arXiv subjects

K. Tsushima

Publications and source records attributed to K. Tsushima.

At least 127 records · Page 7Linked to original sources

arXiv2003

Near-threshold $ω$ and $ϕ$ meson productions in $pp$ collisions

Using a relativistic effective Lagrangian at the hadronic level, near-threshold $ω$ and $ϕ$ meson productions in proton proton ($pp$) collisions, $p p \to p p ω/ϕ$, are studied within the distorted wave Born approximation. Both initial and final state $pp$ interactions are included. In addition to total cross section data, both $ω$ and $ϕ$ angular distribution data are used to constrain further the model parameters. For the $p p \to p p ω$ reaction we consider two different possibilities: with and without the inclusion of nucleon resonances. The nucleon resonances are included in a way to be consistent with the $π^- p \to ωn$ reaction. It is shown that the inclusion of nucleon resonances can describe the data better overall than without their inclusion. However, the SATURNE data in the range of excess energies $Q < 31$ MeV are still underestimated by about a factor of two. As for the $p p \to p p ϕ$ reaction it is found that the presently limited available data from DISTO can be reproduced by four sets of values for the vector and tensor $ϕNN$ coupling constants. Further measurements of the energy dependence of the total cross section near threshold energies should help to constrain better the $ϕNN$ coupling constant.

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arXiv2003

Bound Nucleon Form Factors, Quark-Hadron Duality, and Nuclear EMC Effect

We discuss the electromagnetic form factors, axial form factors, and structure functions of a bound nucleon in the quark-meson coupling (QMC) model. Free space nucleon form factors are calculated using the improved cloudy bag model (ICBM). After describing finite nuclei and nuclear matter in the quark-based QMC model, we compute the in-medium modification of the bound nucleon form factors in the same framework. Finally, limits on the medium modification of the bound nucleon $F_2$ structure function are obtained using the calculated in-medium electromagnetic form factors and local quark-hadron duality.

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arXiv2003

Systematic Regge theory analysis of omega photoproduction

Systematic analysis of available data for $ω$-meson photoproduction is given in frame of Regge theory. At photon energies above 20 GeV the $γ{+}p{\to}ω{+}p$ reaction is entirely dominated by Pomeron exchange. However, it was found that Pomeron exchange model can not reproduce the $γ{+}p{\to}ρ{+}p$ and $γ{+}p{\to}ω{+}p$ data at high energies simultaneously with the same set of parameters. The comparison between $ρ$ and $ω$ data indicates a large room for meson exchange contribution to $ω$-meson photoproduction at low energies. It was found that at low energies the dominant contribution comes from $π$ and $f_2$-meson exchanges. There is smooth transition between the meson exchange model at low energies and Regge theory at high energies.

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arXiv2002

Properties of charmed and bottom hadrons in nuclear medium: Results for $Λ_c^+$ and $Λ_b$ hypernuclei

This reports our recent studies on changes in properties of heavy hadrons containing at least a charm or a bottom quark in nuclear matter, and that the results for the $Λ^+_c$ and $Λ_b$ hypernuclei are studied quantitatively. Comparisons are made with the results for the $Λ$ hypernuclei studied previously in the same approach. It is shown that although the scalar and vector potentials for the $Λ$, $Λ_c^+$ and $Λ_b$ in the hypernuclei multiplet with the same baryon numbers are quite similar, the wave functions obtained, e.g., for $1s_{1/2}$ state, are very different. The $Λ^+_c$ probability density distribution in $^{209}_{Λ^+_c}$Pb is much more pushed away from the center than that for the $Λ$ in $^{209}_Λ$Pb due to the Coulomb force. On the contrary, the $Λ_b$ probability density distributions in $Λ_b$ hypernuclei are much larger near the origin than those for the $Λ$ in the $Λ$ hypernuclei due to its heavy mass. A possibility of $B^-$ nuclear bound (atomic) states is also briefly discussed.

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arXiv2002

Near-threshold $ω$-meson production in proton-proton collisions: With or without resonance excitations ?

