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Kazuo Tsushima

Publications and source records attributed to Kazuo Tsushima.

At least 19 recordsLinked to original sources
arXiv2026

Electromagnetic structure of strange vector mesons in nuclear medium

We investigate the in-medium modifications of the charge (electric) $G_C^{*}(Q^2)$, magnetic $G_M^{*}(Q^2)$, and quadrupole $G_Q^{*}(Q^2)$ form factors of the positively charged vector meson $K^{*+}$ in symmetric nuclear matter at zero temperature within the Schwinger proper-time Nambu-Jona-Lasinio (NJL) model. In this framework, both the nuclear medium effects and the electromagnetic structure of the $K^{*+}$ meson are described consistently in the NJL model at the quark level. We find that the charge, magnetic, and quadrupole form factors are suppressed with increasing nuclear density, indicating substantial in-medium modifications of the strange vector meson's internal structure. We further obtain a charge radius of $r_{K^{*+}}^{*}=0.74~\mathrm{fm}$ at normal nuclear density, which is slightly smaller than the corresponding value for the $ρ^{+}$ meson, $r_{ρ^{+}}^{*}\simeq0.75~\mathrm{fm}$.

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arXiv2026

Polarization-dependent mass modifications of $ϕ$ meson with finite momentum in nuclear matter

We investigate the in-medium properties of the $ϕ$ meson with finite momentum, going beyond the commonly studied case at rest. In a nuclear medium, Lorentz invariance is broken, leading to distinct longitudinal and transverse polarization modes that evolve differently with density and momentum. Within an effective Lagrangian approach, we calculate the polarization-dependent mass shifts and width modifications of the $ϕ$ meson arising from $K\bar{K}$ loops and mean-field interactions. The divergent loop integrals are regulated using two different schemes: a covariant form factor and dimensional regularization. Our results show that the mass shift of the transverse polarization is independent of the $ϕ$-meson momentum, whereas that of the longitudinal polarization decreases quadratically with momentum. This difference originates from the coupling of the longitudinal mode to the vector mean field and derivative-type interactions in the self-energy. These effects have direct implications for experimental observables, especially for upcoming measurements at J-PARC, and provide a new prediction for experiments studying hadron dynamics in dense matter.

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arXiv2026

Quark-meson coupling model and heavy-ion collision

We implement the quark-meson coupling model in Daejeon Boltzmann-Uehling-Uhlenbeck (DJBUU) transport model and perform Au+Au collision simulations at intermediate energies. Results are compared with simulations using a conventional quantum hadrodynamics model. Differences in the maximum density reached during the collisions are interpreted in terms of nuclear matter properties predicted by each model.

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arXiv2026

Quark-Meson Coupling Model in Heavy-Ion Collision Simulations

The quark-meson coupling (QMC) model incorporates quark degrees of freedom into the relativistic mean-field (RMF) framework, distinguishing it from traditional quantum hadrodynamics (QHD), which treats nucleons as point-like particles. In this work, we implement the QMC model within the DaeJeon Boltzmann-Uehling-Uhlenbeck (DJBUU) transport code to investigate its applicability to intermediate-energy heavy-ion collisions. We simulate \textsuperscript{197}Au+\textsuperscript{197}Au collisions at a beam energy of 400 A MeV using both QHD and QMC and find that both approaches yield comparable results for bulk observables such as transverse and directed flow, with good agreement with experimental data. To further assess the model performance, we study pion production in neutron-rich (\textsuperscript{132}Sn+\textsuperscript{124}Sn) and less neutron-rich (\textsuperscript{108}Sn+\textsuperscript{112}Sn) systems at 270 A MeV. In contrast to the QHD case, reproducing the observed pion yields and charge ratios within the QMC framework requires a slightly reduced density-dependent suppression in the in-medium $Δ$ production cross-section. These results demonstrate that the QMC model can be effectively integrated into transport simulations.

