高能Cu+Cu碰撞中末态带电粒子赝快度分布的能量与对心度依赖性

利用Glauber模型,给出了核-核碰撞中的参与者数及二元核子-核子碰撞数与碰撞参数的解析关系,并以此为基础,在考虑到参与者由于多重碰撞而引起的能量损失效应后,通过有效二元核子-核子碰撞末态带电粒子赝快度分布的加权叠加,建立起了以碰撞参数为自变量的核-核碰撞末态带电粒子的赝快度分布函数,并用其分析了RHIC-PHOBOS合作组在不同能量与不同对心度的Cu+Cu碰撞中所做的实验测量。所得结果与实验符合得较好。     

158GeV/c的Pb–Pb碰撞中的J/ψ抑制现象研究

在Blaizot等人建立的模型基础上,我们考虑了核–核碰撞中的参与者数与核子–核子碰撞数的起伏效应对J/?ψ产生的影响,并对NA50合作组在入射动量为158GeV/c的Pb-Pb碰撞中的实验数据进行了分析,理论结果与实验数据符合的很好     

核子发射数的数学模拟:扩展BUU模型

本文在原来BUU方法的基础上,辨认了靶核与炮弹中的中、质子,同时在模拟核子的初始分布中考虑了小液滴模型修正,引入了表面弥散度、中子皮和形变参数等,使核子的分布更真实,这对丰中子、丰质子核反应以及非对称核反应尤为重要.考虑到中能核反应时,自由核子-核截面中,不同核子间的散射截面大约是同类核子间散射截面的三倍的事实,在BUU碰撞项中体现了这一点,这样可合理地取出不同核子-核子间碰撞流的问题.我们在利用这个扩展的BUU模型研究44Mev/A Kr+Au反应之前,有效地检验了模型的稳定性问题,并在文中合理地定义了核子发射的条件,得到了反应过程中弹核擦碎的中子、质子数与碰撞参数的关系以及类弹靶的关联.     

介质中的核子–孩子碰撞截面及其对原子核反应总截面的影响

给出了一个在较大能量范围内描述介质中核子-核子碰撞截面的计算公式,并结合微观修正的Glauber模型,研究了介质效应对原子核反应总截面的影响.     

在非相对论能量下介质中的核子–核子碰撞截面

在非相对论能量下,采用Skyrme有效相互作用,计算了在核介质中的核子-核子(N-N)碰撞截面,研究了它与能量及密度的依赖关系,以及不同Skyrme参数对所计算截面的影响.发现在低密度时,介质中的核子-核子碰撞截面大于经Pauli修正后的自由核子一核子碰撞截面.而当高密度时,总会在高于某一能量以后,介质中的N-N碰撞截面大于经Pauli修正后的自由N-N碰撞截面.     

核子-核子碰撞截面的参数化公式

核子-核子碰撞截面(σ_NN)是描述原子核反应模型中的重要物理量。 核子-核子碰撞截面包括质子-质子碰撞截面（σ_pp）、中子-中子碰撞截面（σ_NN）和中子 质子碰撞截面（σ_np）, 它们随着核子能量与核物质密度的改变而变化。人们在目前研究中提出了多种核子-核子碰撞截面关于能量和核物质密度的参数化公式,通过计算12C+12C的完全反应截面,比较了不同入射能量下使用各种核子-核子碰撞截面参数化公式对12C完全反应截面的影响。 The nucleus-nucleus cross sections(σ_NN) include the proton-proton cross section, neutron-neutron cross section, and neutron-proton cross section. σ_NN change with the variations of the incident energies and the densities. Some parameterizations of σ_NNas a function of incident energy and density have been proposed in the theoretical and experimental study of nuclear reactions. Through the calculation of reaction cross sections of 12C+12C at different energies, the effect of different σ_NN parameterizations in the reaction cross sections of 12 are studied.      

