The two-pion exchange NN potential in nuclear matter and nuclear stability

A meson exchange model of the ππ interaction which fits free ππ scattering data is used to calculate the interactions of pions in nuclear matter as a function of nuclear density. Polarization of the nuclear medium by the pions results in a marked increase in the s-wave ππ attraction at low energy. The influence of this effect on the nucleon-nucleon interaction is a corresponding increase with density of the NN central potential due to the exchange of two correlated pions, resulting in an NN interaction which fails to saturate. A possible mechanism for restoring the theoretical stability of nuclear matter is explored and found to be effective.     

Secondary Collision and the Enhancement of the K+π+ Ratio in the High Energy Heavy-ion Collision

The enhancement of K⁺/π⁺ ratio in the high energy heavy-ion collision is believed to be a possible signature of the quark gluon plasma.Based on the Participant Nucleon Model and the concept of the formation time of secondary particles,the K⁺/π⁺ ratio in the central collision of Si(14.5GeV/c) and Au are calculated by Monte Carlo simulation.The following secondary collisions are considered.πN→K⁺Y、ππ→KK and the single charge exchange reaction between pions and nucleons.The calculation results show that the increased K⁺/π⁺ ratio caused by these secondary collisions is not enough to explain the experimental data observed by the E802 group at BNL.     

Why breakup of photons and pions into forward dijets is so different: Predictions from nonlinear nuclear <Emphasis Type="Italic">k</Emphasis><Subscript>⊥</Subscript> factorization

Based on an approach to non-Abelian propagation of color dipoles in a nuclear medium, we formulate a nonlinear k_⊥ factorization for the breakup of photons and pions into forward hard dijets in terms of the collective Weizsäcker-Williams glue of nuclei. We find quite distinct practical consequences of nonlinear nuclear k_⊥ factorization for interactions of pointlike photons and nonpointlike pions. In the former case, the large transverse momentum p_⊥ of jets comes from the intrinsic momentum of quarks and antiquarks in the photon, and nuclear effects manifest themselves as an azimuthal decorrelation with an acoplanarity momentum of the order of the nuclear saturation momentum Q _A . In the breakup of pions off free nucleons to the leading order in pQCD, the spectator parton has a small transverse momentum and the hard dijet cross section is suppressed. In the breakup of pions off heavy nuclei, the forward hard jets are predicted to be entirely decorrelated. We comment on the sensitivity of the pionic dijet cross section to the pion distribution amplitude. The predicted distinction between the breakup of photons and pions can be tested by the sphericity and thrust analysis of the forward hadronic system in the COMPASS experiment at CERN.     

Research of bb Production in the Topcolor Assisted Technicolor Model

In the framework of topcolor assisted technicolor (TC2) model, we calculate the contributions of cross section from the pseudo Goldstone bosons and new gauge bosons to e⁺e^－ →bb. We find that, within reasonable ranges of the parameters, the corrections arising from technipions and new gauge bosons are negligibly small; and the top pions afford dominate contribution, the maxim cross section corrections can reach 43% with the c.m. energy √s=500GeV; whereas in case of √s=1500GeV, the maxim cross section relative corrections is only 3.1%. It might open a window to detect topcolor assisted technicolor model in the experiment of next generation linear collider.     

Charge-constrained coherent pion states produced in nuclear matter

In a recent article, Bloss, Hone and Scalapino have used a simplified model Hamiltonian and Keldysh field theoretic methods to approximately calculate the power radiated as Cherenkov charged pions whose emission is triggered when a fast nucleon traverses nuclear matter. The model used by these authors conserves charge by flipping the isospin state of the fast nucleon alone for each charged pion emitted. Since it is the nuclear matter excited by the fast nucleon which should be largely reponsible for emitting the Cherenkov charged pions, we investigate an alternate model that allows all the disturbed nuclear matter to change its charge state when a charged pion is emitted—as an idealization, the emitting matter is assumed to have an infinite number of charge states, in contrast to the two of the fast nucleon. This modified model is shown to be exactly solvable in terms of charge-constrained coherent pion states; the power radiated as charged pions is found to be approximately twice that calculated by Bloss et al. from their model.     

Atomic mass dependence of hadron production in semi-inclusive deep inelastic lepton-nucleus scattering

Hadron production in lepton-nucleus deep inelastic scattering is studied in a quark energy loss model. The leading-order computations for hadron multiplicity ratios are presented and compared with the selected HERMES pions production data with the quark hadronization occurring outside the nucleus by means of the hadron formation time. It is found that the obtained energy loss per unit length is 0.440 ± 0.013 GeV/fm for an outgoing quark by the global fit. It is confirmed that the atomic mass number dependence of hadron attenuation is theoretically and experimentally in good agreement with the A^2/3 power law for quark hadronization occurring outside the nucleus.     