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arXiv2025

Medium effects on the electromagnetic form factors of the $ρ$ meson

The dynamics of partons inside the light $ρ$ meson is found to be essential for its properties and internal structure, both in free space and in the nuclear medium. In this paper, we systematically investigate the in-medium structure changes of $ρ^+$ mesons within the covariant Nambu-Jona-Lasinio (NJL) model, utilizing the Schwinger proper-time regularization scheme. We solve the Bethe-Salpeter equations to guarantee the bound meson-state condition. At the quark level, the nuclear medium effects are also derived within the same NJL model to maintain a consistent approach with the in-medium $ρ^+$ meson electromagnetic form factors. To this end, we analyze the spacelike electromagnetic form factors of the $ρ^+$ meson in free space and in a nuclear medium. We find that the charge radius and quadrupole moment of the $ρ^+$ meson increase with increasing nuclear matter density, while the magnetic moment decreases, in agreement with the existing previous theoretical predictions. The enhancement of the $ρ^+$ meson charge radius at normal density relative to that in free space is about 11\% (0.08 fm), while the reduction of $ρ^+$ meson magnetic moment is about 8\% (0.20 $μ_N$). Our predictions for the charge radius, magnetic moment, and quadrupole moment of the $ρ^+$ meson in both free space and nuclear medium, remain challenging to be verified experimentally.

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arXiv2025

Heavy-heavy and heavy-light mesons in cold nuclear matter

We review the in-medium modifications of effective masses (Lorentz scalar potentials or phenomenon of mass shift) of heavy-heavy and heavy-light mesons in symmetric nuclear matter and their nuclear bound states. We use a combined approach with the quark-meson coupling (QMC) model and an effective Lagrangian. As demonstrated by the cases of pionic and kaonic atoms, studies of meson-nucleus bound state can provide us with important information on chiral symmetry in dense nuclear medium. In this review, we treat the mesons, $K, K^*, D, D^*, B, B^*, η, η', ϕ, η_c, J/ψ, η_b, Υ$, and $B_c$, where our emphasys is on the heavy mesons. In addition, we also present some new results for the $B_c$-nucleus bound states.

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arXiv2025

In-medium electromagnetic form factors of pseudoscalar mesons from the quark model

We explore the modifications of hadron structure in a nuclear medium, focusing on the spacelike electromagnetic form factors (EMFFs) of light and heavy-light pseudoscalar mesons. By combining the light-front quark model (LFQM) with the quark-meson coupling (QMC) model, which reasonably reproduces EMFFs in free space and the saturation properties of nuclear matter, respectively, we systematically analyze the in-medium EMFFs and charge radii of mesons with various quark flavors. Our findings show that the EMFFs of charged (neutral) mesons exhibit a faster fall-off (increase) with increasing four-momentum transfer squared and nuclear density. Consequently, the absolute value of the charge radii of mesons increases with nuclear density, where the rate of increase depends on their quark flavor contents. We observe that the EMFFs of pions and kaons undergo significant modifications in the nuclear medium, while heavy-light mesons are only slightly modified. By decomposing the quark flavor contributions to EMFFs, we show that the medium effects primarily impact the light-quark sector, leaving the heavy-quark sector nearly unaffected. The results of this study further suggest the importance of the medium effects at the quark level.

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arXiv2024

$η$ and $η'$ mesons in nuclear matter and nuclei

We present updated and extended results for the $η$- and $η'$-nucleus bound state energies, obtained by solving the Schrödinger and Klein-Gordon equations with complex optical potentials, for a wide range of nuclei. The $η$ and $η'$ nuclear potentials are obtained in the local density approximation from the mass shift of these mesons in nuclear matter, which is calculated within the quark-meson coupling model. Our results show that the $η$ and $η'$ mesons are expected to form mesic nuclei with all the nuclei considered. However, the signal for the formation of the $η$- and $η'$-mesic nuclei may be difficult to identify experimentally due to possible large widths.

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arXiv2023

In-medium properties of the light and heavy-light mesons in a light-front quark model

We investigate the in-medium properties of pseudoscalar and vector mesons with the light-light and heavy-light quarks in a light-front quark model, using the in-medium quark properties computed by the quark-meson coupling model. Both models are constructed on an equal footing with the constituent quark degree of freedom. Here, we particularly focus on the weak decay constants and distribution amplitudes (DAs) of the mesons in symmetric nuclear matter. We find that the weak decay constants decrease as nuclear density increases for $π$, $K$, $D$, and $B$ pseudoscalar as well as $ρ$, $K^{*}$, $D^{*}$, and $B^*$ vector mesons, where their properties in free space have good agreement with the available experimental and lattice QCD data. A larger reduction is found for the light-light quark pseudoscalar mesons, while a smaller reduction is found for the heavy-light quark vector mesons, in particular, with the bottom quark. We discuss the effect of the vector potential on the weak decay constants and present our predictions for the in-medium modifications of DAs. Also, a comparison with the free space lattice QCD data is made.

