arXiv subjects
Publications and source records attributed to K. Saito.
We describe a quantum algorithm to compute the density of states and thermal equilibrium properties of quantum many-body systems. We present results obtained by running this algorithm on a software implementation of a 21-qubit quantum computer for the case of an antiferromagnetic Heisenberg model on triangular lattices of different size.
The binding energy differences of the valence proton and neutron of the mirror nuclei, $^{15}$O -- $^{15}$N, $^{17}$F -- $^{17}$O, $^{39}$Ca -- $^{39}$K and $^{41}$Sc -- $^{41}$Ca, are calculated using the quark-meson coupling (QMC) model. The calculation involves nuclear structure and shell effects explicitly. It is shown that binding energy differences of a few hundred keV arise from the strong interaction, even after subtracting all electromagnetic corrections. The origin of these differences may be ascribed to the charge symmetry breaking effects set in the strong interaction through the u and d current quark mass difference.
We study how energy transport in an integrable system is affected by the spectral densities of heat reservoirs. The model investigated here is the quantum harmonic chain whose both ends are in contact with two heat reservoirs at different temperatures. The master equation for the reduced density matrix is derived on the assumption that the reservoirs are composed of an infinite number of independent harmonic oscillators. We evaluate temperature profile and energy flux in the stationary state for the master equation and discuss how they depend on the types of spectral densities. When we attach the reservoirs of the same type of spectral density, we find that the temperature profile is independent of the types. On the other hand, when the two reservoirs have different types of spectral densities, the energy profile near the ends of the chain depends on the types. When the coupling is finite, the temperature profile near the ends shows wide variation of behavior dependent on spectral densities and temperatures of reservoirs. This dependence is discussed with the Fokker-Planck equations obtained in the classical limit.
The spectra of the scalar ($σ$) and vector ($ω$) mesons in nuclear matter are studied in detail using the quark-meson coupling (QMC) model. It is shown that above normal nuclear matter density the effects of $σ$-$ω$ mixing and the decay of the $σ$ into $2π$ change the spectra of $σ$ and $ω$ mesons considerably. As in Quantum Hadrodynamics we find a remarkable spectral change in the $σ$ meson and the longitudinal mode of the $ω$ meson, namely a two-peak structure.
We discuss the effect of the thermal environment on the low-temperature response of the magnetization of uniaxial magnets to a time-dependent applied magnetic field. At sufficiently low temperatures the staircase magnetization curves observed in molecular magnets such as Mn_{12} and Fe_8 display little temperature dependence. However the changes of the magnetization at each step do not seem to be directly related to the probability for a quantum mechanical nonadiabatic transition. In order to explain this deceptive apparent nonadiabatic behavior, we study the quantum dynamics of the system in a thermal environment and propose a relation between the observed magnetization steps and the quantum mechanical transition probability due to the nonadiabatic transition.
Using the quark-meson coupling (QMC) model, we study the longitudinal response function for quasielastic electron scattering from nuclear matter. In QMC the coupling constant between the scalar ($σ$) meson and the nucleon is expected to decrease with increasing nuclear density, because of the self-consistent modification of the structure of the nucleon. The reduction of the coupling constant then leads to a smaller contribution from relativistic RPA than in the Walecka model. However, since the electromagnetic form factors of the in-medium nucleon are modified at the same time, the longitudinal response function and the Coulomb sum are reduced by a total of about 20% in comparison with the Hartree contribution. We find that the relativistic RPA and the nucleon structure variation both contribute about fifty-fifty to the reduction of the longitudinal response.
We show that the D^- meson will form narrow bound states with ^{208}Pb. Mean field potentials for the D^0, D^0-bar and D^- in ^{208}Pb are calculated self-consistently using the quark-meson coupling (QMC) model in local density approximation. The meson-^{208}Pb bound state energies are then calculated by solving the Klein-Gordon equation with these potentials. The experimental confirmation and comparison with the D^0-bar and D^0 will provide distinctive information on the nature of the interaction between the charmed meson and matter.
Short-range quark-quark correlations are introduced into the quark-meson coupling (QMC) model in a simple way. The effect of these correlations on the structure of the nucleon in dense nuclear matter is studied. We find that the short-range correlations may serve to reduce a serious problem associated with the modified quark-meson coupling model (within which the bag constant is allowed to decrease with increasing density), namely the tendency for the size of the bound nucleon to increase rapidly as the density rises. We also find that, with the addition of correlations, both QMC and modified QMC are consistent with the phenomenological equation of state at high density.
