In-medium NN→NΔ cross section and its dependence on effective Lagrange parameters in isospin-asymmetric nuclear matter

The in-medium NN→N△ cross sections and its differential cross sections in isospin asymmetric nuclear medium are investigated in the framework of the one-boson exchange model by including isovector mesons, i.e., δand p mesons. Our results show that the in-medium NN→N△ cross sections are suppressed when the density increases, and the differential cross sections become isotropic with an increase in the density around the △ threshold energy. The isospin splitting on the medium correction factor, R =σ_(NN→N△)~*/σ_(NN→N△)~(free) is observed for different channels of NN→N△, especially around the threshold energy for all the effective Lagrangian parameters. By analyzing the selected effective Lagrangian parameters, our results show that the larger effective mass is, the weaker medium correction R is.     

The influence of Isospin Dependence of In-Medium NN Cross Sections on the Ratio of Emitted Neutrons to Protons in HICs

The influence of isospin dependence of in-medium nucleon-nucleon cross sections on the nip ratios for emitted nucleons in reactions 96Zr-j-96Zr and 96Ru-j-96RH at Eb z 400 AMeV is investigated by means of an improved quantum molecular dynamics model. Our results show that the high energy part of the spectra of the n//p ratios for emitted nucleons is sensitive to the isospin dependence of in-medium nucleon-nucleon cross sections for neutron-rich reaction systems. Therefore, we propose that the nip ratio of emitted high energy nucleons in a very neutron-rich reaction system at several hundreds of AMeV can be taken as sensitive observables to constrain the isospindependence of in-medium nucleon-nucleon cross sections.     

K^＋ Scattering with the Nuclear Pion from Chiral Effective Lagrangian

The K^ scattering cross section with the in-medium virtual pion is evaluated in the lowest-order chiral perturbation theory with the density-dependent pion decay constant and mass.The contribution of nuclear pions to the total K^ -nucleus cross section is found to be about 5% and 12% when the excess pion numbers per nucleon nπ=0.057 and 0.13 are used.The inclusion of the off-mass-shell behavior of the K^ π amplitude produced a significant improvement in the K^ -nucleus cross section.     

Momentum dependence of pion-induced φ meson production on nuclei near threshold

We study the near-threshold pion-induced production ofφmesons off nuclei in the kinematical conditions of the HADES experiment,recently performed at GSI.The calculations have been performed within a collision model based on the nuclear spectral function.The model accounts for both the primaryπ~-meson–protonπ~-p→φn and the secondary pion–nucleonπN→φNφproduction processes as well as the effects of the nuclearφand nucleon mean-field potentials.We find that the primary reaction channelπ~-p→φn dominates in theφproduction off~(12)C and~(184)W target nuclei in the HADES acceptance window at incident pion momentum of 1.7 Ge V/c.We calculate the momentum dependence of the absolute and relative(transparency ratio)φmeson yields from the above direct channel.The calculations have been performed for this initial pion momentum allowing for different options for theφN absorption cross sectionσ_(φN )and different scenarios for the in-medium mass shifts of theφmeson and secondary neutron,produced together withφin this channel.We demonstrate that the transparency ratio for theφmesons has,contrary to the absolute cross sections,an insignificant sensitivity to theφmeson and secondary neutron in-medium mass shifts atφmomenta studied in the HADES experiment.On the other hand,we show that there are measurable changes in the transparency ratio due to theφN absorption cross section,which means that such a relative observable can be useful to help determine this cross section from the data taken in the HADES experiment.     

Charged Pion Form Factor Determination in the Range of Q2 = 0.6 ～ 1.6 (GeV/c)2

Using the most recent differential cross section data for ep quasi-elastic scattering, the charged pion formation and its form factor Fπ is calculated in the energy range of 2.4 ～ 4 GeV at Q2 = 0.6 ～ 1.6 (GeV/c)2. The functional dependence of the charged pion form factor to the separated cross section σL is investigated and compared to the previously determined result.     

Charged Pion Form Factor Determination in the Range of Q^2= 0.6-1.6 （GeV／c）^2

Using the most recent differential cross section data for e-p quasi-elastic scattering, the charged pion formation and its form factor Fπ is calculated in the energy range of 2.4-4 GeV at Q^2 = 0.6-1.6 (GeV/c)^2. The functional dependence of the charged pion form factor to the separated cross section aL is investigated and compared to the previously determined result.     

Pion Production in Heavy-Ion Collisions in the 1 A GeV Region

Within the framework of the improved isospin dependent quantum molecular dynamics （ImIQMD） model, pion emission in heavy-ion collisions in the region 1 A GeV is investigated systematically, in which the pion is considered to be mainly produced by the decay of resonances △（1232） and N^＊（1440）. The in-medium dependence and Coulomb effects of pion production are included in the calculation. Total pion multiplicity and π^-/π^＋ yields are calculated for the reaction ^197Au＋^197Au in central collisions for selected Skyrme parameters SkP, SLy6, Ska, SⅢ and compared with the measured data of the FOPI collaboration.     