在非相对论能量下介质中的核子－核子碰撞截面

在非相对论能量下，采用Ｓｋｙｒｍｅ有效相互作用，计算了在核介质中的核子－核子（Ｎ－Ｎ）碰撞截面，研究了它与能量及密度的依赖关系，以及不同Ｓｋｙｒｍｅ参数对所计算截面的影响．发现在低密度时，介质中的核子－核子碰撞截面大于经Ｐａｕｌｉ修正后的自由核子一核子碰撞截面．而当高密度时，总会在高于某一能量以后，介质中的Ｎ－Ｎ碰撞截面大于经Ｐａｕｌｉ修正后的自由Ｎ－Ｎ碰撞截面．     

核子-核子碰撞截面对同位素标度参数α的同位旋效应

对两对重离子中心碰撞系统⁴⁰C+⁴⁰Ca 和⁶⁰Ca+⁴⁰Ca以及¹¹²Sn+¹¹²Sn和¹²⁴Sn+¹²⁴Sn反应中就同位素标度参数α对于核子-核子碰撞截面的同位旋效应进行了研究.计算结果表明α对同位旋相关核子-核子碰撞截面σ^med_{NN关键词： 同位旋效应 核子-核子截面 机理 同位素标度}     

核子-核子碰撞截面对同位素标度参数α的介质效应

核子能量在40—60MeV能区范围,对在两对重离子中心碰撞系统⁴⁰Ca+⁴⁸Ca和⁶⁰Ca+⁴⁸Ca以及¹¹²Sn+¹¹²Sn和¹²⁴Sn+¹²⁴Sn 的反应中就同位素标度参数α对于核子-核子碰撞截面σ^med_NN(α_m)的介质 关键词： 同位素标度行为 介质效应 核子-核子碰撞截面 重离子碰撞     

高能强子与原子核碰撞的受伤核子数分布和非弹性截面的计算

在对超高能区宇宙线产生的广延大气簇射的模拟计算中注意到,质子—空气核碰撞中的受伤核子数分布和非弹性截面对于簇射的纵向发展有着重要的影响.本文依据符合现有加速器能区的有关质子—质子作用截面的经验公式,对超高能区的质子与空气核碰撞的受伤核子数分布和非弹性截面做了计算.     

Production of charged pions, kaons and antikaons in relativistic C + C and C + Au collisions

Production cross-sections of charged pions, kaons and antikaons have been measured in C+C and C+Au collisions at beam energies of 1.0 and 1.8 AGeV for different polar emission angles. The kaon and antikaon energy spectra can be described by Boltzmann distributions whereas the pion spectra exhibit an additional enhancement at low energies. The pion multiplicity per participating nucleon M(π⁺)/<A _part> is a factor of about 3 smaller in C+Au than in C+C collisions at 1.0 AGeV whereas it differs only little for the C and the Au target at a beam energy of 1.8 AGeV. The K⁺ multiplicities per participating nucleon M(K⁺)/ <A _part> are independent of the target size at 1 AGeV and at 1.8 AGeV. The K^- multiplicity per participating nucleon M(K^-)/ <A _part> is reduced by a factor of about 2 in C+Au as compared to C+C collisions at 1.8 AGeV. This effect might be caused by the absorption of antikaons in the heavy target nucleus. Transport model calculations underestimate the K^-/K⁺ ratio for C+C collisions at 1.8 AGeV by a factor of about 4 if in-medium modifications of K-mesons are neglected. Received: 10 December 1999 / Accepted: 14 November 2000     

Quasielastic transfer reactions in 238U + 197Au and 197Au + 197Au collisions near the Coulomb barrier

