In core collapse supernovae, large numbers of neutrinos are emitted from the protoneutron star after core bounce. At such high neutrino number densities, neutrino-neutrino coherent scatterings cause a non-linear phenomenon called "collective neutrino oscillation" which results in the dramatic flavor transition in neutrino and antineutrino spectra. There was no quantitative study which took into account the influence of collective neutrino oscillations on the vp-process nucleosynthesis consistently. In this work, we study the impact of collective neutrino oscillations on the vp process nucleosynthesis by combining realistic three flavor multiangle simulations with nucleosynthesis network calculations for the first time. We find that the abundances of p-nuclei which are synthesized in the vp process are enhanced by oscillation effects by 10-10⁴ times in normal mass hierarchy. Our results imply the necessity of collective neutrino oscillations for the precise nucleosynthesis in neutrino-driven winds and also help understand the origin of solar-system isotopic abundances of molybdenum 92, 94 and ruthenium 96, 98.

Dwarf galaxies have the fewer number of stars with high r-process abundances than those of the Milky Way halo. Metal mixing processes in the early phase of galaxy formation deeply relate to these difference of the elemental abundances. However, the efficiency of metal mixing in galaxies is not yet understood. Here we performed a series of chemodynamical simulations of dwarf galaxies with turbulence-induced metal mixing model. We found that the efficiency of metal mixing is larger than 1/10 of the efficiency estimated the turbulence theory. We also found that timescale of metal mixing is less than 40 Myr. This timescale is shorter than that of typical dynamical times of dwarf galaxies.

Type Ia supernovae (SNe Ia) are one of the most energetic phenomena in the Universe, and have ever been used as cosmic ladder to discover the accelerating cosmic expansion. However, the details of the explosion mechanism are still unclear. Under an extremely high temperature and density condition in the celestial phenomena like SNe Ia, electrons are successively captured by radioactive nuclei and affect explosive nucleosynthesis. Recent theoretical prediction of the electron capture (EC) rates (i.e. equivalently Gamow-Teller strengths) of some unstable nuclei has proved to be consistent with precise experimental measurements. According to the theoretical prediction, the EC rates differ from those used in the previous conventional calculations. In the present study therefore we systematically calculate the EC rates using new generation nuclear shell model, and apply the results to network calculations of SN Ia nucleosynthesis. As a result, we find that the overproduction problem in the abundances of neutron-excess Cr-Fe-Ni isotopes is solved and that the solar-system abundances from Si to Ni are well reproduced systematically in our late-detonation wind model of SN Ia.

Abundances of r-process elements such as Eu and Ba in extremely metal-poor stars may reflect the early evolutionary histories of galaxies. However, how the chemo-dynamical evolution of galaxies affects the abundance of r-process elements is not yet understood. In this study, we perform a series of N-body/hydrodynamic simulations of galaxies with different density and mass. We calculate the evolution of r-process abundances in these galaxies. We find that galaxies with dynamical times around 100 Myr have star formation rates of less than 10^-3 Msun/year and they reproduce the observed r-process abundances (Figure A and B). This result does not depend on the mass of halos. On the other hand, r-process elements appear higher metallicity in galaxies with dynamical times around 10 Myr (Figure C and D). We also find that galaxies with lower star formation rates need longer timescale to mix metals, resulting in larger dispersions of r-process abundance ratios. This study demonstrates that future observations of r-process elements in extremely metal-poor stars will be able to constrain the evolutionary histories of galaxies.

The success of a theory of the CMB fluctuation and anisotropies and the observations of cosmic large-scale structure are thought to be a piece of evidence that the Big-Bang theory, which was first proposed by George Gamow in 1948, is the robust theoretical model of the cosmic expansion. However, this model predicts too large primordial abundance of lithium (atomic number Z=3) among all other light elements and contradicts astronomical observations of the early generations of old metal-poor stars. This discrepancy is called "the Big-Bang lithium problem" which might indicate the breakdown of the standard Big-Bang cosmology or suggest the need of a new particle-cosmological theory beyond the standard model. Elementary particles and nuclei in the hot Big-Bang expansion of the early Universe are not necessarily obey the classical Maxwell-Boltzmann distribution but some other class of non-extensive statistics. Tsallis statistics are the one of them which is presumed to describe well the thermodynamic systems showing chaos or fractals. Our international collaboration team demonstrated to solve this long standing Big-Bang lithium problem by applying Tsallis statistics to the primordial nucleosynthesis. If this "elegant solution" is correct, the Big Bang theory is now one step closer to fully describing the formation of our Universe, as referred in AAS NOVA.

The origin of heavy elements like gold and uranium, called r-process elements, is one of the 11 Greatest Unanswered Questions of the Century in Particle & Nuclear Physics and Astronomy (http://discovermagazine.com/2002/feb/cover). Shota Shibagaki and Taka Kajino in COSNAP International Consortium proposed a new theoretical model that the r-process elements were produced in supernova explosions from the early Galactic evolution, followed later by the additional contribution from the binary neutron-star mergers after 100 Myr of cosmic time. In this theoretical model, a long-standing underproduction problem of the isotopic abundances above and below the typical r-process peaks around A = 130, 165 and 195 is resolved, still satisfying the universal r-process abundance pattern between the early generations of metal-deficient stars and the solar-system.

