which ν e tends to be more isotropic than ν ¯ e ; consequently, ELN crossings emerge. The ELN crossing found in this study is clearly associated with rotation, which motivates further investigation on how the subsequent FFC influences explosion dynamics, nucleosynthesis, and neutrino signals in rotating CCSNe.

Supernova Preshock Neutronization Burst as a Probe of Nonstandard Neutrino Interactions

Abstract A core-collapse supernova (CCSN) provides a unique astrophysical site for studying neutrino–matter interactions. Prior to the shock-breakout neutrino burst during the collapse of the iron core, a preshock ν e burst arises from the electron capture of nuclei and it is sensitive to the low-energy coherent elastic neutrino–nucleus scattering (CEνNS) which dominates the neutrino opacity. Since the CEνNS depends strongly on nonstandard neutrino interactions (NSIs), which are completely beyond the standard model and yet to be determined, the detection of the preshock burst thus provides a clean way to extract the NSI information. Within the spherically symmetric general-relativistic hydrodynamic simulation for the CCSN, we investigate the NSI effects on the preshock burst. We find that the NSI can maximally enhance the peak luminosity of the preshock burst almost by a factor of three, reaching a value comparable to that of the shock-breakout burst. Future detection of the preshock burst will have critical implications on astrophysics, neutrino physics, and physics beyond the standard model.

Beyond the MSW effect: Neutrinos in a dense medium

We present a theory of neutrino oscillations in a dense medium which goes beyond the effective matter potential used in the description of the MSW effect. We show how the purity of the neutrino state is degraded by neutrino interactions with the environment and how neutrino–matter interactions can be a source of decoherence. We present new oscillation formulae for neutrinos interacting with leptons and carry out a numerical analysis which exhibits deviations from the MSW formulae for propagation through the Earth of ultra-high energy neutrinos. In particular, we show that at high density and/or high neutrino energy, the vanishing transition probabilities derived for MSW effect, are non zero when the scattering is taken into account.

A DG-IMEX Method for Two-moment Neutrino Transport: Nonlinear Solvers for Neutrino–Matter Coupling*

Abstract Neutrino–matter interactions play an important role in core-collapse supernova (CCSN) explosions, as they contribute to both lepton number and/or four-momentum exchange between neutrinos and matter and thus act as the agent for neutrino-driven explosions. Due to the multiscale nature of neutrino transport in CCSN simulations, an implicit treatment of neutrino–matter interactions is desired, which requires solutions of coupled nonlinear systems in each step of the time integration scheme. In this paper, we design and compare nonlinear iterative solvers for implicit systems with energy-coupling neutrino–matter interactions commonly used in CCSN simulations. Specifically, we consider electron neutrinos and antineutrinos, which interact with static matter configurations through the Bruenn 85 opacity set. The implicit systems arise from the discretization of a nonrelativistic two-moment model for neutrino transport, which employs the discontinuous Galerkin (DG) method for phase-space discretization and an implicit–explicit (IMEX) time integration scheme. In the context of this DG-IMEX scheme, we propose two approaches to formulate the nonlinear systems: a coupled approach and a nested approach. For each approach, the resulting systems are solved with Anderson-accelerated fixed-point iteration and Newton’s method. The performance of these four iterative solvers has been compared on relaxation problems with various degrees of collisionality, as well as proto–neutron star deleptonization problems with several matter profiles adopted from spherically symmetric CCSN simulations. Numerical results suggest that the nested Anderson-accelerated fixed-point solver is more efficient than other tested solvers for solving implicit nonlinear systems with energy-coupling neutrino–matter interactions.

Impact of a Magnetic Field on Neutrino–Matter Interactions in Core-collapse Supernovae

Abstract We explore the impact of a magnetic field on neutrino–matter interactions in core-collapse supernovae. We first derive the modified source terms for neutrino–nucleon scattering and neutrino absorption and emission processes in the moment formalism. Then, we perform full relativistic, three-dimensional, magnetorotational core-collapse supernova simulations of a star with spectral neutrino transport. Our simulations self-consistently treat the parity-violation effects of weak interaction in the presence of an external magnetic field. The result shows significant global asymmetry, mostly confined in the proto-neutron star, clearly reflecting the magnetic field structure. The asymmetric property arises from two factors: the angle between the neutrino flux and magnetic field, and the term that is parallel to the magnetic field and is also proportional to the deviation of the distribution function of neutrinos from thermal equilibrium. The typical correction value amounts to ∼1% relative to the total neutrino–matter interaction rate for the magnetic field strength of G. Although these asymmetric properties do not immediately affect the explosion dynamics, our results imply that they would be significant once the neutrinos diffuse out of the proto-neutron- star core carrying those asymmetries away. We also show that, during our simulation time of ∼370 ms after bounce, our results indicate that the correction value due to the modified inelastic scattering process dominates over that of the modified neutrino absorption and emission process.