Differential cross sections as a function of cm angle were measured for 1n- and 2n-transfer reactions in ²³⁸U + ¹⁹⁷ Au and¹⁹⁷ Au + ¹⁹⁷ Au collisions in the energy range from 0.881 V_c to 1.093 V_c and 0.825 V_c to 0.964 V_c, respectively. For ¹⁹⁸Au and ¹⁹⁹Au from the ²³⁸U + ¹⁹⁷Au collisions, for reduced distances of closest approach d _o 1.55 fm, the angular distributions at all bombarding energies are well described by the semiclassical theory. Equivalently, the transfer probabilities show the expected exponential decrease with increasing d _o over many orders of magnitude. For all other transfer products from ²³⁸U + ¹⁹⁷Au collisions, and for all transfer products from ¹⁹⁷Au + ¹⁹⁷Au collisions, markedly reduced cross sections relative to the semiclassical theory are observed for central collisions at all bombarding energies, even for values of d _o that are well outside the region where absorption is known to set in. Only for the more peripheral collisions, one observes agreement of the angular distributions (transfer probabilities) with the semiclassical expectations. The deviations for central collisions are absent for reactions with positive Q _gg values and scale roughly with increasingly negative values of Q _gg, i.e. with increasing Q-value mismatch. Channel coupling is proposed as the relevant mechanism.     

Characteristics of charged particle production in relativistic heavy-ion collisions

Inclusive spectra of charged particles at midrapidity in Au+Au collisions at $\sqrt {s_{NN} } = 130$ GeV and 200 GeV were measured with the STAR detector at RHIC. The measured mean transverse momentum 〈p _T 〉 shows a characteristic dependence on charged particle multiplicity and beam energy in Au+Au collisions that is distinctly different from pp, $p\bar p$ and e⁺e^? collisions. A 32%±3%(syst) increase in 〈p _T 〉 from pp to Au+Au collisions was observed at 200 GeV. While the charged multiplicity was found to increase by 19%±5%(syst) from $\sqrt {s_{NN} } = 130$ GeV to 200 GeV, no significant difference in 〈p _T 〉 was found between the two energies. A comparison with model predictions is discussed.     

Directed flow of identified particles from Au+Au collisions at RHIC

We present the measurement of directed flow (v ₁) for the identified particles, namely, Λ, $ \bar \Lambda $ \bar \Lambda and K _s ⁰, as a function of rapidity and centrality in Au+Au collisions at $ \sqrt {s_{NN} } $ \sqrt {s_{NN} } = 200 GeV and 62.4 GeV. The measurement is based on the run IV data obtained by the STAR experiment at RHIC. In order to enhance event plane resolution, we use tracks reconstructed from the Forward Time Projection Chambers (FTPCs), together with the sideward deflection of spectator neutrons measured by the STAR’s Shower Maximum Detector at Zero Degree Calorimeters (ZDC-SMDs). We find that for 200 GeV, proton and antiproton v ₁ is less than 1%, the K _s ⁰ Λ, $ \bar \Lambda $ \bar \Lambda v ₁ is less than 2%; for 62 GeV, proton v ₁ is less than 1% and antiproton is less than 2%, v ₁ for K _s ⁰, Λ, $ \bar \Lambda $ \bar \Lambda is less than 2% in Au+Au collisions at 200 GeV.     

Ks0 and Λ elliptic flow from 200 GeV Cu+Cu collisions

Elliptic flow allows us to probe early dynamics in high energy nuclear collisions. The υ ₂ result of charged hadrons and identified hadrons [1,2] from 200 GeV Au+Au collisions at RHIC suggest that the matter with partonic collectivity and thermalization has been formed in central collisions. In this analysis, we present preliminary results of υ ₂ for K _S ⁰ and Λ from 200 GeV Cu+Cu collisions. The partonic collectivity and thermalization assumption are tested in smaller Cu+Cu system comparing with those from Au+Au collisions.     