The determination of the spin distribution functions for the neutron requires an understanding of the changes induced by a nuclear medium. We present the first estimates of the changes induced in the spin-dependent, valence parton distributions of the proton and neutron bound in $^3$He and $^6$Li. For $^3$He the result is quite reassuring, in that the off-shell correction to $g_{1n}$ is very small. On the other hand, in $^6$Li the corrections to the spin distributions in the bound proton are sufficiently large that they should be taken into account if one aims for a precision of better than 10%. We provide a simple parametrization of the off-shell changes in the valence spin distributions of $^3$He and $^6$Li, which can be used with any conventional estimate of nuclear binding and Fermi motion corrections.
We review the quark-meson coupling model, in which the quark degrees of freedom are explicitly involved to describe the properties of not only nuclear matter but also finite nuclei. Then, we present the electric and magnetic form factors for the proton bound in specific orbits for several closed-shell nuclei.
Nature of energy transport around a critical point is studied in the Creutz cellular automaton. Fourier heat law is confirmed to hold in this model by a direct measurement of heat flow under a temperature gradient. The thermal conductivity is carefully investigated near the phase transition by the use of the Kubo formula. As the result, the thermal conductivity is found to take a finite value at the critical point contrary to some previous works. Equal-time correlation of the heat flow is also analyzed by a mean-field type approximation to investigate the temperature dependence of thermal conductivity. A variant of the Creutz cellular automaton called the Q2R is also investigated and similar results are obtained.
We investigate theoretically whether it is feasible to detect $η$- and $ω$-nucleus bound states. As well as the closed shell nuclei, $^{16}$O, $^{40}$Ca, $^{90}$Zr and $^{208}$Pb, we also investigate $^6$He, $^{11}$B and $^{26}$Mg, which are the final nuclei in the proposed experiment involving the (d,$^3$He) reaction at GSI. Potentials for the $η$ and $ω$ mesons in these nuclei are calculated in local density approximation, embedding the mesons in the nucleus described by solving the mean-field equations of motion in the QMC model. Our results suggest that one should expect to find $η$- and $ω$-nucleus bound states in all these nuclei.
We calculate electromagnetic form factors of the proton bound in specified orbits for several closed shell nuclei. The quark structure of the nucleon and the shell structure of the finite nuclei are given by the QMC model. We find that orbital electromagnetic form factors of the bound nucleon deviate significantly from those of the free nucleon.
We review recent calculations using the quark meson coupling model which should be of particular interest at Jefferson Lab. In particular, we discuss the change in the proton electric and magnetic form factors when it is bound in a specific shell model orbit. Modern quasi-elastic electron scattering experiments should be able to detect effects of the size predicted. We also examine the mean field potential felt by an $ω$ in a finite nucleus, concluding that the $ω$ should be bound by between 50 and 100 MeV.
Using the Walecka model, we investigate theoretically whether an omega meson is bound to finite nuclei. We study several nuclei from ^{6}He to ^{208}Pb, and compare the results with those in the quark-meson coupling (QMC) model. Our calculation shows that deeper omega-nucleus bound states are predicted in the Walecka model than in QMC. One can expect to detect such bound states in the proposed experiment involving the (d,^3He) reaction at GSI.
The propagation of the scalar ($σ$) and vector ($ω$) mesons in nuclear matter is studied in detail using the Walecka model over a wide range of densities and including the effects of a finite $σ$ width through the inclusion of a two-pion loop. We calculate the dispersion relation and spectral functions of the $σ$ and (transverse and longitudinal) $ω$ mesons, including the effect of $σ$-$ω$ mixing in matter. It is shown that the mixing effect is quite important in the propagation of the (longitudinal) $ω$ and $σ$ mesons above normal nuclear matter density. We find that there is a two-peak structure in the spectral function of the $σ$ channel, caused by $σ$-$ω$ mixing.
In order to measure the charge form factor of the neutron, $G^n_E(Q^2)$, one needs to use a neutron bound in a nuclear target. We calculate the change in the form factor for a neutron bound in $^3He$, with respect to the free case, using several versions of the quark meson coupling model. It is found that the form factor may be suppressed by as much as 12% at $Q^2 = 0.5{GeV}^2$ with respect to that of the free neutron.
Possible variations of the quark-meson coupling (QMC) model are examined in which the bag constant decreases in the nuclear medium. The reduction is supposed to depend on either the mean scalar field or the effective mass of the nucleon. It is shown that the electric and magnetic radii of the bound nucleon are almost linearly correlated with the bag constant. Using the fact that the size of the bound nucleon inside a nucleus is strongly constrained by $y$-scaling data in quasielastic, electron-nucleus scattering, we set a limit for the reduction allowed in the bag constant for these two models. The present study implies that the bag constant can decrease up to 10--17 % at average nuclear density, depending on the details of the model.