中能重离子碰撞中的碰撞参数研究

在入射能量E为40和100MeV/u时,对¹¹²Sn+¹¹²Sn和¹²⁴Sn+¹²⁴Sn两个反应系统在不同碰撞参数下进行了同位旋相关的量子分子动力学模型计算,系统研究了阻塞率、线性动量转移、荷电粒子多重性、轻荷电粒子多重性、中子多重性以及束缚核总电荷Z_bound随碰撞参数的变化规律,结果表明,中子多重性对碰撞参数的依赖在两个能量下都存在着明显的同位旋效应,Z_bound在E为40MeV/u时存在着同位旋效应.同时讨论了在较低和较高能量时如何更合理地确定反应事件的碰撞参数.     

同位旋效应对中能重离子反应中轻粒子产物的影响

利用同位旋相关的量子分子动力学模型,研究了¹¹²Sn+¹¹²Sn和¹²⁴Sn+¹²⁴Sn两反应系统在不同入射能量、不同碰撞参数、不同势场和不同核子-核子碰撞截面下的粒子发射特征.阐述了发射体系的同位旋对轻粒子产额比的影响.发现轻粒子产额比是同位旋的敏感观测量.另外,还发现中快度区发射的粒子有更高的丰中子程度.同时,轻粒子的比不敏感于核子核子碰撞截面,而敏感于核态方程,这使得从轻粒子比提取同位旋相关的核态方程变成一种可能. 关键词： 同位旋相关的量子分子动力学 同位旋效应 轻粒子产额比     

利用自由中-质比研究对称能的高密行为

基于同位旋和动量依赖的强子输运模型IBUU04, 采用两种具有不同密度依赖的对称能研究了¹³²Sn+¹²⁴Sn反应系统在400MeV/A, 600MeV/A, 800MeV/A三种不同碰撞能量下中快度发射核子中-质比随横动量的变化关系. 发现发射核子中--质比在低横动量端非常敏感于对称能, 发射核子中--质比的强度随碰撞能量的降低而增强. 并且发射核子中-质比对对称能的敏感性随着碰撞能量的增大而降低, 在高横动量端尤为明显, 在碰撞能量为400MeV/A时, 自由核子中-质比对对称能的敏感性最为明显.     

核温度测量与粒子能量的关系

提取了30MeV/u ⁴⁰Ar+natAg反应中后角出射的同位素(^6,7Li,^3,4He)产额比温度为5MeV.观察到这一同位素产额比温度随阈值能量E_cut/A的增加而逐渐由5MeV上升到6MeV.为进一步研究这一变化关系,用Monte Carlo方法模拟了热核蒸发粒子的过程,表明随粒子能量的增加,发射该能量粒子的发射源平均温度也在增加.说明高温热核发射高能量粒子的几率大.     

高能重离子碰撞中正负荷电粒子比单事例起伏研究

用强子和弦级联模型,JPCIAE及相应的Monte Carlo事例产生器,研究相对论性核–核碰撞中有限快度区间内正负荷电粒子比单事例起伏与能量、中心度、共振态衰变及快度间隔的关系.JPCIAE模型能够较好地符合CERN/SPS能区Pb+Pb碰撞的实验结果.本文还用此模型预言了RHIC能区Au+Au碰撞和ALICE能区Pb+Pb碰撞中的正负荷电粒子比单事例起伏.可以看出碰撞能量、中心度、共振态衰变及快度间隔对正负荷电粒子比单事例起伏的影响都不大.     

超相对论Au+Au碰撞中椭圆流的时间依赖

用相对论量子分子动力学(RQMD)模型模拟了质心系束能量为s_NN=200GeV的Au+Au非对心碰撞, 研究了椭圆流对末态粒子冻出时间的关系. 研究了在不同的阶段, 椭圆流对末态粒子位置的关系. 结果表明椭圆流随冻出时间的单调递减, 椭圆流对横向半径的关系随冻出时间发生变化. 用压力梯度对所得结果进行了分析. 径向速度用来表征压力梯度.     

依赖系统大小的能量损失对强子谱压低的影响

基于微扰量子色动力学(pQCD)的部分子模型, 在领头阶(LO)近似下分别计算了质子－质子(p+p)碰撞, Au+Au和Cu+Cu不同对心度碰撞产生π⁰的强子谱和相应的核修正因子R_AA. 在考虑部分子喷注能量损失(即喷注淬火)效应后的理论计算表明, 与相同质心系能量s=200GeV的p+p碰撞相比, Au+Au或Cu+Cu不同对心度碰撞产生的强子谱被压低, 被压低的程度与依赖系统大小的能量损失有强烈的关系. 系统越大, 部分子喷注的能量损失越大, 导致强子谱被压低得越多, R_AA越小. 理论计算的数值结果和实验数据相符合, 说明能量损失机制的可靠性和真实性.     