Pion-source geometry deduced from soft-pion interferometry in heavy-ion collisions at 650 A MeV

We report transverse,R _T , and longitudinal,R _L , source sizes extracted from two-pion interferometry analysis in the Au+Au and Nb + Nb interactions at 650 A MeV. For the Nb+Nb interactions, both,R _T andR _L , do not exceed the niobium nucleus radius. In the case of Au+Au collisions, the transverse size is larger than the longitudinal one being about 12 fm for the selection ofP ⁺ < 120=" mev/c.=" we=" also=" corroborate=" the=" existence=" of=" the=" previously=" reported=" specific=" three-pion=">One of us (T.S.) wishes to thank Professor M.I. Podgoretsky and Professor R.M. Weiner for interesting discussions and comments. We also wish to acknowledge remarks of Dr. B. Lörstad.     

Direct photons in nuclear collisions at fair energies

Using the extrapolation of existing data, estimations of prompt-photon production at FAIR energies have been made. At y = y _c.m. the rapidity density of prompt photons with p _t > 1.5 GeV/c per central Au + Au event at 25 A GeV is estimated as ∼10⁻⁴. With the planned beam intensity 10⁹ per second and 1% interaction probability, for 10% of most central events one can expect the prompt-photon rate ∼10² photons per second. Direct photons from the hadron scenario of ion collisions generated by the Hadron-String-Dynamics (HSD) transport approach with implemented meson scatterings πρ → πγ, ππ → ργ have been analyzed. Photons from short-living resonances (e.g., ω → π ⁰ γ) decaying during the dense phase of the collision should be considered as direct photons. They contribute significantly in the directphoton spectrum at p _t = 0.5–1 GeV/c. At the FAIR energy 25 A GeV in Au + Au central collisions the HSD generator predicts, as a lower estimate, γ _direct/ ≃ 0.5% in the region p _t = 0.5–1 GeV/c. At p _t = 1.5–2 GeV/c γ _prompt/ ≃ 2%. Thermal direct photons have been evaluated with the Bjorken Hydro-Dynamics (BHD) model. The BHD spectra differ strongly from the HSD predictions. The direct-photon spectrumis very sensitive to the initial temperature parameter T ₀ of the model. The 10-MeV increase in the T ₀ value leads to ∼2 times higher photon yield. The text was submitted by the authors in English.     

Transverse energy measurement in Au+Au collisions by the STAR experiment

Transverse energy (E _T ) has been measured with both of its components, namely hadronic (E _T ^had ) and electromagnetic (E _T ^em ) in a common phase space at mid-rapidity for 62.4 GeV Au+Au collisions by the STAR experiment. E _T production with centrality and ?{s_NN }\sqrt {s_{NN} } is studied with similar measurements from SPS to RHIC and is compared with a final state gluon saturation model (EKRT). The most striking feature is the observation of a nearly constant value of E _T /N _ch ∼ 0.8 GeV from AGS, SPS to RHIC. The initial energy density estimated by the boost-invariant Bjorken hydrodynamic model, is well above the critical density for a deconfined matter of quarks and gluons predicted by lattice QCD calculations.     

High energy collisions of protonated water clusters

We have measured attenuation cross sections and fragmentation cross sections for protonated water clusters H(H₂O)_n ⁺ (n = 1 to 100) colliding with noble gas atoms (He and Xe) at a laboratory energy of 50 keV. In collisions with He, a transparency effect in the attenuation cross section was observed. For the case of fragmentation in collisions with Xe, a strong enhancement of small clusters was observed which we attribute to multifragmentation. Received 30 November 2000     

Preequilibrium nucleon jets in TDHF calculations of medium-energy,heavy-ion collisions

We study collisions of ¹²C and ¹⁹⁷Au projectiles at E_lab/A_proj = 30 to 85 MeV/A on ¹⁹⁷Au targets, using the 3-dimensional TDHF model. Most of the qualitative phenomena observed experimentally are reproduced by the TDHF calculations. The results are compared with recent data from the 1 GeV ¹²C beam at CERN and show some quantitative deviations from experiment. Possible reasons for these discrepancies are discussed and give valuable insight into intermediate energy collision processes.