arXiv subjects

Qiang Zhao

Publications and source records attributed to Qiang Zhao.

225 records · Page 13Linked to original sources

arXiv2001

Non-diffractive mechanisms in the $ϕ$ meson photoproduction on nucleons

We examine the non-diffractive mechanisms in the $ϕ$ meson photoproduction from threshold up to a few GeV using an effective Lagrangian in a constituent quark model. The new data from CLAS at large angles can be consistently accounted for in terms of {\it s}- and {\it u}-channel processes. Isotopic effects arising from the reactions $γp\to ϕp$ and $γn\to ϕn$, are investigated by comparing the cross sections and polarized beam asymmetries. Our result highlights an experimental means of studying non-diffractive mechanisms in $ϕ$ meson photoproduction.

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arXiv2001

Isospin Effect on the Process of Multifragmentation and Dissipation at Intermediate Energy Heavy Ion Collisions

In the simulation of intermediate energy heavy ion collisions by using the isospin dependent quantum molecular dynamics, the isospin effect on the process of multifragmentation and dissipation has been studied. It is found that the multiplicity of intermediate mass fragments $N_{imf}$ for the neutron-poor colliding system is always larger than that for the neutron-rich system, while the quadrupole of single particle momentum distribution $Q_{zz}$ for the neutron-poor colliding system is smaller than that of the neutron-rich system for all projectile-target combinations studied at the beam energies from about 50MeV/nucleon to 150MeV/nucleon. Since $Q_{zz}$ depends strongly on isospin dependence of in-medium nucleon-nucleon cross section and weakly on symmetry potential at the above beam energies, it may serve as a good probe to extract the information on the in-medium nucleon-nucleon cross section. The correlation between the multiplicity $N_{imf}$ of intermediate mass fragments and the total numer of charged particles $N_c$ has the behavior similar to $Q_{zz}$, which can be used as a complementary probe to the in-medium nucleon-nucleon cross section.

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arXiv2001

Entrance channel dependence and isospin dependence of preequilibrium nucleon emission in intermediate energy heavy ion collisions

Using isospin dependent quantum molecular dynamical model, the studies of the isospin effect on preequilibrium nucleon emission in heavy ion collisions under different entrance channel conditions show that the ratio of preequilibrium neutron number to proton number depends strongly on symmetry potential, beam energy, and the ratio of neutron to proton of the colliding system, but weakly on isospin dependent in-medium nucleon-nucleon cross sections, impact parameter, Pauli potential, and momentum dependent interaction in the energy region from 45MeV/u up to 150 MeV/u where the dynamics is dominated by nucleon-nucleon collisions. In addition, the ratio of preequilibrium neutron number to proton number for a neutron-rich colliding system is larger than the initial value of the ratio of the colliding system, but the ratio for a neutron-deficient system is less than the initial value.

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arXiv2001

Nuclear Stopping as A Probe to In-medium Nucleon-nucleon Cross Section in Intermediate Energy Heavy Ion Collisions

Using an isospin-dependent quantum molecular dynamics, nuclear stopping in intermediate heavy ion collisions has been studied. The calculation has been done for colliding systems with different neutron-proton ratios in beam energy ranging from 15MeV/u to 150MeV/u. It is found that, in the energy region from above Fermi energy to 150MeV/u, nuclear stopping is very sensitive to the isospin dependence of in-medium nucleon-nucleon cross section, but insensitive to symmetry potential. From this investigation, we propose that nuclear stopping can be used as a new probe to extract the information on the isospin dependence of in-medium nucleon-nucleon cross section in intermediate energy heavy ion collisions.

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arXiv2000

Nucleonic resonance excitations with linearly polarized photon in $γp\to ωp$

In this work, an improved quark model approach to the $ω$ meson photo-production with an effective Lagrangian is presented. The {\it t}-channel {\it natural}-parity exchange is taken into account through the Pomeron exchange, while the {\it unnatural}-parity exchange is described by the $π^0$ exchange. With a very limited number of parameters, the available experimental data in the low energy regime can be consistently accounted for. We find that the beam polarization observables show sensitivities to some {\it s}-channel individual resonances in the $SU(6)\otimes O(3)$ quark model symmetry limit. Especially, the two resonances $P_{13}(1720)$ and $F_{15}(1680)$, which belong to the representation $[{\bf 56, ^2 8}, 2, 2, J]$, have dominant contributions over other excited states. Concerning the essential motivation of searching for "missing resonances" in meson photo-production, this approach provides a feasible framework, on which systematic investigations can be done.

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arXiv1998

Vector Meson Photoproduction with an Effective Lagrangian in the Quark Model

A quark model approach to the photoproduction of vector mesons off nucleons is proposed. Its starting point is an effective Lagrangian of the interaction between the vector meson and the quarks inside the baryon, which generates the non-diffractive s- and u- channel resonance contributions. Additional t-channel $π^0$ and $σ$ exchanges are included for the $ω$ and $ρ^0$ production respectively to account for the large diffractive behavior in the small $t$ region as suggested by Friman and Soyeur. The numerical results are presented for the $ω$ and $ρ$ productions in four isospin channels with the same set of parameters, and they are in good agreement with the available data not only in $ω$ and $ρ^0$ productions but also in the charged $ρ$ productions where the additional t-channel $σ$ exchange does not contribute so that it provides an important test to this approach. The investigation is also extended to the $ϕ$ photoproduction, and the initial results show that the non-diffractive behavior of the $ϕ$ productions in the large $t$ region can be described by the s- and u- channel contributions with significantly smaller coupling constants, which is consistent with the findings in the similar studies in the QHD framework. The numerical investigation has also shown that polarization observables are essential for identifying so-called "missing resonances".

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arXiv1998

Vector Meson Photoproduction with an Effective Lagrangian in the Quark Model II: $ω$ Photoproduction

An investigation of $γp\to ωp$ is presented in a constituent quark model approach. The sparse data in the large $t$ region where the resonances dominate is well described within the model, while the diffractive behavior in the small $t$ region requires an additional t-channel exchange. Taking into account the t-channel $π^0$ exchange, we find a good overall agreement with the available data with only 3 free parameters. Our study shows that the differential cross section is not sensitive to s-channel resonances, however, the polarization observables are demonstrated to be very sensitive. Thus, measuring polarization observables is the crucial part of the vector meson photoproduction program in the search for "missing" resonances.

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arXiv1998

$ω$ and $ρ$ Photoproduction with an Effective Quark Model Lagrangian

An unified approach for vector meson photoproduction is presented in the constit quark model. The s- and u-channel resonance contributions are generated using an effective quark vector-meson Lagrangian. In addition, taking into account $π^0$ and $σ$ t-channel exchanges for diffractive production, the available total and differential cross section data for $ω$, $ρ^0$, $ρ^+$, and $ρ^-$ photoproduction can be well described with the same quark model parameter set. Our results clearly indicate that polarization observables are essential to identify so-called "missing" resonances.

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arXiv1997

Vector Meson Photoproduction with an Effective Lagrangian in the Quark Model I: Formalism

A quark model approach to the photoproduction of vector mesons off nucleons is proposed. Its starting point is an effective Lagrangian of the interaction between the vector meson and the quarks inside the baryon. This paper develops the formalism and highlights the dynamical roles of s-channel resonances. It is demonstrated that the proposed approach is ideal to support searching for the missing resonances in the vector meson photoproduction experiments proposed at TJNAF.

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