Impact parameter dependence of the scaling of anisotropic flowsin intermediate energy heavy-ion collisions

The scaling behaviors of anisotropic flows of light charged particles are studied for 25 MeV/u ⁴⁰Ca+⁴⁰Ca collisions at different impact parameters by the isospin-dependent quantum molecular dynamics model. The nucleon-number scaling of elliptic flow exists and the scaling of the ratios of v₄/v₂² and v₃/(v₁v₂) is applicable to collisions at almost all impact parameters except for peripheral collisions.     

中能重离子碰撞中集体流标度性质研究

利用同位旋相关的量子分子动力学模型模拟了40Ca+40Ca,50Ca+40Ca,64Cu+64Cu和197Au+197Au等多个碰撞系统中快度轻碎片集体流标度的性质。发现,中能区不同碰撞系统横向动量相关的椭圆流存在对碎片核子数的标度,且存在v4/v22标度,对不同碎片该比值都近似为常数0.5。同时探讨了碰撞能量、碰撞参数、核状态方程对标度律的影响,发现这些标度行为不敏感于以上物理量,可能是核碎片形成机制(组合机制)的必然结果。     

Scaling of charged particle multiplicity in Pb–Pb collisions at SPS energies

The charged particle multiplicity distribution dN_ch/dη has been measured by the NA50 experiment in Pb–Pb collisions at the CERN SPS. Measurements were done at incident energies of 40 and 158 GeV per nucleon over a broad impact parameter range. The multiplicity distributions are studied as a function of centrality using the number of participating nucleons (N_part), or the number of binary nucleon–nucleon collisions (N_coll). Their values at midrapidity exhibit a power law scaling behaviour given by N_part^1.00 and N_coll^0.75 at 158 GeV. Compatible results are found for the scaling behaviour at 40 GeV. The width of the dN_ch/dη distributions is larger at 158 than at 40 GeV/nucleon and decreases slightly with centrality at both energies. Our results are compared to similar studies performed by other experiments both at the CERN SPS and at RHIC.     

Multistrange Baryon elliptic flow in Au+Au collisions at square root of sNN=200 GeV

We report on the first measurement of elliptic flow v2(pT) of multistrange baryons Xi- +Xi+ and Omega- + Omega+ in heavy-ion collisions. In minimum-bias Au+Au collisions at square root of s(NN)=200 GeV, a significant amount of elliptic flow, comparable to other nonstrange baryons, is observed for multistrange baryons which are expected to be particularly sensitive to the dynamics of the partonic stage of heavy-ion collisions. The pT dependence of v2 of the multistrange baryons confirms the number of constituent quark scaling previously observed for lighter hadrons. These results support the idea that a substantial fraction of the observed collective motion is developed at the early partonic stage in ultrarelativistic nuclear collisions at the Relativistic Heavy Ion Collider.     

Scaling properties of azimuthal anisotropy in Au+Au and Cu+Cu Collisions at sqrt[s NN]=200 GeV

Differential measurements of elliptic flow (v2) for Au+Au and Cu+Cu collisions at sqrt[sNN]=200 GeV are used to test and validate predictions from perfect fluid hydrodynamics for scaling of v2 with eccentricity, system size, and transverse kinetic energy (KE T). For KE T identical with mT-m up to approximately 1 GeV the scaling is compatible with hydrodynamic expansion of a thermalized fluid. For large values of KE T mesons and baryons scale separately. Quark number scaling reveals a universal scaling of v2 for both mesons and baryons over the full KE T range for Au+Au. For Au+Au and Cu+Cu the scaling is more pronounced in terms of KE T, rather than transverse momentum.     

转动集体流的同位旋效应

在同位旋相关的量子分子动力学模型框架下,通过分析中间快度的方位角分布研究了40MeV/u ⁵⁸Fe+⁵⁸Fe和⁵⁸Ni+⁵⁸Ni反应体系在不同碰撞参数下的转动集体流,并且半经典地计算了转动观测量.发现丰中子体系(⁵⁸Fe+⁵⁸Fe)具有较强的转动集体流,这种现象在半周边碰撞中更加明显.转动集体流的同位旋相关性主要来自于核子–核子碰撞截面的同位旋相关性和对称能的影响.     

