arXiv subjects

A. Takamura

Publications and source records attributed to A. Takamura.

8 recordsLinked to original sources

arXiv2009

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 normal mass hierarchy or 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 and 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.

astro-ph.HE↗

arXiv2008

Neutrino-Nucleus Reaction Cross Sections for Light Element Synthesis in Supernova Explosions

The neutrino-nucleus reaction cross sections of 4He and 12C are evaluated using new shell model Hamiltonians. Branching ratios of various decay channels are calculated to evaluate the yields of Li, Be, and B produced through the nu-process in supernova explosions. The new cross sections enhance the yields of 7Li and 11B produced during the supernova explosion of a 16.2 M_odot star model compared to the case using the conventional cross sections by about 10%. On the other hand, the yield of 10B decreases by a factor of two. The yields of 6Li, 9Be, and the radioactive nucleus 10Be are found at a level of 10^{-11} M_odot. The temperature of nu_{mu,tau}- and bar{nu}_{mu,tau}-neutrinos inferred from the supernova contribution of 11B in Galactic chemical evolution models is constrained to the 4.3-6.5 MeV range. The increase in the 7Li and 11B yields due to neutrino oscillations is demonstrated with the new cross sections.

astro-ph↗

arXiv2006

Neutrino Oscillation Effects on Supernova Light Element Synthesis

Neutrino oscillations affect light element synthesis through the neutrino-process in supernova explosions. The 7Li and 11B yields produced in a supernova explosion of a 16.2 solar-mass star model increase by factors of 1.9 and 1.3 in the case of large mixing angle solution with normal mass hierarchy and sin^{2}2theta_{13} > 0.002 compared with those without the oscillations. In the case of inverted mass hierarchy or nonadiabatic 13-mixing resonance, the increment of their yields is much smaller. Neutrino oscillations raise the reaction rates of charged-current neutrino-process reactions in the region outside oxygen-rich layers. The number ratio of 7Li/11B could be a tracer of normal mass hierarchy and relatively large theta_{13}, still satisfying sin^{2}2theta_{13} < 0.1, through future precise observations in stars having strong supernova component.

astro-ph↗

arXiv2006

Supernova Neutrino Nucleosynthesis of Light Elements with Neutrino Oscillations

Light element synthesis in supernovae through neutrino-nucleus interactions, i.e., the nu-process, is affected by neutrino oscillations in the supernova environment. There is a resonance of 13-mixing in the O/C layer, which increases the rates of charged-current nu-process reactions in the outer He-rich layer. The yields of 7Li and 11B increase by about a factor of 1.9 and 1.3, respectively, for a normal mass hierarchy and an adiabatic 13-mixing resonance, compared to those without neutrino oscillations. In the case of an inverted mass hierarchy and a non-adiabatic 13-mixing resonance, the increase in the 7Li and 11B yields is much smaller. Observations of the 7Li/11B ratio in stars showing signs of supernova enrichment could thus provide a unique test of neutrino oscillations and constrain their parameters and the mass hierarchy.

astro-ph↗

arXiv2002

Exact Formula of Probability and CP Violation for Neutrino Oscillations in Matter

Within the framework of the standard three neutrino scenario, we derive an exact and simple formula of the oscillation probability $P(ν_e \to ν_μ)$ in constant matter by using a new method. From this formula, it is found that the matter effects can be separated from the pure CP violation effects. Furthermore, the oscillation probability can be written in the form, $P(ν_e \to ν_μ)=A\cos δ+B\sinδ+C$, in the standard parametrization without any approximation. We also demonstrate that the approximate formula in high-energy can be easily reproduced from this as an example.

hep-ph↗

arXiv2002

Matter Enhancement of T Violation in Neutrino Oscillation

We study the matter enhancement of T violation in neutrino oscillation with three generations. The magnitude of T violation is proportional to Jarlskog factor J. Recently, the elegant relation, (Δ_m)_{12}(Δ_m)_{23}(Δ_m)_{31}J_{m} = Δ_{12}Δ_{23}Δ_{31}J, was derived, where Δ_{ij}=Δm^2_{ij}/(2E) and subscript m implies the quantities in matter. Using this relation, we reconsider how J_m changes as a function of the matter potential a under the approximation |Δm^2_{12}| \ll |Δm^2_{13}|. We show that the number of maxima for J_{m} depends on the magnitude of \sin^2 2 θ_{13} and there are two maxima considering the constraint on \sin^2 2θ_{13} from the CHOOZ experiment. One maximum of $J_{m} at a = O(Δ_{12}) is given by J/\sin2θ_{12}, which leads to the large enhancement of J_m in the case of the SMA MSW solution. The other maximum at a = O(Δ_{13}) is |Δ_{12}/Δ_{13}|J/\sin2θ_{13}, and the enhancement is possible, if \sin 2 θ_{13} is small enough. These maximal values are consistent with the results obtained by other methods.

hep-ph↗

arXiv2000

Schematic Method for Estimation of CP Asymmetry in Neutrino Oscillation

Within the framework of three generations of leptons the schematical method for estimating CP asymmetry in neutrino oscillations is considered. We introduce a unitarity triangle corresponding to ν_e-ν_μ oscillation, in addition, we show that it is convenient for the estimation to define another new triangle. CP asymmetry is determined by the difference between the shapes of a unitarity triangle and new triangle. As results, (i) we show that CP asymmetry becomes maximal if the shapes of these two triangles are the same. (ii) We can easily estimate L/E which leads almost 100% asymmetry using this. (iii) In ν_e-ν_μ oscillation with \sin^2 2θ_{13} \simeq 0.04, we obtain about 90% asymmetry for LMA MSW scenario and 3% asymmetry for SMA MSW scenario within long baseline neutrino expriments to be realized in near future.

hep-ph↗

arXiv1999

Exact Parametrization of Majorana Neutrino Mass Matrix with Large Mixing

Under the assumption that the neutrinos are Majorana particles we study how the lepton mass matrices can be transformed into the simple form which has the same physical quantities by removing redundant parameters. We propose the exact parametrization of the lepton mass matrices which reflects the small nu_e-nu_mu mixing and the large nu_mu-nu_tau mixing. The relations between the twelve parameters and the physical quantities are shown. Furthermore we calculate the MNS matrix by applying the assumptions used in the quark sector. Finally we also check the validity of these assumptions from the experimental values.

hep-ph↗