Influence of the K－ Mesons Absorption to Their Production in the Heavy-Ion Collisions

We have investigated the yield and the spectrum of the K^－ mesons in the heavy-ion collisions with the relativistic RBUU approach. It's found that the ratio of the yield of K⁺ to K^－ depends on the size of the nuclear systems. One main reason is caused by the absorption effect of the K^－ mesons in the nuclear medium. We have found that the mean-absorption path of the K^－ mesons are different for light nuclear system and for the middle or heavy nuclear systems. The extent of the absorption of K^－ mesons is different for different elementary production channels. It's one important reason to make the channel Bπ→K^－X play an important role in the K^－production, especially for light and middle systems.     

Probing nuclear expansion dynamics with π−/π+-spectra

We study the dynamics of charged pions in the nuclear medium via the ratio of differential π^? - and π⁺-spectra in a coupled-channel BUU (CBUU) approach. The relative energy shift of the charged pions is found to correlate with the pion freeze-out time in nucleus-nucleus collisions as well as with the impact parameter of the heavy-ion reaction. Furthermore, the long-range Coulomb force provides valuable information on the expansion dynamics of the hot nuclear system. Detailed comparisons with experimental data for Au + Au at 1 AGeV and Ni + Ni at 2.0 AGeV are presented.     

Backward hadron production in neutrino-nucleus interactions

The production of backward pions in lepton-nucleus collisions is analyzed. We show that a large yield of high momentum backward pions can be explained by the Regge asymptotic of the distribution of nucleons carrying a large momentum fraction in the nuclear target. The calculated spectra of pions emitted in the ν+ Ne →μ+π⁻+ X and ν+²H →μ+π⁻+ X reactions are in satisfactory agreement with the available experimental data. Received: 22 July 1999 / Revised version: 22 November 1999     

Meson-exchange currents in pion-nucleus double-charge-exchange reactions

The scattering of pions by virtual mesons in the nuclear medium is analysed and its contribution to the process of double-charge exchange in pion-nucleus scattering compared to the conventional mechanism which involves charge exchange in the scattering of the pions with two different nucleons. It is shown that at forward angles the cross section is decreased by about 50% around pion energies T ≈ 130 MeV and increased by a similar amount at energies above the resonance, T ≈ 250 MeV. At large angles the meson-exchange contribution changes the cross section appreciably and can become the dominant mechanism.     

Anisotropy of the Pion Emission and the Nuclear Equation of State

The anisotropy of the pion emission in the collisions of 1.8AGeV Ar+Pb is studied with the VUU model to obtain the information of the nuclear equation of state.We point out that the collective motion of pions in the final state has the final state has the characteristic similar to that of collective correlations of baryons,and the azimuthal anisotropy of pions is a sensitive parameter to the nuclear equation of state in the projectile rapidity region where the absorption of target spectator is weak.Collisions of 1.8AGeV Ar+Pb in the Bevalac streamer chamber are analyzed using the azimuthal correlation function for pions and the results favor the hard nuclear incompressibility.     

Three pion production on nuclei at 200 GeV

We have carried out a systematic study of the coherent dissociation of pions into 3 pions using nuclear targets. The experiment was performed at Fermilab using a high resolution forward spectrometer. Data were taken with carbon, copper and lead targets at an incident momentum of 202.5 GeV/c. Results are presented on momentum transfers, 3-pion masses, and on the nuclearA-dependence of the production cross section.     

Fission studies of gold induced by (1665 MeV) π- using a CR-39 detector

The fission cross section and fission probability of ¹⁹⁷Au, induced by (1665 MeV) π^-, have been studied using CR-39 track detectors. A 4π-geometry was used to count track statistics. A beam of negative pions of 1665 MeV was produced at AGS of Brookhaven National Laboratory, USA, and allowed to fall normally on the stack. Two detectors from the stack were scanned for fission fragment tracks after etching in 6N NaOH at 70℃. The statistics of fission fragment tracks in both detectors were obtained. It was found that there was a marked asymmetry of registered tracks with respect to the forward and backward hemispheres. This asymmetry could be partly accounted for on the basis of momentum transfer to the struck nucleus. On the basis of counting statistics fission cross section was measured, and fission probability was determined by dividing the fission cross section with the reaction cross section. The fission cross-section and fission probability were compared with the computed values using the cascade-exciton model code CEM95.     