Partonic Flow and phi-Meson production in Au+Au collisions at sqrt radical sNN = 200 GeV

We present first measurements of the phi-meson elliptic flow (v2(pT)) and high-statistics pT distributions for different centralities from radical sNN=200 GeV Au+Au collisions at RHIC. In minimum bias collisions the v2 of the phi meson is consistent with the trend observed for mesons. The ratio of the yields of the Omega to those of the phi as a function of transverse momentum is consistent with a model based on the recombination of thermal s quarks up to pT approximately 4 GeV/c, but disagrees at higher momenta. The nuclear modification factor (R CP) of phi follows the trend observed in the K S 0 mesons rather than in Lambda baryons, supporting baryon-meson scaling. These data are consistent with phi mesons in central Au+Au collisions being created via coalescence of thermalized s quarks and the formation of a hot and dense matter with partonic collectivity at RHIC.     

中能Ca+Ca反应中的对称能效应

采用把重离子碰撞的动力学和统计衰变过程相结合的一个两步模型,再现了INDRA Collaborations 对入射能为35 AMeV40Ca+40Ca 和48Ca+48Ca 反应的实验结果。进而得到了反应碎块同位素分布的Isoscaling 参数α(Z) 和β(N),约化对称能系数ζ(Z) 及与平均场中的对称能强度系数Cs 之间的关系。结果表明,Isoscaling 参数强烈依赖于系统的丰中子程度。随着质子数的增大,ζ(Z) 在动力学阶段呈现平缓趋势但在衰变之后呈现上升趋势,且该结果不依赖于所选取的反应系统。对称能效应在统计衰变过程之后的末级碎块分布中有明显表现。A two-step model for combining the dynamical and statistical decay processes in heavy-ion collisions is used to reproduce the experimental results of the INDRA Collaborations on 40Ca+40Ca and 48Ca+48Ca reactions at 35 AMeV. We obtain the isoscaling parameters α(Z) and β(N) of isotopic distributions of the fragments, the reduced symmetry energy coefficient ζ(Z) and its dependence on the symmetry energy strength coefficient Cs in the mean field. Our results suggest that Isoscaling parameters strongly depend on the degree of neutron-rich. With the increasing atomic number, ζ(Z) represents a smoothly flat tendency during the dynamical process but shows the increasing tendency after decay, and such results are independent on the selected reaction systems. The effect of symmetry energy shows evidently in the isotopic distributions of fragments after statistical decay process.     

High-p T results from PHOBOS

We present results from the PHOBOS experiment on charged hadron production over a large range in , from Data from d + Au collisions at GeV and Au + Au collisions at and 200 GeV are shown. The data indicate that in Au + Au collisions a strongly interacting state of matter is formed, that appears qualitatively different from the system formed in pp or d + Au collisions. The systematics of particle production in d + Au and Au + Au collisions exhibit remarkably simple scaling rules, including an apparent factorization of the energy and centrality dependence over a wide range of collision energies. Arrival of the final proofs: 29 July 2005 PACS: 25.75.-q, 25.75.Dw, 25.75.Gz     

Measurement of shape and absolute value of 1sσ excitation probability in 4.7 MeV/u Xe+Au collisions

The impact parameter dependence of the 1sσ excitation probability for 4.7 MeV/u Xe + Au collisions has been measured for impact parameters between 40 and 200 fm. A determination of the total 1sσ excitation cross section from the data gives (10,8 ± 2)b in good agreement with results of an independent measurement.     

Particle-type dependence of azimuthal anisotropy and nuclear modification of particle production in Au+Au collisions at square root of sNN=200 GeV

We present STAR measurements of the azimuthal anisotropy parameter v(2) and the binary-collision scaled centrality ratio R(CP) for kaons and lambdas (Lambda+Lambda) at midrapidity in Au+Au collisions at square root of s(NN)=200 GeV. In combination, the v(2) and R(CP) particle-type dependencies contradict expectations from partonic energy loss followed by standard fragmentation in vacuum. We establish p(T) approximately 5 GeV/c as the value where the centrality dependent baryon enhancement ends. The K(0)(S) and Lambda+Lambda v(2) values are consistent with expectations of constituent-quark-number scaling from models of hadron formation by parton coalescence or recombination.     

相对论重离子碰撞的趋平衡问题

从相对论ＢＵＵ理论出发,研究了１ＧｅＶ／ｕ能区重离子碰撞全局和局域平衡性质．研究结果表明,在该能区平均场对反应动力学过程仍有相当的作用;对有限核系统的反应动力学过程的时间演化的研究表明,有限核系统未能达到全局平衡,在中心区基本达到局域平衡．因而在该能区的重离子碰撞中引入完全热平衡概念时,需谨慎考虑．The equilibration in relativistic heavy ion collisions for systems 16 O+ 16 O, 40 Ca+ 40 La and 139 La+ 139 La is studied with RBUU theory. We have found that the mean field still plays a role in addition to the collision term in the equilibration process in relativistic heavy ion collisions at energy around 1 GeV/u. For finite systems, the systems do not reach complete equilibrium. But at the center zone, the local equilibrium is almost reached.     

