Resonant Spin-Flavor Conversion of Supernova Neutrinos: Dependence on Electron Mole Fraction

Detailed dependence of resonant spin-flavor (RSF) conversion of supernova neutrinos on electron mole fraction Ye is investigated. Supernova explosion forms a hot-bubble and neutrino-driven wind region of which electron mole fraction exceeds 0.5 in several seconds after the core collapse. When a higher resonance of the RSF conversion is located in the innermost region, flavor change of the neutrinos strongly depends on the sign of 1-2Ye. At an adiabatic high RSF resonance the flavor conversion of bar{nu}_e -> nu_{mu,tau} occurs in Ye 0.5 and inverted mass hierarchy. In other cases of Ye values and mass hierarchies, the conversion of nu_e -> bar{nu}_{mu,tau} occurs. The final bar{nu}_e spectrum is evaluated in the cases of Ye 0.5 taking account of the RSF conversion. Based on the obtained result, time variation of the event number ratios of low bar{nu}_e energy to high bar{nu}_e energy is discussed. In normal mass hierarchy, an enhancement of the event ratio should be seen in the period when the electron fraction in the innermost region exceeds 0.5. In inverted mass hierarchy, on the other hand, a dip of the event ratio should be observed. Therefore, the time variation of the event number ratio is useful to investigate the effect of the RSF conversion.Comment: 16 pages, 33 figures, accepted for publication in Physical Review

Neutrino oscillation and expected event rate of supernova neutrinos in adiabatic explosion model

We study how the influence of the shock wave appears in neutrino oscillations and the neutrino spectrum using density profile of adiabatic explosion model of a core-collapse supernova which is calculated in an implicit Lagrangian code for general relativistic spherical hydrodynamics. We calculate expected event rates of neutrino detection at SK and SNO for various theta_{13} values and both normal and inverted hierarchies. The predicted event rates of bar{nu}_e and nu_e depend on the mixing angle theta_{13} for the inverted and normal hierarchies, respectively, and the influence of the shock appears for about 2 - 8 s when sin^2 2 theta_{13} is larger than 10^{-3}. These neutrino signals for the shock propagation is decreased by < 30 % for bar{nu}_e in inverted (SK) or by < 15 % for nu_e in normal hierarchy (SNO) compared with the case without shock. The obtained ratio of the total event for high-energy neutrinos (20 MeV < E_{nu} < 60 MeV) to low-energy neutrinos (5 MeV < E_{nu} < 20 MeV) is consistent with the previous studies in schematic semi-analytic or other hydrodynamic models of the shock propagation. The time dependence of the calculated ratio of the event rates of high-energy to low-energy neutrinos is a very useful observable which is sensitive to theta_{13} and hierarchies. Namely, time-dependent ratio shows clearer signal of the shock propagation that exhibits remarkable decrease by at most factor \sim 2 for bar{nu}_e in inverted (SK), whereas it exhibits smaller change by \sim 10 % for nu_e in normal hierarchy (SNO). Observing time-dependent high-energy to low-energy ratio of the neutrino events thus would provide a piece of very useful information to constrain theta_{13} and mass hierarchy, and eventually help understanding the propagation how the shock wave propagates inside the star.Comment: 19 pages, 9 figures, accepted for publication in Physical Review

Multimessengers from Core-Collapse Supernovae: Multidimensionality as a Key to Bridge Theory and Observation

Core-collapse supernovae are dramatic explosions marking the catastrophic end of massive stars. The only means to get direct information about the supernova engine is from observations of neutrinos emitted by the forming neutron star, and through gravitational waves which are produced when the hydrodynamic flow or the neutrino flux is not perfectly spherically symmetric. The multidimensionality of the supernova engine, which breaks the sphericity of the central core such as convection, rotation, magnetic fields, and hydrodynamic instabilities of the supernova shock, is attracting great attention as the most important ingredient to understand the long-veiled explosion mechanism. Based on our recent work, we summarize properties of gravitational waves, neutrinos, and explosive nucleosynthesis obtained in a series of our multidimensional hydrodynamic simulations and discuss how the mystery of the central engines can be unraveled by deciphering these multimessengers produced under the thick veils of massive stars