The pion mass difference in threshold pion reactions on light nuclei

We investigate the effect of the pion mass difference in scattering and photoproduction of pions on light nuclei near threshold. The effect is found to be most easily described in a nuclear K-matrix formalism where the nuclear K-matrix elements are shown to be independent of the mass difference.     

Possible existence of finite formation time of strongly interacting plasma in nuclear collisions at RHIC and LHC

We argue that, on the basis of recent experimental data, there is possible existence of a finite formation time of strongly interacting plasma in nuclear collisions at RHIC. To show this, we construct a simple model based on a Monte Carlo simulation of nucleus—nucleus collisions with a realistic nuclear density distribution. The most striking feature of the experimental data—an absence of absorption of high-transverse-momentum pions in the reaction-plane direction for midperipheral collisions—points to the presence of a surface zone with no absorption and strong suppression in the inner core. A natural interpretation of such a zone could be the plasma formation time T ≃ 2–3 fm/c. With this assumption, we describe the angular anisotropy of high-transverse-momentum pions with respect to the reaction plane and the centrality dependence of the nuclear modification factor in Au + Au and Cu + Cu collisions.We present predictions for LHC. The text was submitted by the author in English.     

Nuclear relativistic Hartree-Fock approximation with weakened pion tensor force

Properties linked to the single-particle energies, as nuclear spectra, spin-orbit splittings and shell gaps are investigated in the framework of the relativistic Hartree-Fock approximation with pseudovector coupling for the πN vertex. The role of an effective mass of pions moving in the nuclear medium and its relationship with the strength of pion tensor force is discussed. A simple method to reduce the contribution of this tensor force that considerably improves the single-particle spectrum of nuclei is proposed.     

Nuclear shadowing effect in relativistic heavy-ion collisions

Two complementary approaches are used in studying the nuclear shadowing effect in heavy-ion collisions at SIS/GSI beam energies within a hadronic transport model. By analysing simultaneously the average transverse momentum of nucleons and pions in the reaction plane as a function of rapidity, the shadowing effect is revealed as an anticorrelation of the average transverse momentum distributions of nucleons and pions in semicentral and peripheral collisions. While by studying the azimuthal angle distribution of pions with respect to the reaction plane, the shadowing effect appears as an azimuthal anisotropy with a preferential emission of pions perpendicular to the reaction plane. The dependence of the nuclear shadowing effect on the impact parameter and the beam energy is also studied.     

Pion superfluidity in nuclear matter

We utilize analogies with theories and properties of both liquid He⁴ and electrons in solids and liquids in constructing a model of nuclear matter in which the presence of stabilized pions is assumed. This model is then used to predict relationships between various thermodynamic parameters of a nuclear matter system, such as that between its “free” pion density and the characteristic transition temperature at which a Bose-Einstein condensation will commence.     

Jet absorption and the corona effect at RHIC: Extracting collision geometry from experimental data

The possible existence of a finite formation time of strongly interacting plasma in nuclear collisions at RHIC is demonstrated from recent experimental data. To show this, we use a simple model based on Monte Carlo simulation of nucleus-nucleus collisions with realistic nuclear density distribution. The most striking feature of the experimental data—an absence of absorption of high transverse momentum pions in the reaction plane direction for mid-peripheral collisions—points to the presence of a surface zone with no absorption and strong suppression in the inner core. A natural interpretation of such a zone could be the plasma formation time T ~ 2–3 fm/c. The existence of a formation time could dramatically change our understanding of many experimentally observed features. With this assumption, we describe the angular anisotropy of high transverse momentum pions with respect to the reaction plane and the centrality dependence of the nuclear modification factor in Au + Au and Cu + Cu collisions. The text was submitted by the author in English.     

Optical Potential Parameters of Weakly Bound Nuclear System 17F+13C

Elastic scattering angular distributions of the 14^N＋16^O system and the angular distributions of transfer reaction 16^O（14^N,13^C）17^F at ELab=76.2 MeV and 57 MeV have been measured and calculated by means of the exact finiterange distorted-wave Born approximation with the PTOLEMY code. The optical potential parameters for the weakly bound nuclear system 17^F＋13^C have been deduced and applied to analyse the elastic scattering angular distributions of the similar systems 17^F＋12^C and 17^F＋14^N which are taken from literature. The result shows that the transfer reaction with stable projectile and target combination can be used as an alternative method to extract the optical potential parameters for the weakly bound nuclear system.