Precise measurement of the e^＋e^- →π^＋π^-（γ） cross section with the initial state radiation method at BABAR

We present a precise BABAR measurement on the cross section of the process e^＋e^- →π^＋π^-（γ） from threshold to an energy of 3 GeV with the initial state radiation （ISR） technique, using 232 fb^-1 of data collected with the BABAR detector at e^＋e^- center-of-mass energies near 10.58 GeV. The ISR luminosity is determined from a study of the leptonic process e^＋e^- →μ^＋μ^-γ（γ）. The leading-order hadronic contribution to the muon magnetic anomaly calculated using the ππ cross section measured from threshold to 1.8 GeV is （514.1± 2.2（stat） ± 3.1（syst）） × 10^-10.     

Isospin Effect on Nuclear Stopping in Intermediate Energy Heavy Ion Collisions

By using the isospin-dependent quantum molecular dynamics model,we study the dependence of nuclear stopping Qzz/A and R in intermediate energy heavy ion collisions on system size,initial N/Z,isospin symmetry potential and the medium correction of two-body cross sections.We find the effect of the initial N/Z ratio and isospin symmetry potential on stopping is weak.The excitation function of Qzz/A and R depends on the form of medium correction of two-body cross sections,the equation of state of nuclear matter.Our results show that the behaviour of the excitation function of Qzz/A and R can provide clearer information of the isospin dependence of the medium correction of two-body cross sections.     

Precise measurement of the e~+e~- →π~+π~-(γ) crosssection with the initial state radiation method at BABAR

We present a precise BABAR measurement on the cross section of the process e+e-→π+π-(γ) from threshold to an energy of 3 GeV with the initial state radiation(ISR) technique,using 232 fb-1 of data collected with the BABAR detector at e+e-center-of-mass energies near 10.58 GeV.The ISR luminosity is determined from a study of the leptonic process e+e-→μ+μ-γ(γ).The leading-order hadronic contribution to the muon magnetic anomaly calculated using the ππ cross section measured from threshold to 1.8 GeV is(514.1±2.2(stat)±3.1(syst))×10-10.     

Dependence of Isoscaling Parameters on Nucleon-Nucleon Cross Section and Momentum-Dependent Interaction

Inltuences of the isospin dependence of the in-medium nucleon-nucleon cross section and the momentum-dependent interaction （MDI） on the isotope scaling are investigated by using the isospin-dependent quantum molecular dynamics model （IQMD）. The results show that both the isospin dependence of the in-medium nucleon-nucleon cross section and the momentum-dependent interaction affect the isoscaling parameters appreciably and indepen- dently. The inltuence caused by the isospin dependence of two-body collision is relatively larger than that from the MDI in the mean tield. Aiming at exploring the implication of isoscaling behaviour, which the statistical equilibrium in the reaction is reached, the statistical properties in the mass distribution and the kinetic energy distribution of the fragments simulated by IQMD are presented.     

Isospin Effect of Coulomb Interaction on Momentum Dissipation in Intermediate Energy Heavy Ion Collisions

We investigate the isospin effect of Coulomb interaction on the momentum dissipation or nuclear stopping in the intermediate energy heavy ion collisions by using the isospin-dependent quantum molecular dynamics model. The calculated results show that the Coulomb interaction induces obviously the reductions of the momentum dissipation. We also find that the variation amplitude of momentum dissipation induced by the Coulomb interaction depends sensitively on the form and strength of symmetry potential. However, the isospin effect of Coulomb interaction on the momentum dissipation is less than that induced by the in-medium nucleon-nucleon cross section.In this case, Coulomb interaction does not change obviously the isospin effect of momentum dissipation induced by the in-medium two-body collision. In particular, the Coulomb interaction is preferable for standing up the isospin effect of in-medium nucleon-nucleon cross section on the momentum dissipation and reducing the isospin effect of symmetry potential on it, which is important for obtaining the feature about the sensitive dependence of momentum dissipation on the in-medium nucleon-nucleon cross section and weakly on the symmetry potential.     

Emission of Neutral Pions with High Transverse Momenta in A-A and p-p Collisions at Energies Available at the BNL Relativistic Heavy Ion Collider

Invariant neutral pion （π^0） yields in Au-Au and Cu-Cu collisions and invariant π^0 cross sections in p-p collisions are studied in the framework of a two-cylinder model. The considered distributions of neutral pions at the maximum energy of the relativistic heavy ion eollider （RFIIC） have a tail part in the region of high transverse momentum. A two-component distribution based upon the two-cylinder model is used to describe the experimental data of the PHENIX collaboration.     

