Lorenz Hüdepohl

Supernova Neutrinos: Production, Oscillations and Detection

Neutrinos play a crucial role in the collapse and explosion of massive stars, governing the infall dynamics of the stellar core, triggering and fueling the explosion and driving the cooling and deleptonization of the newly formed neutron star. Due to their role neutrinos carry information from the heart of the explosion and, due to their weakly interacting nature, offer the only direct probe of the dynamics and thermodynamics at the center of a supernova. In this paper, we review the present status of modelling the neutrino physics and signal formation in collapsing and exploding stars. We assess the capability of current and planned large underground neutrino detectors to yield faithful information of the time and flavor dependent neutrino signal from a future Galactic supernova. We show how the observable neutrino burst would provide a benchmark for fundamental supernova physics with unprecedented richness of detail. Exploiting the treasure of the measured neutrino events requires a careful discrimination of source-generated properties from signal features that originate on the way to the detector. As for the latter, we discuss self-induced flavor conversions associated with neutrino-neutrino interactions that occur in the deepest stellar regions; matter effects that modify the pattern of flavor conversions in the dynamical stellar envelope; neutrino-oscillation signatures that result from structural features associated with the shock-wave propagation as well as turbulent mass motions in post-shock layers. Finally, we highlight our current understanding of the formation of the diffuse supernova neutrino background and we analyse the perspectives for a detection of this relic signal that integrates the contributions from all past core-collapse supernovae in the Universe.

Suppression of self-induced flavor conversion in the supernova accretion phase

Self-induced flavor conversions of supernova (SN) neutrinos can strongly modify the flavor-dependent fluxes. We perform a linearized flavor stability analysis with accretion-phase matter profiles of a 15M[symbol: see text] spherically symmetric model and corresponding neutrino fluxes. We use realistic energy and angle distributions, the latter deviating strongly from quasi-isotropic emission, thus accounting for both multiangle and multienergy effects. For our matter and neutrino density profile we always find stable conditions: flavor conversions are limited to the usual Mikheyev-Smirnov-Wolfenstein effect. In this case one may distinguish the neutrino mass hierarchy in a SN neutrino signal if the mixing angle θ13 is as large as suggested by recent experiments.

High-resolution supernova neutrino spectra represented by a simple fit

To study the capabilities of supernova neutrino detectors, the instantaneous spectra are often represented by a quasi-thermal distribution of the form f(E) = E^alpha e^{-(alpha+1)E/E_{av}} where E_{av} is the average energy and alpha a numerical parameter. Based on a spherically symmetric supernova model with full Boltzmann neutrino transport we have, at a few representative post-bounce times, re-converged the models with vastly increased energy resolution to test the fit quality. For our examples, the spectra are well represented by such a fit in the sense that the counting rates for a broad range of target nuclei, sensitive to different parts of the spectrum, are reproduced very well. Therefore, the mean energy and root-mean-square energy of numerical spectra hold enough information to provide the correct alpha and to forecast the response of multi-channel supernova neutrino detection.

IMPACT OF NEUTRINO FLAVOR OSCILLATIONS ON THE NEUTRINO-DRIVEN WIND NUCLEOSYNTHESIS OF AN ELECTRON-CAPTURE SUPERNOVA

Neutrino oscillations, especially to light sterile states , can affect the nucleosynthesis yields because of their possible feedback effect on the electron fraction (Ye). For the first time, we perform nucleosynthesis calculations for neutrino-driven wind trajectories from the neu trino-cooling phase of an 8.8 M⊙ electron-capture supernova, whose hydrodynamic evolution was computed in spherical symmetry with sophisticated neutrino transport and whose Ye evolution was post-processed by including neutrino oscillations both between active and active-sterile flavors. We also take into account the α-effect as well as weak magnetism and recoil corrections in the neutrino absorption and emission processes. We observe effects on the Ye evolution which depend in a subtle way on the relative radial positions of the sterile MS W resonances, of collective flavor transformations, and on the formation ofα-particles. For the adopted supernova progenitor, we find th at neutrino oscillations, also to a sterile state with eV-mass, do not significantly a ffect the element formation and in particular cannot make the post-explosion wind outflow neutron rich enough to a ctivate a strong r-process. Our conclusions become even more robust when, in order to mimic equation-of-state dependent corrections due to nucleon potential effects in the dense-medium neutrino opacities, four cases with reduced Ye in the wind are considered. In these cases, despite the conversion of neutrinos to steri le neutrinos, Ye increases compared to the values obtained without oscillations and active flavor transformati ons. This is a consequence of a complicated interplay between sterile-neutrino production, neutrino-neutrino interactions, andα-effect. Subject headings: supernovae: general — nuclear reactions, nucleosynthesis, abundances — neutrinos

Flavor-dependent neutrino angular distribution in core-collapse supernovae

According to recent studies, the collective flavor evolution of neutrinos in core-collapse supernovae depends strongly on the flavor-dependent angular distribution of the local neutrino radiation field, notably on the angular intensity of the electron-lepton number carried by neutrinos. To facilitate further investigations of this subject, we study the energy and angle distributions of the neutrino radiation field computed with the Vertex neutrinotransport code for several spherically symmetric (1D) supernova simulations (of progenitor masses 11.2, 15 and 25 M ) and explain how to extract this information from additional models of the Garching group. Beginning in the decoupling region (“neutrino sphere”), the distributions are more and more forward peaked in the radial direction with an angular spread that is largest for νe, smaller for ν̄e, and smallest for νx, where x = μ or τ. While the energy-integrated νe minus ν̄e angle distribution has a dip in the forward direction, it does not turn negative in any of our investigated cases.

The SuperN-Project: neutrino hydrodynamics simulations of core-collapse supernovae

We give an overview of the challenges and the current status of our two-dimensional (core collapse) supernova modelling, and present the system of equations and the algorithm for its solution that are employed in our code Vertex. We also discuss the parallelisation of Vertex, give scaling results on different architectures, and report on our ongoing efforts to increase the computational efficiency of the code. Furthermore, we outline some of the recent results obtained from simulations performed on the NEC SX-8 at the HLRS. Specifically, we report our findings about the role of general relativity in core-collapse supernovae, about nucleosynthesis conditions in O-Ne-Mg core supernovae, and about the proto-neutron star cooling phase.

The SuperN-Project: an update on core-collapse supernova simulations

We give an overview of the challenges and the current status of our two-dimensional (core collapse) supernova modelling, and present the system of equations and the algorithm for its solution that are employed in our code, and report on our continuing efforts to improve the physics in our supernova code VERTEX as well as its the computational efficiency. We also discuss recent results of simulations performed on the NEC SX-8 at the HLRS, which include the first multi-dimensional general-relativistic neutrino transport simulations—conducted with a new extension of the VERTEX code—as well as simulations of neutron star cooling over several seconds for different nuclear equations of state.