We present results for the $p p \to p p ω$ reaction studied by considering two different scenarios: with and without the inclusion of nucleon resonance excitations. The recently measured angular distribution by the COSY-TOF Collaboration at an excess energy of $Q = 173$ MeV and the energy dependence of the total cross section data for $π^- p \to ωn$ are used to calibrate the model parameters. The inclusion of nucleon resonances improves the theoretical prediction for the energy dependence of the total cross section in $pp \to ppω$ at excess energies $Q < 31$ MeV. However, it still underestimates the data by about a factor of two, and remains a problem in understanding the reaction mechanism.

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arXiv2002

$Λ^+_c$- and $Λ_b$-hypernuclei

$Λ^+_c$- and $Λ_b$-hypernuclei are studied in the quark-meson coupling (QMC) model. Comparisons are made with the results for $Λ$-hypernuclei studied in the same model previously. Although the scalar and vector potentials felt by the $Λ$, $Λ_c^+$ and $Λ_b$ in the corresponding hypernuclei multiplet which has the same baryon numbers are quite similar, the wave functions obtained, e.g., for $1s_{1/2}$ state, are very different. The $Λ^+_c$ baryon density distribution in $^{209}_{Λ^+_c}$Pb is much more pushed away from the center than that for the $Λ$ in $^{209}_Λ$Pb due to the Coulomb force. On the contrary, the $Λ_b$ baryon density distributions in $Λ_b$-hypernuclei are much larger near the origin than those for the $Λ$ in the corresponding $Λ$-hypernuclei due to its heavy mass. It is also found that level spacing for the $Λ_b$ single-particle energies is much smaller than that for the $Λ$ and $Λ^+_c$.

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arXiv2002

Properties of charmed and bottom hadrons in nuclear matter: A plausible study

Changes in properties of heavy hadrons with a charm or a bottom quark are studied in nuclear matter. Effective masses (scalar potentials) for the hadrons are calculated using quark-meson coupling model. Our results also suggest that the heavy baryons containing a charm or a bottom quark will form charmed or bottom hypernuclei, which was first predicted in mid 70's. In addition a possibility of $B^-$-nuclear bound (atomic) states is briefly discussed.

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arXiv2002

Omega photoproduction

Photoproduction of omega-meson is analyzed within meson exchange model and Regge model and compared to rho-meson photoproduction. An interplay between two models and uncertainties in data reproduction are discussed.

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arXiv2002

Hadron properties in nuclear medium and their impacts on observables

The effect of changes in hadron properties in a nuclear medium on physical observables is discussed. Highlighted results are, (1) hypernuclei, (2) meosn-nuclear bound states, (3) $K$-meson production in heavy ion collisions, and (4) $J/Ψ$ dissociation in a nuclear medium. In addition, results for the near-threshold $ω$- and $ϕ$-meson productions in proton proton collisions are reported.

hep-ph↗

arXiv2002

Quark-hadron duality and the nuclear EMC effect

Recent data on polarized proton knockout reactions off He-4 nuclei suggest a small but nonzero modification of proton electromagnetic form factors in medium. Using model independent relations derived on the basis of quark-hadron duality, we relate the medium modification of the form factors to the modification at large x of the deep-inelastic structure function of a bound proton. This places strong constraints on models of the nuclear EMC effect which assume a large deformation of the intrinsic structure of the nucleon in medium.

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arXiv2002

Omega photoproduction beyond the resonance region at small u

Within the meson exchange model we study $ω$ meson photoproduction at energies above the s channel resonance region. Different model prescriptions for the $ω{NN}$ vertex function are investigated imposing gauge invariance as well as crossing symmetry. The calculations reproduce the energy dependence of the differential $ω$ photoproduction cross sections at moderate $|u|$ for $E_γ{\le}4.7$ GeV, which previously were discussed as an indication of the hard interaction between the photon and quarks of the nucleon.

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arXiv2001

Medium modification of the nucleon axial form factor

We study the modification of the nucleon axial form factor in nuclear matter. The internal quark substructure of the nucleon is self-consistently described by the quark meson coupling model. We find that the axial form factor of the bound nucleon is quenched considerably from that of the free nucleon. The axial vector coupling constant is reduced by roughly 10% at normal nuclear matter density and the axial form factor varies within 8% for moderate momentum transfer.