The r-process is one of the main processes to synthesize elements heavier than iron. The r-process elements have been observed in metal-poor stars in dwarf galaxies and the Milky-Way halo, but astrophysical site(s) of r-process is not identified yet. Nucleosynthesis calculations suggest that binary neutron star mergers are the promising astrophysical site of r-process. In contrast, galactic chemical evolution studies without considering the formation process of galaxies pointed out that it is difficult to reproduce the observed r-process abundance in extremely metal-poor stars by neutron star mergers due to their long merger time and low occurrence rate. In this study, we performed a series of hydrodynamical simulations of dwarf galaxies assuming that neutron star mergers are the major astrophysical site of r-process. Our simulations reproduce the observed r-process abundance in extremely metal-poor stars by neutron star mergers with merger time of 100 Myr. In addition, we find that the metallicity is constant over ~ 300 Myr from the onset of star formation due to low star formation efficiency in dwarf galaxies. We moreover find that metal mixing in star-forming region avoids producing extremely r-process rich stars, which are inconsistent with the observation, due to the low rate of neutron star mergers. The r-process elements observed in the Milky-Way halo might originate in accreted dwarf galaxies.

International collaboration team directed by Dr. G. Lorusso and Dr. S. Nishimura (P.I., NAOJ visiting professor of 2013) at the RIKEN-Nishina Center for Accelerator-Based Science (chief scientist, Dr. H. Sakurai) produced 110 extremely neutron-rich, heavy radioactive isotopes by using the world highest-performance accelerator at RIKEN-RIBF and succeeded for the first time in measuring their beta-decay half-lives. Although these isotopes called r-process elements are presumed to originate in stellar explosions such as supernovae or binary neutron-star mergers, their astrophysical site has not yet been identified uniquely.  Mr. S. Shibagaki (PhD graduate student) and Prof. T. Kajino in the Division of Theoretical Astronomy at NAOJ and the University of Tokyo applied these high precision data to the theoretical calculation of the r-process nucleosynthesis and succeeded in reproducing the observed abundances of heavy neutron-capture elements (i.e. r-process elements) consistent with those expected from the core-collapse supernovae. It was also found that their supernova r-process calculation can explain the "universality" of the elemental-abundance pattern (as a function of atomic number) between the solar-system and the metal-poor halo stars which had been discovered by spectroscopic observations using SUBARU and Hubble Space Telescopes, etc.  These results are published in Physical Review Letters, 2015 on line on May 11 (doi: 23.40.-s 26.30.Hj 27.60.+j).

Neutrino scattering provides a unique signal for understanding the weak interaction processes in an extremely high-temperature and high-density matter inside the proto-neutron stars born in core-collapse supernova explosions. COSNAP* international collaboration group conducted by Prof. M.-K. Cheoun (Soongsil Univ./NAOJ) and Prof. T. Kajino (NAOJ/Univ. of Tokyo) carried out a quantum mechanical calculation of the neutrino scattering cross sections on nucleons and nuclei at E < 100 GeV in the quark meson coupling (QMC) model and compared the theoretical result with neutrino data detected at high-energy particle accelerators MiniBooNE and NOMAD. It is found that below 1 GeV at energies relevant for applications to the neutrino processes in the supernova matter, one needs to include nucleonic form factor and nuclear structure effects as well as the Fermi motion of nucleons.

T. Kajino, A. Tokuhisa at NAOJ and The University of Tokyo and their international collaboration team have recently proposed a new theory for UHECR tau-neutrinos that could keep ultra-high energy > 10¹⁵ eV. Although two neutrino events, whose energies are higher than 10¹⁵ eV, were detected in IceCube Neutrino Observatory last year, origin of the UHECR is still an unanswered biggest mystery of the century. Although Ginzburg & Syrovatskii (1965) and Waxman & Bahcall (1997) theoretically proposed a model to produce neutral mesons pi, D_s, J/ψ as the source of high-energy neutrinos, they easily lose energy during the passage in space for relatively longer life, and neutrino energy cannot be as high as 10¹⁵ eV. Kajino and his collaborators discovered that the strong interaction between UHECR hadrons like protons or irons can copiously produce heavy neutral mesons like upsilons in synchrotron emission near the strongly magnetized neutron star (i.e. magnetar) or active galactic nuclei (AGN), which quickly decay to produce tau-lepton pairs and create eventually UHECR tau-neutrinos.

Three flavors of active (electron-, mu- and tau-type) neutrinos are known to have finite masses and show the oscillation phenomena during their passage through space or matter inside the sun and supernovae (SNe). It is one of the ultimate purposes in modern physics and cosmology to determine their masses and oscillation properties to construct a unified theory of elementary particles and fields beyond the standard model because neutrino mass is the unique experimental evidence which indicates the breakdown of the standard model. There are still two unknown parameters, i.e. the mass hierarchy (normal or inverted) and the CP violation phase. Toshio Suzuki (Nihon University/NAOJ) and Toshitaka Kajino (NAOJ) have proposed an astronomical method to determine the mass hierarchy in terms of SN nucleosynthesis of specific elements that were produced by the energetic neutrinos interacting with abundant nuclei inside the SNe under the MSW effects of neutrino oscillation. They also prove theoretically that the proposed method is robust almost independent of the Hamiltonians assumed in the theoretical calculations of quantum neutrino-nucleus interaction cross-sections. This paper was selected as a highlight topical review of 2013 in Journal of Physics G.