核子诱导的χcJ抑制

本文主要运用夸克交换模型计算所得的核子—χ_cJ离解截面研究Pb+Pb和S+Pb中心碰撞过程中强子物质中的χ_cJ抑制.并定性地研究了核子动量和核子气温度对热平均截面地影响.Pb+Pb和S+Pb中心碰撞中χ_cJ存活率表明核子与反核子所诱导的χ_cJ抑制是很明显的.     

轻粒子发射时间随测量角的变化

利用小相对角关联方法测量了25MeV/u⁴⁰Ar+⁹³Nb反应中关联出射的轻粒子.通过与三体弹道模型的比较,从关联函数提取了轻粒子的发射时间.结果表明,轻粒子的发射时间随粒子能量的升高而降低,从低能的约600fm/c下降至高能的约50fm/c.将此数据与⁴⁰Ar+¹⁹⁷Au反应中的发射时间进行了比较,发现随着实验室系角度的增加,提取的轻粒子的发射时间变长,并认为其原因在于随发射角度的增加,前平衡发射成份越来越少,平衡发射成份增加.     

LHC高能Pb—Pb和Ca—Ca碰撞中的直接硬光生J/ψ及γ

研究了LHC中,Pb–Pb以质心系能量5.5ATeV以及Ca–Ca以质心系能量7ATeV碰撞过程中的硬直接光生J/ψ和γ过程:A+A→A+J/ψ(γ)+X.结果表明该过程不仅可用于测量原子核中的胶子分布,还可以用以研究大R_t胶子以及重夸克的J/ψ碎裂机制.     

Energy Dependence of String Fragmentation Function and φ Meson Production

The φ meson productions in Au+Au and/or Pb+Pb collisions at AGS, SPS, RHIC, and LHC energies have been studied systematically with a hadron and string cascade model LUCIAE. After considering the energy dependence of the model parameter α in string fragmentation function and adjusting it to the experimental data of charged multiplicity to a certain extent, the model predictions for φ meson yield, rapidity, and transverse mass distributions are compatible with the experimental data at AGS, SPS and RHIC energies. A calculation for Pb+Pb collisions at LHC energy is given as well. The obtained fractional variable in string fragmentation function shows a saturation in energy dependence. It is discussed that the saturation of fractional variable in string fragmentation function might be a qualitative representation of the energy dependence of nuclear transparency.     

Results of the NA49 experiment on the search for the quark-gluon plasma at the CERN SPS

Experiment NA 49 at the CERN SPS has performed large acceptance measurements of hadron production in Pb+Pb collisions at 40 and 158 A-GeV beam energies. These data allowed a study of the space-time evolution, the hadro-chemical composition and event-by-event fluctuations of average event properties of the created hadron system. Results at top SPS energy are consistent with the existence of a transient deconfined phase in the early stage of the reaction.     

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.     

RHIC和LHC能区两粒子关联函数研究

应用多源热模型研究了相对论性重离子对撞机（RHIC）上PHENIX合作组测得的每核子对质心能量√sNN=200 GeV的氘-金（d+Au）碰撞中快度长程方位角关联,和大型强子对撞机（LHC）上CMS合作组测得的质心能量√s=0.9,2.36,7 TeV的质子质子（p+p）碰撞中高多重数短程赝快度关联,在误差范围内模型结果很好地重现了实验数据。用4个参数分析了长程方位角关联,结果表明,参数可以清楚地显示出脊结构的变化趋势,说明脊结构与集体效应、流体力学流效应、粒子对的横动量及信号等有关。用3个参数分析了小系统高多重数短程赝快度关联,结果表明,参数σ和c₃与能量相关,能量越大,两粒子越靠近,关联越强。The multi-source thermal model is used in this paper to analyze the long-range azimuth correlation in deutron-gold (d+Au) collisions at center-of-mass energy per nucleon pair √sNN=200 GeV measured by the PHENIX Collaboration at the relativistic heavy ion collider (RHIC), and the short-range pseudorapidity correlation in high multiplicity event in proton-proton (p+p) collisions at center-of-mass energy √s=0.9, 2.36, 7 TeV measured by the CMS Collaboration at the large hadron collider (LHC). The modeling results are in approximately agreement with the experimental data in error ranges. Four parameters are used in the analysis of long-range azimuth correlation, which show that the parameters describe the change trend of ridge structure. The ridge structure is related to the collective effect, hydrodynamic flow effect, transverse momentum and signal of particle pair, and so forth. Three parameters are used in the analysis of the short-range pseudorapidity correlation in high multiplicity event in small system, which show that the parameters σ and c3 are related to collision energy. Large collision energy results in strong correlation due to close distance between particles.     

