arXiv subjects

K. Tsushima

Publications and source records attributed to K. Tsushima.

At least 19 recordsLinked to original sources

arXiv2026

Local-nuclear-density dependent calculation of nucleon electromagnetic form factor ratios in finite nuclei

We calculate the electromagnetic form factors of nucleons bound in finite nuclei and make predictions to the ratio between the electric ($G_E^*$) and magnetic ($G_M^*$) form factors in terms of the square of the four-momentum transfer $Q^2$. We extend our previous constant-density calculations within the covariant spectator-QMC framework by incorporating the spatial nuclear density profiles of finite nuclei and quantify the deviations from the average-density approximation used in our previous applications. The impact of the medium effects can then be observed considering the ratio between $G_E^*/G_M^*$ and the ratio in free space $G_E/G_M$, that define the proton electromagnetic double ratio associated with a given nucleus. The double ratio is expected to remove some systematic uncertainties. There is the expectation that ratios $G_E^*/G_M^*$ associated with the bound protons inside the nucleus will be measured in the near future in polarization-transfer experiments $(\vec{e} A, e'\! A' \vec{p})$. Anticipating future measurements on the subject, we make predictions for the double ratios, to be compared with the average measurements on the energy states of protons bound to nuclei. We consider the nuclei $^{12}$C, $^{16}$O and $^{40}$Ca. We conclude that the form factors calculated using nuclear density profile function $\rho(r)$ are less suppressed than in the case of the results calculated using the average nuclear density. The results indicate that the relative importance of the low-density surface region increases with $Q^2$, leading to a weaker suppression than that predicted by the average-density approximation. We also make predictions for the ratios associated with neutrons bound to nuclei.

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arXiv2026

Octet baryon electroweak form factors in dense nuclear matter

Motivated by the necessity of developing theoretical models for studying the electroweak structure of baryons in a nuclear medium, we apply a covariant quark model to study interactions of baryons with nuclear matter. The electromagnetic and axial form factors of the octet baryons are determined by combining a covariant quark model that takes into account the meson cloud dressing of the baryon cores, developed for free space, with the quark-meson coupling model in the extension to the nuclear medium. We discuss the medium modifications on the electroweak form factors of octet baryons for the range of densities from $\rho=0$ up to $\rho=2 \rho_0$, where $\rho_0= 0.15$ fm$^{-3}$ is the normal nuclear matter density. We also study how the shape of the form factors is modified in finite nuclei due to the profile of the nuclear density distributions compared with calculations using the average density of the nucleus

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arXiv2026

Electric and magnetic timelike form factors of hyperons at large transfer momentum

There has been considerable progress in the study of the electromagnetic form factors of baryons in the timelike region, through electron-positron scattering reactions ($e^+ e^- \to B \bar B$), in the last two decades. Timelike experiments reveal information about the distribution of charge and magnetism inside the hyperons that cannot be obtained in spacelike experiments (electron scattering on baryons). Motivated by the novel data, we extend to the timelike region, without any further parameter fitting, a covariant quark model developed for the spacelike region that takes into account the meson cloud excitations of the baryon cores. We use the formalism to calculate the electric ($G_E$) and magnetic ($G_M$) form factors of spin 1/2 baryons in the large square transfer momentum $q^2$ region. Our calculations are compared with the available data from CLEO and BESIII above $q^2=10$ GeV$^2$. We conclude that our predictions for the effective form factors (combination between $G_E$ and $G_M$) are in good agreement with the $q^2 > 15$ GeV$^2$ data for $\Lambda$, $\Sigma^+$, $\Sigma^0$, $\Xi^-$ and $\Xi^0$. Upcoming data for $\Sigma^-$ can be used to further test our predictions. We also compare our model calculations with the available data for ratio $|G_E/G_M|$. We conclude that the present $q^2$ data range is not large enough to test our calculations, but that a more definitive test can be performed by upcoming data above $q^2=20$ GeV$^2$.