Improved neutrino-nucleon interactions in dense and hot matter for numerical simulations

Neutrinos play an important role in compact star astrophysics: neutrino-heating is one of the main ingredients in core-collapse supernovae, neutrino-matter interactions determine the composition of matter in binary neutron star mergers and have among others a strong impact on conditions for heavy element nucleosynthesis and neutron star cooling is dominated by neutrino emission except for very old stars. Many works in the last decades have shown that in dense matter medium effects considerably change the neutrino-matter interaction rates, whereas many astrophysical simulations use analytic approximations which are often far from reproducing more complete calculations. In this work we present a scheme which allows to incorporate improved rates, for charged current interactions, into simulations and show as an example some results for core-collapse supernovae, where a noticeable difference is found in the location of the neutrinospheres of the low-energy neutrinos in the early post-bounce phase.

Complete predictions for high-energy neutrino propagation in matter

We present predictions for the interactions of energetic neutrinos with matter as they propagate through Earth towards large-volume detectors. Our results are based on state-of-the-art calculations of the high-energy neutrino-matter interaction cross-sections, which we have implemented in the HEDIS module of GENIE. In addition to the dominant interaction process, deep inelastic scattering off quarks and gluons, we include the relevant subdominant channels: (in)elastic scattering off the photon field of nucleons, coherent scattering off the photon field of nuclei, as well as the scattering on atomic electrons via the Glashow resonance. Our predictions for the neutrino attenuation rates are provided by a new software package, NuPropEarth. We quantify the dependence of our results on the cross-section model, including nuclear corrections, the incidence angle, and the spectral index, and compare them with other publicly available tools.

Chimera: A Massively Parallel Code for Core-collapse Supernova Simulations

We provide a detailed description of the Chimera code, a code developed to model core collapse supernovae in multiple spatial dimensions. The core collapse supernova explosion mechanism remains the subject of intense research. Progress to date demonstrates that it involves a complex interplay of neutrino production, transport, and interaction in the stellar core, three-dimensional stellar core fluid dynamics and its associated instabilities, nuclear burning, and the foundational physics of the neutrino-stellar core weak interactions and the equations of state of all stellar core constituents -particularly, the nuclear equation of state associated with nucleons, both free and bound in nuclei. Chimera, by incorporating detailed neutrino transport, realistic neutrino-matter interactions, three-dimensional hydrodynamics, realistic nuclear, leptonic, and photonic equations of state, and a nuclear reaction network, along with other refinements, can be used to study the role of neutrino radiation, hydrodynamic instabilities, and a variety of input physics in the explosion mechanism itself. It can also be used to compute observables such as neutrino signatures, gravitational radiation, and the products of nucleosynthesis associated with core collapse supernovae. The code contains modules for neutrino transport, multidimensional compressible hydrodynamics, nuclear reactions, a variety of neutrino interactions, equations of state, and modules to provide data for post-processing observables such as the products of nucleosynthesis, and gravitational radiation. Chimera is an evolving code, being updated periodically with improved input physics and numerical refinements. We detail here the current version of the code, from which future improvements will stem, which can in turn be described as needed in future publications.

Triangulation pointing to core-collapse supernovae with next-generation neutrino detectors

A core-collapse supernova releases the vast majority of the gravitational binding energy of its compact remnant in the form of neutrinos over an interval of a few tens of seconds. In the event of a core-collapse supernova within our galaxy, multiple current and future neutrino detectors would see a large burst in activity. Neutrinos escape a supernova hours before light does, so any prompt information about the supernova's direction that can be inferred via the neutrino signal will help to enable early electromagnetic observations of the supernova. While there are methods to determine the direction via intrinsic directionality of some neutrino-matter interaction channels, a complementary method which will reach maturity with the next generation of large neutrino detectors is the use of relative neutrino arrival times at different detectors around the globe. To evaluate this triangulation method for realistic detector configurations of the next few decades, we generate random supernova neutrino signals with realistic detector assumptions, and quantify the error in expected time delay between detections. We investigate a practical and robust method of estimating the time differences between burst detections, also correcting for detection efficiency bias. With this method, we determine the pointing precision of supernova neutrino triangulation as a function of supernova distance and location, detectors used, detector background level and neutrino mass ordering assumption. Under favorable conditions, the 1$\sigma$ supernova search area from triangulation could be reduced to a few percent of the sky. It should be possible to implement this method with low latency under realistic conditions.