Charged Multiplicity Density and Number of Participant Nucleons in Relativistic Nuclear Collisions

The energy and centrality dependencies of charged particle pseudorapidity density in relativistic nuclearcollisions were studied using a hadron and string cascade model, JPCIAE. Both the relativistic p+p experimental dataand the PHOBOS and PHENIX Au+Au data at RHIC energy could be fairly reproduced within the framework ofJPCIAE model and without retuning the model parameters. The predictions for Pb + Pb collisions at the LHC energywere also given. We computed the participant nucleon distributions using different methods. It was found that thenumber of participant nucleons is not a well defined variable both experimentally and theoretically. Thus it may beinappropriate to use the charged particle pseudorapidity density per participant pair .as a function of the number ofparticipant nucleons for distinguishing various theoretical models.     

Multiplicity studies and effective energy in ALICE at the LHC

In this work we explore the possibility to perform “effective energy” studies in very high energy collisions at the CERN large hadron collider (LHC). In particular, we focus on the possibility to measure in pp collisions the average charged multiplicity as a function of the effective energy with the ALICE experiment, using its capability to measure the energy of the leading baryons with the zero degree calorimeters. Analyses of this kind have been done at lower centre-of-mass energies and have shown that, once the appropriate kinematic variables are chosen, particle production is characterized by universal properties: no matter the nature of the interacting particles, the final states have identical features. Assuming that this universality picture can be extended to ion–ion collisions, as suggested by recent results from RHIC experiments, a novel approach based on the scaling hypothesis for limiting fragmentation has been used to derive the expected charged event multiplicity in AA interactions at LHC. This leads to scenarios where the multiplicity is significantly lower compared to most of the predictions from the models currently used to describe high energy AA collisions. A mean charged multiplicity of about 1000–2000 per rapidity unit (at η∼0) is expected for the most central Pb–Pb collisions at . In memory of A. Smirnitskiy     

Energy dependent growth of nucleon and inclusive charged hadron distributions

In the Color Glass Condensate formalism, charged hadron p T spectra in p+p and p+Pb collisions are studied by considering an energy-dependent broadening of nucleon density distribution. Then, in the glasma flux tube picture, the n-particle multiplicity distributions at different pseudo-rapidity ranges are investigated. Both the theoretical results show good agreement with the recent experimental data from ALICE and CMS at LHC energies.The predictive results for p T or multiplicity distributions in p+p and p+Pb collisions at the Large Hadron Collider are also given in this paper.     

On chemical equilibrium in nuclear collisions

The data on average hadron multiplicities in central A+A collisions measured at CERN SPS are analysed with the ideal hadron gas model. It is shown that the full chemical equilibrium version of the model fails to describe the experimental results. The agreement of the data with the off–equilibrium version allowing for partial strangeness saturation is significantly better. The chemical freeze–out temperature of about 180 MeV seems to be independent of the system size (from S+S to Pb+Pb) and in agreement with that extracted in , and collisions. The strangeness suppression is discussed at both hadron and valence quark level. It is found that the hadronic strangeness saturation factor increases from about 0.45 for interactions to about 0.7 for central A+A collisions with no significant change from S+S to Pb+Pb collisions indicating that the strangeness enhancement in heavy ion collisions cannot be fully attributed to the increased system size. The quark strangeness suppression factor is found to be about 0.2 for elementary collisions and about 0.4 for heavy ion collisions independently of collision energy and type of colliding system. Received: 31 October 1997 / Published online: 26 February 1998     

Hadron Multiplicities in Pb＋Pb Collisions at the Large Hadron Collider and Pomeron Loop Effects

We study the pseudo-rapidity distribution of hadron multiplicities of high energy Pb＋Pb collisions by using color glass condensate dynamics at LHC/ALICE in the fixed coupling case. It is found that after including the pomeron loop effects the charged hadron multiplicities at central rapidity are about 1500 for central Pb＋Pb collisions, which are significantly smaller than the saturation based calculations, ～ 1700 ÷ 2500 and compatible with that based on a study of multiplicities in the fragmentation region.     

Thermodynamics of hadronic matter at high density

We calculate thermodynamics observables for an interacting relativistic hadron gas. Hadronic states are taken into account by the use of a sizeable portion of the experimental hadron spectrum, supplemented in some cases by an exponentially rising continuum. Calculations with non-zero baryon number densities, subject to the additional requirement of zero net strangeness, show structure in the heat capacity per unit volume of the baryon sector at a temperature of approximately 140 MeV. This structure also becomes visible in the total heat capacity per unit volume at large baryon number densities, and provides a signature for the change of the thermal response of the hadron gas from baryon- to meson-dominated, even though the meson number density is lower than the baryon number density. Furthermore, this structure is not seen in calculations with a massless hadron gas. Its origin is therefore associated with information contained in the hadronic mass spectrum, and thus with the sub-hadronic degrees of freedom of the hadrons.