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arXiv2025

In-medium mass shifts of $B_c^{()}, B_s^{()}$ and $D_s^{(*)}$ mesons

We present our predictions for the Lorentz scalar mass shifts of two-flavored heavy mesons, $B_c^{(*)}, B_s^{(*)}$ and $D_s^{(*)}$ in symmetric nuclear matter. The in-medium mass shifts are estimated by evaluating the lowest order one-loop self-energies of the mesons based on a flavor-SU(5) effective Lagrangian approach. In-medium properties necessary for the estimates are calculated by the quark-meson coupling (QMC) model. The enhanced self-energies of the mesons in symmetric nuclear matter relative to those in free space, yield the negative mass shifts of these mesons.

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arXiv2025

Two-flavored heavy mesons' nuclear bound states

We calculate the $B_c$- and $B_s$-nucleus bound state energies and coordinate space radial wave functions by solving the Klein-Gordon equation in momentum space. The attractive strong potentials for the $B_c$ and $B_s$ mesons in nuclei are calculated from the respective mass shifts of these mesons in nuclear matter using a local density approximation. This negative mass shift may be regarded as a signature of partial restoration of chiral symmetry in medium in an empirical sense, because the origin of the negative mass shift in the present study is not directly related to the chiral symmetry mechanism.

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arXiv2025

Two-flavored heavy-light mesons' nuclear bound states

We calculate the $B^-$-, $\overline{B^0}$-, $D^+$-, $D^0$-, $K^-$-, and $\overline{K^0}$-$^{12}$C bound state energies by solving the Klein-Gordon (K;G.) equation in momentum space, and also obtain the corresponding coordinate space radial wave functions. The strong Lorentz scalar and vector potentials in $^{12}$C are calculated using a local density approximation, where the scalar potentials are obtained based on the mass shifts of the respective mesons in nuclear matter. The mesons' mass shifts, the $^{12}$C nuclear density distributions, the strong potentials as well as the Coulomb potentials, are calculated by the quark-meson coupling (QMC) model.

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arXiv2025

Timelike electromagnetic form factors of hyperons at large $q^2$

In the last few years there has been considerable progress in the study of the electromagnetic form factors of baryons in the timelike region, through electron-positron scattering, with increasing squared transfer momentum $q^2$. The modulus of the electric ($G_E$) and magnetic ($G_M$) form factors has been measured for nucleons, hyperons and other baryons at BaBar, CLEO, Belle and BESIII. The novel measurements motivated the extension of a covariant quark model, developed to the spacelike region ($q^2 \le 0$), to the timelike region, without any further parameter fitting. The extension is based on asymptotic relations derived from analyticity and unitarity, valid for the large-$q^2$ region. We use the model to make predictions for the effective form factor $|G|$ (combination of $G_E$ and $G_M$) and the ratio $|G_E/G_M|$ for spin 1/2 hyperons at large $q^2$ (above 10 GeV$^2$). Our calculations are in good agreement with the data from CLEO and BESIII for $\Lambda$, $\Sigma^+$ and $\Xi^-$ above $q^2=15$ GeV$^2$. Upcoming data for $\Sigma^0$, $\Sigma^-$ and $\Xi^-$ at large $q^2$ may be used to further test our predictions. We also compare our model calculations with the scarce available data for $|G_E/G_M|$. We conclude that the present $q^2$ range is not large enough to test our calculations, but that a more definitive test can be made by experiments above $q^2=20$ GeV$^2$.