A multidimensional implementation of the Advanced Spectral neutrino Leakage scheme

ABSTRACT We present a new, multidimensional implementation of the Advanced Spectral Leakage (ASL) scheme with the purpose of modelling neutrino–matter interactions in neutron star mergers. A major challenge is the neutrino absorption in the semitransparent regime, which is responsible for driving winds from the merger remnant. The composition of such winds is crucial in the understanding of the electromagnetic emission in the recently observed macronova following GW170817. Compared to the original version, we introduce an optical-depth-dependent flux factor to model the average angle of neutrino propagation, and a modulation that accounts for flux anisotropies in non-spherical geometries. We scrutinize our approach by first comparing the new scheme against the original one for a spherically symmetric core-collapse supernova snapshot, both in 1D and in 3D, and additionally against a two-moment (M1) scheme as implemented in 1D into the code GR1D. The luminosities and mean energies agree to a few per cents in most tests. Finally, for the case of a binary merger remnant snapshot we compare the new ASL scheme with the M1 scheme that is implemented in the Eulerian adaptive mesh refinement code flash. We find that the neutrino absorption distribution in the semitransparent regime is overall well reproduced. Both approaches agree to within $\lesssim 15{{\ \rm per\ cent}}$ for the average energies and to better than $\sim 35 {{\ \rm per\ cent}}$ in the total luminosities.

Neutrino quantum kinetics in compact objects

Neutrinos play a critical role of transporting energy and changing the lepton density within core-collapse supernovae and neutron star mergers. The quantum kinetic equations (QKEs) combine the effects of neutrino-matter interactions treated in classical Boltzmann transport with the neutrino flavor-changing effects treated in neutrino oscillation calculations. We present a method for extending existing neutrino interaction rates to full QKE source terms for use in numerical calculations. We demonstrate the effects of absorption and emission by nucleons and nuclei, electron scattering, electron-positron pair annihilation, nucleon-nucleon bremsstrahlung, neutrino-neutrino scattering. For the first time, we include all these collision terms self-consistently in a simulation of the full isotropic QKEs in conditions relevant to core-collapse supernovae and neutron star mergers. For our choice of parameters, the long-term evolution of the neutrino distribution function proceeds similarly with and without the oscillation term, though with measurable differences. We demonstrate that electron scattering, nucleon-nucleon bremsstrahlung processes, and four-neutrino processes dominate flavor decoherence in the protoneutron star (PNS), absorption dominates near the shock, and all of the considered processes except elastic nucleon scattering are relevant in the decoupling region. Finally, we propose an effective decoherence opacity that at most energies predicts decoherence rates to within a factor of 10 in our model PNS and within 20% outside of the PNS.

Asymmetric nuclear light clusters in supernova matter

We explore the appearance of light clusters at high densities of collapsing stellar cores. Special attention is paid to the unstable isotope H4, which was not included in previous studies. The importance of light clusters in the calculation of rates for neutrino matter interaction is discussed. The main conclusion is that thermodynamic quantities are only weakly sensitive to the chemical composition. The change in pressure and hence the direct change in collapse dynamics will be minor. But the change in neutrino heating and neutronization processes can be significant.

Evaluating radiation transport errors in merger simulations using a Monte Carlo algorithm

Neutrino-matter interactions play an important role in the post-merger evolution of neutron star-neutron star and black hole-neutron star mergers. Most notably, they determine the properties of the bright optical/infrared transients observable after a merger. Unfortunately, Boltzmann's equations of radiation transport remain too costly to be evolved directly in merger simulations. Simulations rely instead on approximate transport algorithms with unquantified modeling errors. In this paper, we use for the first time a time-dependent general relativistic Monte-Carlo (MC) algorithm to solve Boltzmann's equations and estimate important properties of the neutrino distribution function ~10ms after a neutron star merger. We do not fully couple the MC algorithm to the fluid evolution, but use a short evolution of the merger remnant to critically assess errors in our approximate gray two-moment transport scheme. We demonstrate that the analytical closure used by the moment scheme is highly inaccurate in the polar regions, but performs well elsewhere. While the average energy of polar neutrinos is reasonably well captured by the two-moment scheme, estimates for the neutrino energy become less accurate at lower latitudes. The two-moment formalism also overestimates the density of neutrinos in the polar regions by ~50%, and underestimates the neutrino pair-annihilation rate at the poles by factors of 2-3. Although the latter is significantly more accurate than one might have expected before this study, our results indicate that predictions for the properties of polar outflows and for the creation of a baryon-free region at the poles are likely to be affected by errors in the two-moment scheme, thus limiting our ability to reliably model kilonovae and gamma-ray bursts.