arXiv subjects

K. Tsushima

Publications and source records attributed to K. Tsushima.

At least 163 records · Page 9Linked to original sources

arXiv1999

A quark-meson coupling model with short-range quark-quark correlations

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.

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arXiv1998

The role of the $P_{11}(1710)$ in the $NN{\to}NΣK$ reaction

Using the resonance model, which was successfully applied for the study of the $pp{\to}pΛK^+$ reaction, we investigate $NN{\to}NΣK$ reactions that should provide cleaner information about resonance excitations and meson exchange contributions. For this purpose we demonstrate that the invariant mass distribution for the $ΣK$ system, as well as the Dalitz plot for the $NN{\to}NΣK$ reaction, provide direct information about the $ΣK$ production mechanism, which can be tested in the near future by experiments at COSY.

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arXiv1998

Spin Dependent Parton Distributions in a Bound Nucleon

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.

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arXiv1998

Eta, Omega and Charmed Mesic Nuclei

Using the quark-meson coupling (QMC) model, we investigate theoretically whether $η$ and $ω$ mesons form meson-nucleus bound states. We study several nuclei from $^{6}$He to $^{208}$Pb, including those which are the final nuclei in the proposed experiment at GSI. Results for the $ω$ are compared with those of the Walecka model. Our results suggest that one should expect to find $η$- and $ω$-nucleus bound states in all these nuclei. Furthermore, we investigate the possibility of charmed mesic nuclei. It is shown that the $D^-$ meson will inevitably form narrow bound states with $^{208}$Pb.

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arXiv1998

J/Ψproduction in πN collisions

We calculate the total cross section for $πN \to J/ΨN$ based on $ρ$-meson exchange. On the basis of this calculation we predict a maximal reaction rate arising from the OZI-violating $J/Ψ{\leftrightarrow}ρπ$ vertex in tree-level. Our estimate for the maximum reaction rate is still one order of magnitude smaller than the existing predictions which were based on the other OZI-violating mechanisms.

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arXiv1998

Are eta- and omega-nuclear states bound ?

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.

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arXiv1998

Electromagnetic form factors of the bound nucleon

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.

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arXiv1998

Recent results from QMC relevant to TJNAF

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.

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arXiv1998

omega-nucleus bound states in the Walecka model

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.

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arXiv1998

Sigma and omega meson propagation in a dense nuclear medium

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.

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arXiv1998

The neutron charge form factor in helium-3

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.

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arXiv1998

Medium dependence of the bag constant in the quark-meson coupling model

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.

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arXiv1998

Quark and gluon condensates in the quark-meson coupling model

Using the quark-meson coupling (QMC) model, we study the density dependence of the quark and gluon condensates in nuclear matter. We show that the change of the quark condensate is mainly driven by the scalar field in the medium and that the reduction of the quark condensate is suppressed at high density, even in the mean-field approximation. The gluon condensate decreases by 4 - 6 % at nuclear saturation density. We also give a simple relationship between the change of the quark condensate and that of a hadron mass in the medium.

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arXiv1997

A clue to the mechanism of $ΛK^+$ production in $pp$-reactions

We analyse the $p p \to p ΛK^+$ total cross section data which were recently measured by the COSY-11 collaboration at an energy 2~MeV above the reaction threshold. Our analysis suggests that the measurement of the invariant mass spectrum for the $ΛK^+$ at energies around 100 MeV above the threshold can provide a new constraint on the theoretical calculations for this reaction. In particular, the measurement can give a clue as to whether the reaction is dominated by resonance excitation or not. Thus, it should contribute to further understanding of the strangeness production mechanism. We show the invariant mass spectra for the $ΛK^+$ system in these optimal kinematic conditions calculated by several approaches, and demonstrate that our suggestion is experimentally feasible.

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arXiv1997

The quark-meson coupling model for Lambda, Sigma and Xi hypernuclei

The quark-meson coupling (QMC) model, which has been successfully used to describe the properties of both infinite nuclear matter and finite nuclei, is applied to a systematic study of $Λ, Σ$ and $Ξ$ hypernuclei. Assumptions made in the present study are, (i) the (self-consistent) exchanged scalar, and vector, mesons couple only to the u and d quarks, and (ii) an SU(6) valence quark model for the bound nucleons and hyperon. The model automatically leads to a very weak spin-orbit interaction for the $Λ$ in a hypernucleus. Effects of the Pauli blocking at the quark level, particularly in the open, coupled, $ΣN - ΛN$ channel (strong conversion), is also taken into account in a phenomenological way.

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arXiv1997

Self-consistent description of $Λ$ hypernuclei in the quark-meson coupling model

The quark-meson coupling (QMC) model, which has been successfully used to describe the properties of both finite nuclei and infinite nuclear matter, is applied to a study of $Λ$ hypernuclei. With the assumption that the (self-consistent) exchanged scalar, and vector, mesons couple only to the u and d quarks, a very weak spin-orbit force in the $Λ$-nucleus interaction is achieved automatically. This is a direct consequence of the SU(6) quark model wave function of the $Λ$ used in the QMC model. Possible implications and extensions of the present investigation are also discussed.

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arXiv1997

In-medium electron-nucleon scattering

In-medium nucleon electromagnetic form factors are calculated in the quark meson coupling model. The form factors are typically found to be suppressed as the density increases. For example, at normal nuclear density and $Q^2 \sim 0.3 { GeV}^2$, the nucleon electric form factors are reduced by approximately 8% while the magnetic form factors are reduced by only 1 - 2%. These variations are consistent with current experimental limits but should be tested by more precise experiments in the near future.

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arXiv1997

Naturalness in the quark-meson coupling model

The quark-meson coupling (QMC) model is examined using Georgi's ``naive dimensional analysis''. We argue that the QMC model is quite natural as an effective field theory for nuclei.

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