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arXiv2022

Magnetic moments of the octet, decuplet, low-lying charm, and low-lying bottom baryons in a nuclear medium

We study the magnetic moments of the octet, decuplet, low-lying charm, and low-lying bottom baryons with nonzero light quarks in symmetric nuclear matter using the quark-meson coupling (QMC) model, which satisfies the constraint for the allowed maximum change (swelling) of the in-medium nucleon size derived from the $y$-scaling data for $^3$He$(e,e')$ and $^{56}$Fe$(e,e')$. The present QMC model also satisfies the expected allowed maximum enhancement of the nucleon magnetic moments in nuclear matter. Moreover, it has been proven that the calculated in-medium to free proton electromagnetic form factor (EMFF) ratios calculated within the QMC model, reproduce well the proton EMFF super ratio extracted from $^4{\rm He}(\vec{e},e'\vec{p})^3{\rm H}$ at Jefferson Laboratory (JLab). The medium modifications of the magnetic moments are estimated by evaluating the in-medium to free space baryon magnetic moment ratios to compensate the MIT bag deficiency to describe the free space octet baryon magnetic moments, where ratios are often measured directly in experiments even without knowing the absolute values, such as the free and bound proton electromagnetic form factors, as well as the European Muon Collaboration (EMC) effect to extract the structure function $F_2$ ratio of the bound to free nucleons by the corresponding cross section ratio. We also present the results calculated with the different current quark mass values for the strange and bottom quarks to see the possible impact. Furthermore, for a practical use, we give the explicit density dependent parametrizations for the vector potentials of the baryons and light-$(u, d)$ quarks, as well as for the effective masses of the baryons treated in this study, and of the mesons, $ω,ρ,K,K^*,η,η',D,D^*,B$, and $B^*$.

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arXiv2022

Nucleon Electromagnetic and Axial Form Factors in a Light-front Constituent Quark Model

In the present work we study the effect of the scalar spin coupling of constituent quarks on the nucleon electroweak properties by introducing a valence light-front wave function with two momentum scales. By comparing the results obtained with the one scale and two scale wave function models, we have found that the last one has shown a reasonable description of the static observables and $μ_pG_{Ep}/G_{Mp}$ ratio in which the position of the zero appears around 10~[GeV/c]$^2$ or for higher squared momentum transfers. We have also shown results for the axial-vector coupling $g_{A}$ and the nucleon axial-vector form factor. The best result for $g_A$ was obtained when the parameters of the nucleon wave function model were such that the experimental value of the neutron magnetic moment was described.

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arXiv2020

In-medium $J/ψ$ mass shift by the $D$ meson loop effect

By an effective Lagrangian plus QCD sum-rule approach, we investigate the mass shift of the $J/ψ$ state in medium, in symmetric nuclear matter with zero and finite temperature, and cold strange matter. The in-medium mass of the $J/ψ$ state is evaluated through the intermediate pseudoscalar $D$-meson loop for the $J/ψ$ self-energy. The effect of medium is incorporated through the in-medium mass of $D$ meson calculated using chiral SU(3) model plus QCD sum-rule approach. The self energy loop integral is regularized using the phenomenological form factor of the dipole form. We compare our results with some of the results in the literature. The present results should be helpful to understand better the expected data from heavy ion collision experiments, such as CBM and PANDA.

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arXiv2016

Pion Electromagnetic Form Factor at Lower and Higher Momentum Transfer

The pion electromagnetic form factor is calculated at lower and higher momentum transfer in order to explore constituent quark models and the differences among those models. In particular, the light-front constituent quark model is utilized here to calculate the pion electromagnetic form factor at lower and higher energies. The matrix elements of the electromagnetic current, are calculated with both "plus" and "minus" components of the electromagnetic current in the light-front. Further, the electromagnetic form factor is compared with other models in the literature and experimental data.

