arXiv subjects

T. Asaka

Publications and source records attributed to T. Asaka.

At least 19 recordsLinked to original sources

arXiv2020

An electron linac with high beam intensity over 1-Ampere for disposal of radioactive waste

In order to dissipate long-lived radioactive waste, not only high ux proton accelerator but also electron linac have been proposed. A proton accelerator directly induces nuclear fission and mutation. On the other hand, electron beam has two processes: production of gamma rays through bremsstrahlung and produced gamma rays are available for the (gamma ; n) reaction. Protons dissipate nuclei and simultaneously newly produce radioisotopes of by-products. Since an electron linac produces less radioactive wastes, we focus only an electron linac with high beam intensity more than 1-Ampere. To reduce the yield of by-products for radioisotopes as less as possible, we accelerate electron beam with the energy less than 30 MeV. The accelerated beam intensity is designed to be more than 1-Ampere. To realize an electron linac with high intensity, the operation for the linac is not pulsed one, but continuous wave. To accelerate electron beam, we install higher-order-modes (HOMs) free normal conducting cavities to suppress beam instabilities.

physics.acc-ph↗

arXiv2018

Precision electroweak shift of muonium hyperfine splitting

Electroweak second order shifts of muonium ($μ^+e^-$ bound state) energy levels are calculated for the first time. Calculation starts from on-shell one-loop elastic $μ^+ e^-$ scattering amplitudes in the center of mass frame, proceed to renormalization and to derivation of muonium matrix elements by using the momentum space wave functions. This is a reliable method unlike the unjustified four-Fermi approximation in the literature. Corrections of order $αG_F$ (with $α\sim 1/137$ the fine structure constant and $G_F$ the Fermi constant) and of order $αG_F /(m_Z a_B)$ (with $m_Z$ the Z boson mass and $a_B$ the Bohr radius) are derived from three classes of Feynman diagrams, Z self-energy, vertex and box diagrams. The ground state muonium hyperfine splitting is given in terms of the only experimentally unknown parameter, the smallest neutrino mass. It is however found that the neutrino mass dependence is very weak, making its detection difficult.

hep-ph↗

arXiv2015

Basic oscillation measurables in the neutrino pair beam

It is shown that the vector current contribution of neutrino interaction with electrons in ion gives rise to oscillating component, which is absent for the axial-vector contribution, when a single neutrino is detected in the recently proposed neutrino pair beam. CP violation measurements are thus possible with high precision along with determination of mass hierarchical patterns.

hep-ph↗

arXiv2007

Variation of charge/orbital ordering in layered manganites Pr1-xCa1+xMnO4 investigated by transmission electron microscopy

Structural features of the charge/orbital ordering (CO/OO) in single-layered manganites Pr1-xCa1+xMnO4 have been investigated systematically by transmission electron microscopy. Analyses of electron diffraction patterns as well as dark-field images have revealed that the CO/OO shows a striking asymmetric behavior as the hole doping x deviates from x = 0.5. The modulation wavenumber linearly decreases with increasing x in the over-hole-doped (x > 0.5) crystals, while much less dependent on x in the under-hole-doped (x < 0.5) crystals. A temperature-induced incommensurate-commensurate crossover is observed in 0.35 < x < 0.5 and x = 0.65. The correlation length of CO/OO in x = 0.3 was proven to become shorter than that in x > 0.3.

cond-mat.str-el↗

arXiv2005

Roles of two successive phase transitions in new spin-Peierls system TiOBr

In this sturdy, we determine the roles of two successive phase transitions in the new spin-Peierls system TiOBr by electron and synchrotron X-ray diffraction analyses. Results show an incommensurate superstructure along the h- and k-directions between Tc1=27K and Tc2=47K, and a twofold superstructure which is related to a spin-Peierls lattice distortion below Tc1. The diffuse scattering observed above Tc2 indicates that a structural correlation develops at a high temperature. We conclude that Tc2 is a second-order lock-in temperature, which is related to the spin-Peierls lattice distortion with the incommensurate structure, and that Tc1 is from incommensurate to commensurate phase transition temperature accompanying the first-order spin-Peierls lattice distortion.