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arXiv2025

Manifestation of quark effects in nuclei via bremsstrahlung analysis in the proton-nucleus scattering

\textbf{Background} (1) The incoherent emission of photons is dominant comparing to coherent one in proton-nucleus scattering. The incoherent bremsstrahlung is very sensitive to the magnetic moments of nucleons in nuclei. (2) According to the quark-meson coupling (QMC) model, the nucleon magnetic moments in nuclei are enhanced relative to those in vacuum, originating from the quark structure of nucleons. \textbf{Purpose} Investigate possibilities of observing quark effects in nuclei by the analysis of bremsstrahlung in nuclear reactions. \textbf{Methods} Analyse the bremsstrahlung cross sections with established model in proton-nucleus scattering, by extending with inclusion of in-medium modified nucleon magnetic moments in nuclei by the QMC model. \textbf{Results} (1) After calibrating the model without the quark effects for experimental data (TAPS Collaboration data for $p + \isotope[197]{Au}$), we calculate the cross sections and observe the slight difference between the spectra for models with and without quark effects. Such result is found for the first time, confirming possibilities of observing the quark effects in the spectra of bremsstrahlung. (2) As found, quark effects are not enough to be observed in middle and heavy nuclei, as they have dominant incoherent contributions. (3) \isotope[18]{C} has minimal incoherent contribution concerning other carbon isotopes, where the quark effects should be minimal. In ratios between the spectra for \isotope[18]{C} and \isotope[12]{C} with and without the quark effects the difference is clearly observed. \textbf{Conclusions} We establish the new physical observable for the quark effects in nuclei in the bremsstrahlung accompanied in the nuclear reactions, which can be measured. The present suggestion is for the first time in both theoretically and experimentally to study the quark effects in nuclei via the bremsstrahlung.

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arXiv2025

$B_c^{\pm}$-$^{12}$C states and detailed study of momentum space method for $Υ$- and $η_b$-nucleus bound states

We perform a detailed study of the $Υ$-, $η_b$-, and $B_c$-nucleus systems in momentum space to calculate the bound-state energies and the corresponding coordinate-space radial wave functions. The attractive strong potentials for the meson-nucleus systems are calculated from the Lorentz scalar mass modifications of these mesons in nuclear matter in the local density approximation in the nucleus. The downward shift of the meson masses may be regarded as a signature of partial restoration of chiral symmetry in a nuclear medium applied in the present study in an empirical sense, because the origin of the negative mass shift in this study is not directly related to the chiral symmetry mechanism. Furthermore, as an initial and realistic study, the $B_c^{\pm}$-$^{12}$C bound states are studied for the first time, with the effects of self-consistently calculated Coulomb potentials in $^{12}$C (when the $B_c^{\pm}$ mesons are absent).

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arXiv2025

Electroweak form factors of baryons in dense nuclear matter

There is evidence that the properties of hadrons are modified in a nuclear medium. Information about the medium modifications of the internal structure of hadrons is fundamental for the study of dense nuclear matter and high-energy processes, including heavy-ion and nucleus--nucleus collisions. At the moment, however, empirical information about medium modifications of hadrons is limited; therefore, theoretical studies are essential for progress in the field. In the present work, we review theoretical studies of the electromagnetic and axial form factors of octet baryons in symmetric nuclear matter. The calculations are based on a model that takes into account the degrees of freedom revealed in experimental studies of low and intermediate square transfer momentum $q^2=-Q^2$: valence quarks and meson cloud excitations of baryon cores. The formalism combines a covariant constituent quark model, developed for a free space (vacuum) with the quark--meson coupling model for extension to the nuclear medium. We conclude that the nuclear medium modifies the baryon properties differently according to the flavor content of the baryons and the medium density. The effects of the medium increase with density and are stronger (quenched or enhanced) for light baryons than for heavy baryons. In particular, the in-medium neutrino--nucleon and antineutrino--nucleon cross-sections are reduced compared to the values in free space. The proposed formalism can be extended to densities above the normal nuclear density and applied to neutrino--hyperon and antineutrino--hyperon scattering in dense nuclear matter.

