Final Report Summary - DENSE NEUTRINOS (Neutrino Oscillations in Dense Medium: Probing Particle Physics together with Astrophysics and Cosmology)
Extensive studies of atmospheric, solar, accelerator and reactor neutrinos have confirmed the idea that neutrinos have mass and and different neutrino flavors mix among them. The focus is now shifted to precision measurements of the neutrino mixing parameters. In the effort to understand these elusive particles, the cosmos has always been a favorite laboratory. While detecting particles from cosmological and astrophysical sources is a severe challenge, a new era has begun as many experiments with the capability to detect neutrinos from astrophysical sources have reached maturity. The increased accuracy of neutrino experiments demands a much deeper theoretical understanding of astrophysical neutrino fluxes. These sources are unique in the sense that all particles and even neutrinos themselves are exceptionally dense. In fact, the feeble neutrino-neutrino interactions can become crucial as the dense neutrino gas becomes a "background medium" to itself. The neutrino flavor evolution becomes nonlinear, resulting in a surprisingly rich and interesting phenomenology. With the project "Dense Neutrinos", we studied the oscillations of dense neutrinos streaming from astrophysical sources, with an emphasis on core collapse supernovae.
During a supernova (SN) explosion, about 99% of the gravitational binding energy of the newborn neutron star is released in the form of neutrinos and anti-neutrinos. Details of the SN neutrino signal may reveal the neutrino mass matrix with its underlying symmetries. In addition, the time structure of the SN neutrino signal will carry peculiar signatures of the SN shock-wave propagation, representing the interface between particle, nuclear and astrophysics. Moreover, close to the SN core up to a few hundred kilometers, the dense neutrino flux itself modifies neutrino propagation. The resulting nonlinear evolution shows counter-intuitive behavior like the neutrinos and anti-neutrinos of different energies collectively oscillating to some other flavor. One complexity of the nonlinear evolution is that the SN neutrinos are not emitted from a point source and that their angular divergence is crucial. Every neutrino mode is described by its energy and direction of motion, introducing "multi-angle effects" that can completely modify the solution. Understanding the multi-angle effects in collective neutrino oscillations is one of the crucial topics of the subject as the features of the emitted neutrinos change when the multi-angle effects are included. Disentangling such effects is an intriguing task and crucial for the interpretation of the neutrino signal of the next nearby SN.
Regarding the "multi-angle effects", the previous studies were mostly based on the spherically symmetric "bulb" models. The actual neutrino fluxes and the angular distributions in a SN deviate from these idealized model. The main focus of this project remained to deviate from the simple "bulb" models and cylindrical symmetry and interpret the multi-angle effects. Also another focus was to study the extremely dense matter (electron) background close to SN core and to probe its effect on the nonlinear evolution.
In this regard, SN models in the absence of azimuthal symmetry were studied to analyze the flavor instabilities and the effect on neutrino conversion in SN. The effect of the multi-energy study in such scenarios were probed. Another such study based on the Basel SN simulations, has shown how the extremely dense matter in the central region of the star and the nonuniform angular distributions of the different neutrino flavors can suppress the growth of the flavor instabilities. During this project, a simulation from the Garching group reported surprising features in their models: the neutrino emission in one hemisphere of the star is found to be strongly dominated by electron flavor neutrinos whereas in the opposite hemisphere the electron flavor anti-neutrinos and neutrinos are found to have have similar fluxes i.e. asymmetry in neutrino (lepton) emission. Studies were taken up for a model motivated by these fascinating 3 dimensional SN simulations. These studies found that flavor conversion even for such asymmetric lepton emission scenarios were suppressed due to the large electron background close to the SN core.
The Dense Neutrinos project studied another important aspect of the "non-linear" flavor conversions i.e. the influence of the assumption of homogeneity in the neutrino evolution. The impact of the inhomogeneity on the flavor instabilities were studied. These studies showed that the instabilities move to larger neutrino densities close to the denser core as the probe goes to larger inhomogeneous modes.
Another study under this project probed how the oscillation effects can influence the neutrino signatures of the SN shock wave. This study found that the matter anisotropy motivated by the recent 2-dimensional SN simulations are not enough to depolarize the flavor hierarchy of the SN neutrinos.
The project was also involved in studies regarding the LAGUNA collaboration. Analysis on the detectability of neutrino mass ordering from SN neutrinos for the proposed future detectors, like the 50 kton liquid Ar detector, the 100 kton liquid scintillator detector and Mton water Cherenkov detector, were taken up. In this regard, another phenomenological study probed the capabilities of the planned ton scale direct dark matter experiments to detect the time structure of the SN neutrinos.