cond-mat.str-el↗

arXiv2003

Quarks and Leptons between Branes and Bulk

We study a supersymmetric SO(10) gauge theory in six dimensions compactified on an orbifold. Three sequential quark-lepton families are localized at the three fixpoints where SO(10) is broken to its three GUT subgroups. Split bulk multiplets yield the Higgs doublets of the standard model and as additional states lepton doublets and down-quark singlets. The physical quarks and leptons are mixtures of brane and bulk states. The model naturally explains small quark mixings together with large lepton mixings in the charged current. A small hierarchy of neutrino masses is obtained due to the different down-quark and up-quark mass hierarchies. None of the usual GUT relations between fermion masses holds exactly.

hep-ph↗

arXiv2003

Lopsided Mass Matrices and Leptogenesis in SO(10) GUT

Lopsided structure in mass matrices of down quarks and leptons gives a simple explanation for the observed large angles of neutrino mixings. We realize such mass matrices by the Froggatt-Nielsen mechanism in the framework of supersymmetric SO(10) grand unified theory (GUT). It is shown that the model can reproduce the successful mass matrices which have been obtained in SU(5) models. Cosmological implication of the model is also discussed. We show that the hybrid inflation occurs naturally in the model and it offers non-thermal leptogenesis by decays of the next-to-lightest right-handed neutrinos. The present baryon asymmetry is explained by just the oscillation mass scale in the atmospheric neutrinos.

hep-ph↗

arXiv2002

Non-thermal Leptogenesis from the Heavier Majorana Neutrinos

We investigate a scheme for making leptogenesis by means of the CP violating decays of the seesaw Majorana neutrinos proposed by Fukugita and Yanagida. However, in order to avoid the wash-out of the produced lepton number we propose the production of the Majorana neutrinos to occur non-thermally and sufficiently late. After this time, in consequence, the B-L (baryon minus lepton) quantum number becomes a good ``accidental symmetry'' protecting the asymmetry produced. This non-thermal leptogenesis at late time is realized by a boson decaying into the Majorana neutrinos with a long lifetime. Suggestively this boson could correspond to a scalar field which causes the cosmic inflation, the inflaton, and thus its decay means really the reheating of the Universe. We find that this mechanism works well even if the lightest Majorana neutrinos are not produced sufficiently or not present, and the decays of the heavier seesaw Majorana neutrinos can be responsible to the baryon asymmetry in the present Universe, as we illustrate by the example of the family replicated gauge group model.

hep-ph↗

arXiv2002

Bulk and brane anomalies in six dimensions

We study anomalies of six-dimensional gauge theories compactified on orbifolds. In addition to the known bulk anomalies, brane anomalies appear on orbifold fixpoints in the case of chiral boundary conditions. At a fixpoint, where the bulk gauge group G is broken to a subgroup H, the non-abelian G-anomaly in the bulk reduces to a H-anomaly which depends in a simple manner on the chiral boundary conditions. We illustrate this mechanism by means of a SO(10) GUT model.

hep-ph↗

arXiv2002

Exceptional Coset Spaces and Unification in Six Dimensions

The coset spaces E$_8$/SO(10)$\times$H$_F$ allow complex structures which can account for three quark-lepton generations including right-handed neutrinos. We show that in the context of supersymmetric SO(10) gauge theories in 6 dimensions they also provide the Higgs fields which are needed to break the electroweak and ${B-L}$ gauge symmetries, and to generate small neutrino masses via the seesaw mechanism.

hep-ph↗

arXiv2001

Upgrade of Linac Control System with New Vme Controllers at Spring-8

We integrated an injector linac control system to the SPring-8 standard system on September 2000. As a result of this integration, the SPring-8 accelerator complex was controlled by one unified system. Because the linac was continuously running as the electron beam injector not only for the SPring-8 storage ring but also for New SUBARU, we had to minimize the hardware modification to reduce the time for the development and testing of the new control system. The integration method was almost the same as that of the integration of the booster synchrotron. We report here on the integration of the linac control system with emphasis on the upgrade of the VMEbus controllers and software involving the operating system Solaris 7 as the real-time OS.

physics.acc-ph↗

arXiv2001

Timing System of SPring-8 Booster Synchrotron

The timing system of SPring-8 booster synchrotron generates various timing signals concerning beam injection, acceleration from 1 GeV to 8 GeV and ejection. We have improved the timing system of the synchrotron giving it better stability and flexibility. This improvement results the other merits of advanced operations, for example, storing electron beam in the synchrotron, changing the injection cycle from 1 Hz to the slower frequency to increase the RF knock-out (RF-KO) operation period and ejecting the low energy beam during ramping up.