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arXiv2001

Hadron Mass Extraction from Lattice QCD

The extraction of quantities from lattice QCD calculations at realistic quark masses is of considerable importance. Whilst physical quark masses are some way off, the recent advances in the calculation of hadron masses within full QCD now invite improved extrapolation methods. We show that, provided the correct chiral behaviour of QCD is respected in the extrapolation to realistic quark masses, one can indeed obtain a fairly reliable determination of masses, the sigma commutator and the J parameter. We summarise these findings by presenting the nonanalytic behaviour of nucleon and rho masses in the standard Edinburgh plot.

hep-lat↗

arXiv2001

Structure Functions of Unstable Lithium Isotopes

We study both the spin-average and spin-dependent structure functions of the lithium isotopes, $^{6-11}$Li, which could be measured at RIKEN and other nuclear facilities in the future. It is found that the light-cone momentum distribution of the valence neutron in the halo of $^{11}$Li is very sharp and symmetric around y = 1, because of the weak binding. The EMC ratios for Li isotopes are then calculated. We study the possibility of extracting the neutron structure function from data for the nuclear structure functions of the Li isotopes. Next we calculate the spin-dependent structure functions of $^{7,9,11}$Li isotopes, which have spin of 3/2. The effect of the nuclear binding and Fermi motion on the multipole spin structure function, $^{3/2}_{~1}g_1$, is about 10% in the region x < 0.7, but it becomes quite important at large x. The spin structure function of $^{3/2}_{~3}g_1$ is also investigated. Finally, we discuss the modification of the Gottfried and Bjorken integrals in a nuclear medium and point out several candidates for a pair of mirror nuclei to study the flavor-nonsinglet quark distributions in nuclei.

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arXiv2001

Distortions in the negative energy Dirac sea: violation of the Gottfried sum rule and $Δ\bar{u}$ in proton

We report on recent work concerning the effect which the change in vacuum structure (negative energy Dirac sea), in the presence of a confining scalar field, has on the nucleon structure functions and parton distributions. Using the Dirac equation in 1+1 dimensions, we show that distortions in the Dirac sea are responsible for part of the violation of the Gottfried sum rule -- i.e., part of the flavor asymmetry in the proton sea. Our basic argument is that, even if isospin is an exact symmetry, the presence of a confining potential changes the vacuum structure, and inevitably leads to a violation of SU(2) flavour symmetry in a hadron with a different number of valence $u$ and $d$ quarks. The same mechanism also leads to a prediction for $Δ\bar{u}$ and $Δ\bar{d}$.

hep-ph↗

arXiv2001

Chiral Behaviour of the Rho Meson in Lattice QCD

In order to guide the extrapolation of the mass of the rho meson calculated in lattice QCD with dynamical fermions, we study the contributions to its self-energy which vary most rapidly as the quark mass approaches zero; from the processes $ρ\to ωπ$ and $ρ\to ππ$. It turns out that in analysing the most recent data from CP-PACS it is crucial to estimate the self-energy from $ρ\to ππ$ using the same grid of discrete momenta as included implicitly in the lattice simulation. The correction associated with the continuum, infinite volume limit can then be found by calculating the corresponding integrals exactly. Our error analysis suggests that a factor of 10 improvement in statistics at the lowest quark mass for which data currently exists would allow one to determine the physical rho mass to within 5%. Finally, our analysis throws new light on a long-standing problem with the J-parameter.

hep-lat↗

arXiv2001

Pions in isospin asymmetric nuclei

Using a pair of the lightest mirror nuclei, $^3$He and $^3$H, we study the effect of the medium modification of pion fields on the flavor non-singlet structure function. The change of the pion fields leads to an enhancement of the flavor asymmetry of the antiquark distributions in a nucleus.

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arXiv2001

Non-singlet structure function of the 3He-3H system and divergence of the Gottfried integral

We study shadowing and antishadowing corrections to the flavor non-singlet structure function F_2(3He)-F_2(3H) and show that the difference between the one-particle density distributions of 3He and 3H plays an important role at very small x. We find that the flavor non-singlet structure function in these mirror nuclei is enhanced at small x by nuclear shadowing, which increases the nuclear Gottfried integral, integrated from 10^{-4} to 1, by 11-36 %. When integrated from zero, the Gottfried integral is divergent for these mirror nuclei. It seems likely that, as a consequence of charge symmetry breaking, this may also apply to the proton-neutron system.

hep-ph↗