Saddle points and rare collisions under scaling approach in a Fermi accelerator with two nonlinear terms

Rare collisions of a classical particle bouncing between two walls are studied. The dynamics is described by a two-dimensional, nonlinear and area-preserving mapping in the variables velocity and time at the instant that the particle collides with the moving wall. The phase space is of mixed type preventing diffusion of the particle to high energy. Successive and therefore rare collisions are shown to have a histogram of frequency which is scaling invariant with respect to the control parameters. The saddle fixed points are studied and shown to be scaling invariant with respect to the control parameters too.     

用各向异性流v2和v4测试超相对论性重离子碰撞的动力学描述

We present various predictions for the anisotropic collective flow of particles in heavy-ion collisions, in particular scaling laws of the second and fourthharmonics v₂ and v₄, derived within ideal fluid dynamics. We also discuss qualitatively the deviation from the ideal behaviour expected in an out-of-equilibrium scenario.     

A simple geometrical approach to particle production in collisions with nuclei

It is argued that hadron collisions with nuclei are similar to hadron-hadron collisions, having similar properties for the impact parameter distributions and the leading particle spectra. The relevant existing high-energy data, including the universality of multiplicity distributions and the possibility of geometrical scaling in reactions with nuclei, are easily understood in the framework of geometrical models by extending to p-nucleus collisions what was learnt about impact parameter and leading particles in pp collisions. The questions of forward-backward correlations and photo- and electroproduction are briefly discussed.     

Energetic Protons and Cold Nuclear Multifragmentation Pattern

Multifragmentation is studied in correlation with energetic protons.A cold multifragmentation pattern is proposed based on the numerical simulation of the stochastic Boltzmann-Langevin model for describing the intermediate heavy ion collisions,i.e.,a sizable fraction of the available energy is released from the system by fast non-equilibrium proton emission,in which the explosion is caused to happen by a violent,but cold expansion of the system.Some typical examples simulated for ⁴⁰Ca+⁴⁰Ca central collisions at 90MeV/u are presented.A possible experimental signature which could be detected with the help of a 4π detector system is suggested.     

Open charm measurements at STAR

We report on the measurements of D⁰ meson production via direct reconstruction through the hadronic decay channel D⁰→Kπ in minimum bias d+Au and Au+Au collisions at with p_T up to ∼3 GeV/c. We derive the charm production total cross-section per nucleon–nucleon collision from a combination of three measurements: the D⁰ meson spectra, the non-photonic electron spectra from charm semi-leptonic decays obtained in p+p, d+Au, and Au+Au collisions, and the charm-decayed single muon (prompt muon) spectra at low p_T in Au+Au collisions. The cross-section is found to follow binary scaling, which is a signature of charm production exclusively at the initial impact. The implications of charm quark energy-loss and thermalization in the strongly interacting matter are discussed. PACS 25.75.Dw; 13.20.Fc; 13.25.Ft; 24.85.+p     

Roles of N?→NN and πN→? Reactions in Heavy-Ion Collisions at Intermediate Energies

Within the framework of the isospin-dependent transport model, the roles of the reactions N? → NN and πN→ ? are investigated through simulating heavy-ion collisions at 1000 MeV/nucleon. The absorption process N? → NN plays an important role for heavy impact systems and small impact parameters than for light impact systems and large impact parameters. The resorption process πN→ ? is of importance for heavy impact systems and large impact parameters than for light impact systems and small impact parameters. Thus the influences of the reaction N? → N N(πN→ ?) on pion production dynamics can be neglected in heavy-ion collisions for smaller(larger) impact parameters and light systems. It is the reaction πN→ ? that causes the anti-correlation of pions and nucleons in the rapidity dependence of the directed flow.     

中能重离子反应中的在平面流和椭圆流

利用同位旋相关的量子分子动力学模型研究了⁴⁰Ca+⁴⁰Ca系统在能量从30MeV/u到150MeV/u的在平面流和椭圆流.发现随着入射能量的增加,在平面流出现从负到正的跃迁,而椭圆流则随入射能量的增大而减少并且出现从正到负的跃迁.椭圆流和在平面流对于入射能量和碰撞参数较为敏感,相反对于对称能和核态方程却不太敏感.最后对于不同质量的碎块的依赖性关系我们还作了进一步仔细的研究.