physics.acc-ph↗

arXiv2001

Gauge Unification In Six Dimensions

We study the breaking of a supersymmetric SO(10) GUT in 6 dimensions by orbifold compactification. In 4 dimensions we obtain a N=1 supersymmetric theory with the standard model gauge group enlarged by an additional U(1) symmetry. The 4-dimensional gauge symmetry is obtained as intersection of the Pati-Salam and the Georgi-Glashow subgroups of SO(10), which appear as unbroken subgroups in the two 5 dimensional subspaces, respectively. The doublet-triplet splitting arises as in the recently discussed SU(5) GUTs in 5 dimensions.

hep-ph↗

arXiv2001

Affleck-Dine Leptogenesis and Low Scale Inflation

We study Affleck-Dine leptogenesis via the L H_u flat direction in supersymmetric theories. We find that the baryon asymmetry is enhanced when the energy scale of the inflation is sufficiently low. Especially, we consider models of low scale inflation in which the Hubble parameter during inflation is comparable to (but slightly larger than) the gravitino mass m_{3/2} \sim 1 \TeV. The observed cosmological baryon asymmetry is obtained with the lightest neutrino mass m_{ν_1} \sim 10^{-4} eV, if the reheating process is suddenly terminated after inflation.

hep-ph↗

arXiv2001

False Vacuum Decay after Inflation

Inflation is terminated by a non-equilibrium process which finally leads to a thermal state. We study the onset of this transition in a class of hybrid inflation models. The exponential growth of tachyonic modes leads to decoherence and spinodal decomposition. We compute the decoherence time, the spinodal time, the size of the formed domains and the homogeneous classical fields within a single domain.

hep-ph↗

arXiv2000

Solving the Gravitino Problem by Axino

In a large class of supersymmetric (SUSY) axion model the mass of axino $\axino$ (a fermionic superpartner of the axion) is predicted as $m_{\axino} \lesssim {\cal O}(1)$ keV. Thus, the axino is the lightest SUSY particle (LSP). We pointed out that such a light axino provides a natural solution to the gravitino problem, if the gravitino is the next LSP. We derive a constraint on the reheating temperature $T_R$ of inflation, $T_R \lesssim 10^{15}$ GeV for the gravitino mass $m_{3/2} \simeq 100$ GeV, which is much weaker than that obtained in the minimal SUSY standard model.

hep-ph↗

arXiv2000

Cosmological Gravitino Problem in Gauge-Mediated Supersymmetry Breaking Models

We investigate the cosmological gravitino problem in gauge-mediated supersymmetry breaking models, where the gravitino becomes in general the lightest supersymmetric particle (LSP). In order to avoid the overclosure of the stable gravitino, the reheating temperature of inflation $T_R$ should be low enough. Furthermore, if the gravitino mass is larger than about 100 MeV, the decay of the next-to-LSP (NLSP) into the gravitino may modify disastrously the abundances of the light elements predicted by the big-bang nucleosynthesis (BBN). We consider the case in which the lighter stau is the NLSP and derive cosmological constraints from the BBN on the stau NLSP decay. We obtain a lower bound on the mass of stau $m_{\staul}$, which is more stringent than the current experimental limit $m_{\staul} > 90$ GeV for the gravitino mass region $m_{3/2} \gsim 5$ GeV. This lower bound, together with the overclosure constraint on the stable gravitino, gives an upper bound on $T_{R}$. We find that the reheating temperature can be as high as $10^9$--$10^{10}$ GeV for $m_{3/2} \simeq 5$--100 GeV.

hep-ph↗

arXiv2000

Affleck-Dine Leptogenesis with an Ultralight Neutrino

We perform a detailed analysis on Affleck-Dine leptogenesis taking into account the thermal effects on the dynamics of the flat direction field $ϕ$. We find that an extremely small mass for the lightest neutrino $ν_1$, ${m_ν}_1\lsim 10^{-8}$ eV, is required to produce enough lepton-number asymmetry to explain the baryon asymmetry in the present universe. We impose here the reheating temperature after inflation $T_R$ to be $T_R\lsim 10^8$ GeV to solve the cosmological gravitino problem. The required value of neutrino mass seems to be very unlikely the case since the recent Superkamiokande experiments suggest the masses of heavier two neutrinos $ν_2$ and $ν_3$ to be in a range of $10^{-1}$--$10^{-3}$ eV. We also propose a model to avoid this difficulty based on the Peccei-Quinn symmetry, where the required neutrino mass can be as large as ${m_ν}_1\simeq 10^{-4}$ eV.

hep-ph↗