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arXiv2016

Parton Distribution in Pseudoscalar Mesons with a Light-Front Constituent Quark Model

We compute the distribution amplitudes of the pion and kaon in the light-front constituent quark model with the symmetric quark-bound state vertex function. In the calculation we explicitly include the flavor-SU(3) symmetry breaking effect in terms of the constituent quark masses of the up (down) and strange quarks. To calculate the kaon parton distribution functions~(PDFs), we use both the conditions in the light-cone wave function, i.e., when $\bar{s}$ quark is on-shell, and when $u$ quark is on-shell, and make a comparison between them. The kaon PDFs calculated in the two different conditions clearly show asymmetric behaviour due to the flavor SU(3)-symmetry breaking implemented by the quark masses.

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arXiv2015

Pion in the Medium with a Light-Front Model

The pion properties in symmetric nuclear matter are investigated with the Quark-Meson Coupling (QMC) Model plus the light-front constituent quark model~(LFCQM). The LFCQM has been quite successful in describing the properties of pseudoscalar mesons in vacuum, such as the electromagnetic elastic form factors, electromagnetic radii, and decay constants. We study the pion properties in symmetric nuclear matter with the in-medium input recalculated through the QMC model, which provides the in-medium modification of the LFCQM.

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arXiv2013

Effects of the density-dependent weak form factors on the neutrino reaction via neutral current for the nucleon in nuclear medium and $^{12}$C

The nucleon form factors in free space are usually thought to be modified when a nucleon is bound in a nucleus or immersed in a nuclear medium. We investigate effects of the density-dependent axial and weak-vector form factors on the electro-neutrino ($ν_e$) and anti-electro-neutrino $({\bar ν_e})$ reactions via neutral current (NC) for a nucleon in nuclear medium or $^{12}$C. For the density-dependent form factors, we exploit the quark-meson-coupling (QMC) model, and apply them to the $ν_e$ and ${\bar ν_e}$ induced reactions by NC. About 12% decrease of the total cross section by $ν_e$ reaction on the nucleon is obtained at normal density, $ρ= ρ_0 \sim 0.15 {fm}^{-3} $, as well as about 18% reduction of total $ν_e$ cross section on $^{12}$C, by the modification of the weak form factors of the bound nucleon. However, similarly to the charged current reaction, effects of the nucleon property change in the ${\bar ν}_e$ reaction reduce significantly the cross sections about 30% for the nucleon in matter and $^{12}$C cases. Such a large asymmetry in the ${\bar ν}_e$ cross sections is addressed to originate from the different helicities of ${\bar ν}_e$ and $ν_e$.

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arXiv2013

Asymmetry in the neutrino and anti-neutrino reactions in a nuclear medium

We study the effect of the density-dependent axial and vector form factors on the electro-neutrino ($ν_e$) and anti-neutrino $({\bar ν}_e)$ reactions for a nucleon in nuclear matter or in $^{12}$C. The nucleon form factors in free space are presumed to be modified for a bound nucleon in a nuclear medium. We adopt the density-dependent form factors calculated by the quark-meson coupling (QMC) model, and apply them to the $ν_e$ and ${\bar ν}_e$ induced reactions with the initial energy $E = $ 8 $\sim$ 80 MeV. We find that the total $ν_e$ cross sections on $^{12}$C as well as a nucleon in nuclear matter are reduced by about 5% at the nuclear saturation density, $ρ_0$. This reduction is caused by the modification of the nucleon structure in matter. Although the density effect for both cases is relatively small, it is comparable with the effect of Coulomb distortion on the outgoing lepton in the $ν$-reaction. In contrast, the density effect on the ${\bar ν}_e$ reaction reduces the cross section significantly in both nuclear matter and $^{12}$C cases, and the amount maximally becomes of about 35% around $ρ_0$. Such large asymmetry in the $ν_e$ and ${\bar ν}_e$ cross sections, which seems to be nearly independent of the target, is originated from the difference in the helicities of ${\bar ν}_e$ and $ν_e$. It is expected that the asymmetry influences the r-process and also the neutrino-process nucleosynthesis in core-collapse supernovae.

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