In light of the recent discovery of the IceCube PeV neutrino events, another investigation was made to estimate the high-energy diffuse neutrino background from the stellar remnants in the star burst galaxies. The results showed excellent agreement with the detected IceCube events flux normalization and the spectral shape. The results also pointed out a natural bump in the neutrino spectra below 100 TeV for such models of the star burst galaxies. Future IceCube data should be able to probe this prediction.
During the period the researcher worked extensively with the graduate students in the group and published papers together with them. He got invited for talks in prestigious conferences and in other research institutes.
During a supernova (SN) explosion, about 99% of the gravitational binding energy of the newborn neutron star is released in the form of neutrinos and anti-neutrinos. Details of the SN neutrino signal may reveal the neutrino mass matrix with its underlying symmetries. In addition, the time structure of the SN neutrino signal will carry peculiar signatures of the SN shock-wave propagation, representing the interface between particle, nuclear and astrophysics. Moreover, close to the SN core up to a few hundred kilometers, the dense neutrino flux itself modifies neutrino propagation. The resulting nonlinear evolution shows counter-intuitive behavior like the neutrinos and anti-neutrinos of different energies collectively oscillating to some other flavor. One complexity of the nonlinear evolution is that the SN neutrinos are not emitted from a point source and that their angular divergence is crucial. Every neutrino mode is described by its energy and direction of motion, introducing "multi-angle effects" that can completely modify the solution. Understanding the multi-angle effects in collective neutrino oscillations is one of the crucial topics of the subject as the features of the emitted neutrinos change when the multi-angle effects are included. Disentangling such effects is an intriguing task and crucial for the interpretation of the neutrino signal of the next nearby SN.
Regarding the "multi-angle effects", the previous studies were mostly based on the spherically symmetric "bulb" models. The actual neutrino fluxes and the angular distributions in a SN deviate from these idealized model. The main focus of this project remained to deviate from the simple "bulb" models and cylindrical symmetry and interpret the multi-angle effects. Also another focus was to study the extremely dense matter (electron) background close to SN core and to probe its effect on the nonlinear evolution.
In this regard, SN models in the absence of azimuthal symmetry were studied to analyze the flavor instabilities and the effect on neutrino conversion in SN. The effect of the multi-energy study in such scenarios were probed. Another such study based on the Basel SN simulations, has shown how the extremely dense matter in the central region of the star and the nonuniform angular distributions of the different neutrino flavors can suppress the growth of the flavor instabilities. During this project, a simulation from the Garching group reported surprising features in their models: the neutrino emission in one hemisphere of the star is found to be strongly dominated by electron flavor neutrinos whereas in the opposite hemisphere the electron flavor anti-neutrinos and neutrinos are found to have have similar fluxes i.e. asymmetry in neutrino (lepton) emission. Studies were taken up for a model motivated by these fascinating 3 dimensional SN simulations. These studies found that flavor conversion even for such asymmetric lepton emission scenarios were suppressed due to the large electron background close to the SN core.
The Dense Neutrinos project studied another important aspect of the "non-linear" flavor conversions i.e. the influence of the assumption of homogeneity in the neutrino evolution. The impact of the inhomogeneity on the flavor instabilities were studied. These studies showed that the instabilities move to larger neutrino densities close to the denser core as the probe goes to larger inhomogeneous modes.
Another study under this project probed how the oscillation effects can influence the neutrino signatures of the SN shock wave. This study found that the matter anisotropy motivated by the recent 2-dimensional SN simulations are not enough to depolarize the flavor hierarchy of the SN neutrinos.
The project was also involved in studies regarding the LAGUNA collaboration. Analysis on the detectability of neutrino mass ordering from SN neutrinos for the proposed future detectors, like the 50 kton liquid Ar detector, the 100 kton liquid scintillator detector and Mton water Cherenkov detector, were taken up. In this regard, another phenomenological study probed the capabilities of the planned ton scale direct dark matter experiments to detect the time structure of the SN neutrinos.
In light of the recent discovery of the IceCube PeV neutrino events, another investigation was made to estimate the high-energy diffuse neutrino background from the stellar remnants in the star burst galaxies. The results showed excellent agreement with the detected IceCube events flux normalization and the spectral shape. The results also pointed out a natural bump in the neutrino spectra below 100 TeV for such models of the star burst galaxies. Future IceCube data should be able to probe this prediction.
During the period the researcher worked extensively with the graduate students in the group and published papers together with them. He got invited for talks in prestigious conferences and in other research institutes.