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arXiv2025

Weak interaction axial form factors of the octet baryons in nuclear medium

We study the axial-vector and the induced pseudoscalar form factors associated with the weak transitions between the octet baryon members in nuclear medium, using a covariant constituent quark model. We extend previous calculations of the axial transition form factors from the vacuum (free space) to the nuclear medium (symmetric nuclear matter). The extension of the model to the nuclear medium takes into account the modifications of the properties of hadrons in the medium (masses and coupling constants), as determined by the quark-meson coupling model. The axial-vector ($G_A$) and the induced pseudoscalar ($G_P$) form factors are evaluated for different values of the nuclear density $ρ$ in terms of the square transfer momentum $q^2= -Q^2$. We conclude that, in general, the $G_A$ and $G_P$ form factors are reduced in the nuclear medium. The reduction is stronger for light baryons and high densities. The medium modifications are milder for the heavier octet baryons, particularly at large $Q^2$. The calculations presented here can be used to estimate the cross sections of neutrino and antineutrino scattering with nucleus, and neutrino and antineutrino scattering with hyperons bound to a nucleus, as well as those in the cores of compact stars.

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arXiv2024

In-medium mass shift of two-flavored heavy mesons, $B_c$, $B^_c$, $B_s$, $B^_s$, $D_s$ and $D^*_s$

For the first time, we estimate the in-medium mass shift of the two-flavored heavy mesons $B_c, B_c^*, B_s, B_s^*, D_s$ and $D_s^*$ in symmetric nuclear matter. The estimates are made by evaluating the lowest order one-loop self-energies. The enhanced excitations of intermediate state heavy-light mesons in symmetric nuclear matter are the origin of their negative mass shift. This negative mass shift may be regarded as a signature of partial restoration of chiral symmetry in an empirical sense because the origin of the negative mass shift in the study is not directly related to the chiral symmetry mechanism. Our results show that the magnitude of the mass shift for the $B_c$ meson ($\bar{b} c$ or $b \bar{c}$) is larger than those of the $η_c (\bar{c} c)$ and $η_b (\bar{b} b)$, different from a naive expectation that it would be in between them. While, that of the $B_c^*$ shows the in between of the $J/ψ$ and $Υ$. We observe that the lighter vector meson excitation in each meson self-energy gives a dominant contribution for the corresponding meson mass shift, $B_c, B_s,$ and $D_s$.

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arXiv2024

Electromagnetic $|G_E/G_M|$ ratios of hyperons at large timelike $q^2$

In recent years, it has become possible to measure not only the magnitude of the electric ($G_E$) and magnetic ($G_M$) form factors of spin $\frac{1}{2}$ baryons, but also to measure the relative phases of those quantities in the timelike kinematic region. Aiming to interpret present $|G_E/G_M|$ data on hyperons of the baryon octet, as well as to predict future data, we present model calculations of that ratio for large invariant 4-momentum square $q^2$ in the timelike region ($q^2>0$). Without any further parameter fitting,we extend to the timelike region a covariant quark model previously developed to describe the kinematic spacelike region ($q^2 \le 0$) of the baryon octet form factors. The model takes into account both the effects of valence quarks and the excitations of the meson cloud which dresses the baryons. This application to the timelike region assumes an approximation based on unitarity and analyticity that is valid only in the large $q^2$ region. Using the recent data from BESIII we establish the regime of validity of this approximation. We report here that our results for the effective form factor (combination of $|G_E|$ and $|G_M|$) are in good agreement with the data already for $q^2$ values above 15 GeV$^2$. In addition, a more conservative onset of the validity of the approximation is provided by the newly available $|G_E/G_M|$ data which suggest that our predictions may be compared against data for $q^2 \ge $ 20 GeV$^2$. This is expected in the near future, when the range of the present measurements is expanded to the 20--50 GeV$^2$ region.

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arXiv2024

Kaon Structure in the Nuclear Medium within the Light Front Approach

We study the properties of the charged kaon in symmetric nuclear matter using a Bethe-Salpeter amplitude to model the quark-anti-quark bound state, which is well constrained by previous studies of its vacuum properties. The electromagnetic form factor, charge radius, decay constant and the light-front valence component probability are investigated in symmetric nuclear matter. In order to describe the constituent up and anti-strange quarks in nuclear matter, we adopt the ``quark-meson coupling (QMC) model", which has been widely applied to various hadronic and nuclear phenomena in the nuclear medium.