Neutrino oscillations in magnetically driven supernova explosions

We investigate neutrino oscillations from core-collapse supernovae that produce magnetohydrodynamic (MHD) explosions. By calculating numerically the flavor conversion of neutrinos in the highly non-spherical envelope, we study how the explosion anisotropy has impacts on the emergent neutrino spectra through the Mikheyev-Smirnov-Wolfenstein effect. In the case of the inverted mass hierarchy with a relatively large theta_(13), we show that survival probabilities of electron type neutrinos and antineutrinos seen from the rotational axis of the MHD supernovae (i.e., polar direction), can be significantly different from those along the equatorial direction. The event numbers of electron type antineutrinos observed from the polar direction are predicted to show steepest decrease, reflecting the passage of the magneto-driven shock to the so-called high-resonance regions. Furthermore we point out that such a shock effect, depending on the original neutrino spectra, appears also for the low-resonance regions, which leads to a noticeable decrease in the electron type neutrino signals. This reflects a unique nature of the magnetic explosion featuring a very early shock-arrival to the resonance regions, which is in sharp contrast to the neutrino-driven delayed supernova models. Our results suggest that the two features in the electron type antineutrinos and neutrinos signals, if visible to the Super-Kamiokande for a Galactic supernova, could mark an observational signature of the magnetically driven explosions, presumably linked to the formation of magnetars and/or long-duration gamma-ray bursts.Comment: 25 pages, 21 figures, JCAP in pres

Improved Supernova Model and Neutrino Oscillation

　There are a lot of unresolved problems concerning to the mechanism of core-collapsed supernova explosions and supernova neutrinos. Though the explosions succeed in nature, the shock wave stalls and optical supernovae do not occur in most numerical supernova simulations. How the core-collapsed supernovae explode is one of the biggest mysteries in astrophysics. Since 99 % of the gravitational energy of the collapsed core is released as neutrinos, it is expected that the neutrinos are important keys to solve how supernova explosions succeed. The neutrino oscillation was discovered in various neutrino experiments, for example Super Kamiokande (SK) and Sudbury Neutrino Observatory(SNO). However, it is still very diffcult to determine three neutrino oscillation parameters of the mass difference between 1-3 mass eigenstates, &Delta;m132, the 1-3 mixing angle, &theta;13, and the CP violating phase, &delta;. It is one of the most important research topics of particle physics, nuclear physics and astrophysics to determine these parameter values.　　The purpose of this thesis is to constrain the neutrino oscillation parameters theoretically from the supernova neutrinos by studying the neutrino matter effect on the neutrino oscillation which is called the MSW (Mikheyev-Sminov-Wolfenstein) effect. We examine the influence of the shock wave on the neutrinos in the MSW effect, and the influence on the dependence of direction in 2-Dimensional model. The supernova neutrinos are generated in the supernova core and propagate through the envelope. It is pointed out that shock wave propagation has strong influences on the supernova neutrino oscillation through changing density profile.　We studied in this thesis how the influence of the shock wave appears in the neutrino spectrum using density profile of adiabatic explosion model of a core-collapse supernova which is calculated in an implicit Lagrangian code for general relativistic spherical hydrodynamics. We found that the influence of the shock wave appears from low-energy side and moves toward high-energy side according to the shock propagation. In addition, we found that this manner of the neutrino signal depends remarkably on the neutrino oscillation parameters.　We calculated the expected event rate of neutrino detection at Super-Kamiokande. The time evolution of the event rate was calculated for various &theta;13 values. The observed event rate of anti-electron type neutrino depends on the mixing angle &theta; 13 in the case of the inverted mass hierarchy, while the event rate of electron type neutrino depends on &theta; 13in the case of the normal mass hierarchy. When sin2 2&theta; 13 is larger than 1×10-3, the influence of the shock wave appears after 3 seconds in the observation of neutrinos. Therefore, observing the time evolution of the event rate would constrain the mixing parameter &theta; 13, and eventually helps understanding the propagation of the shock wave inside the star if sin 2 2 &theta; 13 is larger than 1×10-3.　We studied how non-spherical symmetric supernovae (Type-Ic supernovae) affect the neutrino spectrum. In order to clarify the difference of the neutrino spectrum which depends on the direction, we calculated the neutrino spectra in two typical directions of the equator and the pole, and compared them with each other. Moreover, we predicted the event rates of the supernova neutrinos to be observed in the Super-Kamiokande by assuming a supernova at the center of Milky Way. We found that the survival probabilities and neutrino spectra are different from one another depending on the direction from the axis for asymmetric Type-Ic supernova explosion. The event rate of the polar direction decreases when the shock wave is propagating the H-resonance (～103g/cm 3). If we obtain the inclination from axis of the supernova by optical observation, we can find the asymmetric diverse of core explosion from the neutrino observation, and the explosion mechanism in detail.　From these theoretical studies it is expected to obtain detailed information of supernova neutrinos in future supernova events. If information of the mass hierarchy and mixing angle are understood, we can examine by using the neutrino where the shock wave in the star is. In addition, if the non-spherical supernova explodes and the angle from the polar axis of the supernova will be determined from the optical observation, we can examine by using event rate how the asymmetry of the core explosion is. Moreover, it would provide many feed back on the construction of theoretical models of supernovae to tie up with clarification of explosion mechanism