Study of the process e^＋e^- →π^＋π^-π^0π^0 at energies √s 〈 1 GeV

In an experiment with the Spherical Neutral Detector at VEPP-2M collider the cross section of the process e^＋e^- →π^＋π^-π^0π^0 was measured. At energies √s 〈 920 MeV this cross section was measured for the first time. The energy dependence of the cross section is well discribed by the vector dominance model with contributions from p, p＇, p＂ mesons. The decay probability ρ→π^＋π^-π^0π^0 was found to be Bρ = （1.60±0.74±0.18） × 10^-5. The upper limit for the decay ω→π^＋π^-π^0π^0 was improved by two orders of magnitude compared to the previous measurements and is Bω 〈 2 × 10^-4 at 90% confidence level.     

Isospin Effects of the Mean Field and Two—Body Collision on the Fragmentation Process

Isospin effects of the mean field and two-body collision on the fragmentation as well as their dependences on the momentum-dependent interaction at intermediate energy heavy ion collisions are studied by using an isospindependent quantum molecular dynamics model.We find the prominent isospin effects of the multiplicity of the intermediate mass fragments Ninf,where Nimf depends sensitively on the isospin effect of the in-medium nucleonnucleon cross section and weakly on the variation of symmetry potential in the intermediate energy region.The momentum dependence interaction enhances the sensitivity of Nimf on the isospin effect of two-body collision.     

η production in proton-nucleus collisions near threshold

The η-meson production in proton-nucleus (pA) collisions near threshold is studied within a relativistic meson-exchange model. The primary production amplitude is presented in the distorted-wave impulse approximation for the nucleus with isospin 0 or 1 by assuming that N^*(1535) is excited via a meson exchange and then decays into η and nucleon pair(ηN). Taking ¹⁸O and ¹²C nuclei as examples, we evaluate the production cross sections as a function of the incident proton energy, and analyze the effects of nuclear medium and various meson-exchange contributions. Finally we discuss implications for further     

Energy Loss of Fast Quarks in Nuclear Drell-Yan Dimuon Production

The energy loss effect in nuclear matter, which is another nuclear effect apart from the nuclear effects on the patton distribution as in the deep inelastic scattering process, can be measured best by the nuclear dependence of the high energy nuclear Drell-Yan process. By means of the quark energy loss parametrization given in literature and the nuclear patton distribution extracted only with lepton-nucleus deep inelastic scattering experimental data, measured Drell-Yan production cross sections are analysed for 800-GeV protons incident on a variety of nuclear targets from FNAL E866. The average energy loss of quarks are given by fitting the Fe/Be and W/Be Drell-Yan cross section ratios versus the incident patton momentum fraction.     

Measurement of the pion form factor for Mππ^2 between 0.1 and 0.85 GeV2 with the KLOE detector

The KLOE experiment at the φ-factory DAФPNE has measured the pion form factor in the range between 0.1 〈 Mππ^2 〈 0.85 GeV^2 using events taken at √s = 1 GeV with a photon emitted at large polar angles in the initial state. This measurement extends the Mππ^2 region covered by KLOE ISR measurements of the pion form factor down to the two pion production threshold. The value obtained in this measurement of the dipion contribution to the muon anomalous magnetic moment of △αμ^ππ= （478.5±2-0stat±4.8syst±2.9theo）·10^-10 further confirms the discrepancy between the Standard Model evaluation for αμ and the experimental value measured by the （g-2） collaboration at BNL.     

Sigma Meson Production in Nuclear Reactions

The sigma meson production in p ＋ ^12C and p A- ^40Ca reactions at the incident energy Ep = 1.5 GeV is investigated within the Quantum Molecular Dynamics model. The simulation results indicate a distinctive A dependence of the sigma production, that is, the increase of A is followed by an increase of the production cross sections. We find that the σ meson production in proton-induced reactions is strongly medium-dependent, and the produced σ mesons decaying in a denser medium experience a stronger mass shift towards lower masses. This mass shift is an experimentally accessible observable in the final state pion pairs, which do not suffer from reabsorption by the surrounding nucleons. It is pointed out that the ratio of measured sigma cross sections as a function of the sigma invariant-mass from various reactions is a good probe to explore the existence of the σ meson in a dense nuclear environment.     

Hyperon Dynamics in Heavy-Ion Collisions near Threshold Energy

Within the framework of quantum molecular dynamics transport model,the isospin and in-medium effects on the hyperon production in the reaction of ~(197)Au+~(197)Au are investigated thoroughly.A repulsive hyperon-nucleon potential from the chiral effective field theory is implemented into the model,which is related to the hyperon momentum and baryon density.The correction on threshold energy of the elementary hyperon cross section is taken into account.It is found that the Σ yields are suppressed in the domain of midrapidity and kinetic energy spectra with the potential.The hyperons are emitted in the reaction plane because of the strangeness exchange reaction and reabsorption process in the nuclear medium.The Σ~-/Σ~+ ratio depends on the stiffness of nuclear symmetry energy,in particular in the high-energy region(above 500 MeV).