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arXiv2023

Meson cloud contributions to the Dalitz decays of decuplet to octet baryons

We study the role of the meson cloud on the electromagnetic transitions from decuplet ($B'$) to octet ($B$) baryons in terms of the squared four-momentum transfer $q^2$. In the quark model framework, the meson cloud dressing of the quark cores gives important contributions to the $γ^\ast N \to Δ(1232)$ transition form factors. In the present work, we estimate the meson cloud contributions of all decuplet to octet baryon transitions ($γ^\ast B \to B'$ or $B' \to γ^\ast B$). Models that combine valence quark effects with pion and kaon cloud dressing provide a fair description of the radiative decays of decuplet to octet baryons, namely the $Σ^0(1385) \to γΛ(1116)$ and $Σ^+(1385) \to γΣ^+ (1193)$ decays. Previous studies indicated the relevance of the pion cloud effects on the $B^\prime \to γ^\ast B$ transition, but also suggested that the kaon cloud contributions may be important in the timelike region. We combine then the contributions of the bare core, estimated by a covariant quark model, with $q^2$-dependent contributions of pion and kaon clouds. We use the framework to calculate the Dalitz decay rates and the Dalitz decay widths of decuplet baryons in octet baryons with di-electrons ($B' \to e^+ e^- B$) or di-muons ($B' \to μ^+ μ^- B$). We conclude, based on the magnitude of our results, that most estimates of the $B' \to e^+ e^- B$ Dalitz decay widths may be tested at HADES and PANDA (GSI) in a near future. We discuss also the possibility of measuring the $Δ(1232) \to μ^+ μ^- N$ and $Σ^0 (1385) \to μ^+ μ^- Λ(1116)$ decay widths in some facilities, based on the estimated branching ratios.

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arXiv2022

$Υ$ and $η_b$ nuclear bound states

$Υ$ and $η_b$ nuclear bound state energies are calculated for various nuclei neglecting any possible effects of the widths. Essential input for the calculations, namely the medium-modified $B$ and $B^{*}$ meson masses, as well as the density distributions in nuclei, are calculated within the quark-meson coupling (QMC) model. The attractive potentials for the $Υ$ and $η_b$ mesons in nuclei are calculated from the mass shifts of these mesons in nuclear matter in the local density approximation. These potentials originate from the in-medium enhanced $B\overline{B}$ and $BB^{*}$ loops in their respective self energy. After an extensive analysis we conclude that our results suggest that the $Υ$ and $η_b$ mesons should form bound states with all the nuclei considered.

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arXiv2021

Hidden charm mesons in nuclear matter and nuclei

Recent results for the $η_c$- and $J/ψ$-nucleus bound state energies for various nuclei are presented. The attractive potentials for the $η_c$ and $J/ψ$ mesons in the nuclear medium originate, respectively, from the in-medium enhanced $DD^{*}$ and $D\bar{D}$ loops in the $η_c$ and $J/ψ$ self energies. Our results suggest that the $η_c$ and $J/ψ$ mesons should form bound states with all the nuclei considered.

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arXiv2021

Exploring the flavor content of light and heavy-light pseudoscalars

The electroweak properties of light and charmed D and Ds pseudoscalar mesons are investigated within a unified covariant constituent quark model. The quark-antiquark-meson vertices are assumed to have a symmetric form by the exchange of quark momenta, which is successful in describing the light pseudoscalar meson properties. The flavor decomposition of the elastic electromagnetic form factors, electromagnetic charge radii, and weak decay constants are calculated. Based on the results a discussion on the SU(3) and SU(4) symmetry breaking is made and a comparison with the pion and kaon properties to highlight the Higgs contribution to the structure of these mesons.

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