高能核—核碰撞中新物质形态及其强子化物理研究
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摘要
量子色动力学预言在极高能量密度下,会产生一种由夸克,反夸克和胶子组成的新物质形态,这是夸克胶子等离子体(QGP)。而相对论重离子碰撞就是深入研究这种新物质形态的重要实验手段,人们希望通过这种方法,实现从强子物质到夸克物质的相变。研究高能碰撞的末态观测量,比如：末态粒子的分布谱、粒子关联、起伏……,是了解高能重离子碰撞演化过程及粒子产生机制的重要途径。本文利用在布鲁克海汶国家实验室的相对论重离子对撞机(RHIC)上四个实验组的数据,深入探讨了末态粒子的产生机制及其分布的特征。
本文首先简单回顾了相对论重离子碰撞的实验和理论的现状。然后介绍计算中所需的基本物理概念和唯象模型,包括描述核-核碰撞中,核几何形状的Glauber模型；重组合模型(Recombination/Coalescence(ReCo) Model),特别是Oregon小组(Hwa/Yang)模型的基本思想；以及Peter Levai小组的ALgebraic COalescence Rehadronization模型(ALCOR)。
首先利用统计力学方法探讨了高能重离子碰撞中产生的QGP的热力学性质。在高温下,由于渐近自由,部分子之间的相互作用很弱,所以可以把夸克和胶子当作自由气体来处理。但是,在临界温度(Tc)附近,由于偶合常数比较大,所以很难再利用解析的方法。如果从第一原理出发,利用格点QCD计算,又需要大量时间和计算机资源。因此,在本文第二章中,将会用一个自洽的准粒子唯象模型研究QGP的热化性质。假设部分子间的相互作用可以全部包含在其质量之中,这样就可以把系统当作理想气态来处理,然后再利用统计力学,计算各种热力学量。在T>Tc区域,可以很好的拟合格点QCD数据。
基于ALCOR模型讨论奇异粒子在不同波函数组合下的产生率。结果表明强子产生率依赖于胶子质量,但对温度并不敏感；而产生率之比几乎与胶子质量无关。再选择能量为s~(1/2)=200AGeV, Au+Au中心碰撞中的粒子比Φ／K*=0.60±0.15作为出发点,计算在不同波函数组合的强子比,结果表明粒子之比对波函数的选择不敏感。另外,本文还利用最新的实验数据p／π和p/p,在夸克重组合模型(Recombination Model)的框架下,重新探讨了高能重离子碰撞中,在向前快度区域(η=3.2)末态强子的产生。重新考虑了系统中部分子动量衰减和夸克再生效应后,得到与实验数据相符的带电强子谱和较大的质子与π介子之比,而且还预言了反质子与质子之比。
在深入研究核-核碰撞的重叠区域几何结构基础上,统一描述了在低横动量区域(pT<2GeV／c),π0介子所有与轴向方位角有关的观测量：核修正因子RAA(φ,Np),椭圆流v2(Np)和ridge产量YR(φs,Np),并与实验数据符合的很好。两个基本的出发点是：系统表面的半硬散射导致轴向异性和ridge粒子的产生与触发粒子的选择无关。虽然RAA是单粒子分布的量度,而YR是触发粒子和其伴随粒子关联的量度,但两者之间存在着紧密的联系。在整个物理图像中,最关键一点就是把单粒子分布dNAAπ／pTdptdφ分为两部分：不依赖于方位角φ的bulk组分(B(pT,Np))和依赖于φ的ridge组分(R(pT,φ,Np))。
最后,本文利用RHIC实验数据,深入研究不同的中心度,(赝)快度和碰撞系统下,π介子,质子和反质子横动量谱的标度行为。结果表明：π介子的标度行为不依赖与中心度、(赝)快度、碰撞系统和质心能量而存在。而对于质子和反质子,在SNN~(1/2)=200 GeV下,Au+Au碰撞中也存在不依赖于中心度和快度的标度行为。但三者的标度行为存在着差别,这与粒子的夸克组成有着紧密的联系。在这些过程中,表征这些粒子标度行为的参数只有一个：粒子的平均横动量〈pT〉,其依赖于中心度、快度、碰撞能量及其碰撞系统。一旦知道了〈pT〉,那么低pT区的软过程部分和高pT区的硬过程部分就可以由粒子标度函数决定了。然后再利用π介子和质子标度行为,研究p+p碰撞中带电强子在不同多重数下横动量分布的标度行为。发现不同多重数下的分布谱仍具有标度行为,并可以用π介子和质子标度函数的线性叠加来表示。与动量不同,动能是一个标量,并直接与热密物质的温度相关。由于质量效应,对于不同的粒子,相同的动量对应着不同的动能。这样,在超相对论重离子碰撞中,动能的分布更能有效地揭示系统的热化性质。因此,第七章还进一步研究质子和π介子横能分布的标度行为,并与其横动量的标度行为进行了比较,结果表明在低和高动量(横能)区域,横能的标度性更好。标度律可以作为探寻粒子产生的机制的一个重要信息。通过分析,发现弦碎裂和团碎裂机制可以分别描述π介子和质子的标度行为,但是不能同时得到这两种粒子标度行为,因此这两种机制并不是普适的粒子产生机制,需要寻找新的机制来解释末态粒子的产生。
At extremely high energy densities, Quantum Chromodynamics (QCD) predicts a new form of matter, consisting of an extended volume of interacting quarks, antiquarks and gluons. This is quark-gluon plasma (QGP). The relativistic heavy ion collision is a significant experimental tool for deep understanding of this new form of matter, through which scientists expect to realize the transition from nuclear matter to quark matter. The research of final observables in nucleus-nucleus collision, such as:the particle distribution, correlation and fluctuation, etc, is greatly important for understanding the evolution of the high energy heavy ion collision and the particle production mechanisms. This dissertation mainly focuss on the particle production mechanisms and properties of particle distributions, with the data from the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory.
In the first part, the status of heavy ion collision is briefly reviewed, both in experimental and theoretical aspects. Then some important definitions and phenomenological models are introduced, including the Glauber model, Recombination/Coalescence (ReCo) Model by the Oregon group and ALgebraic COalescence Rehadronization model (ALCOR).
At very high temperature, due to the asymptotic freedom, the interactions among the partons are so weak that one could approximately treat the system as an ideal gas of quarks and gluons. However, near the critical temperature Tc, it is very difficult to solve QCD by analytical methods, such as the perturbative theory, because the coupling constant is not small enough. Lattice QCD can be used to calculate, from the first principle, the properties of simple systems. But this method is time consuming and needs big computing capacity. Hence, a self-consistent quasiparticle model is presented to investigate the thermal properties of quark-gluon plasma above the critical temperature. All effects of the interactions among the partons are contained in the thermal mass of partons, so the system may be treated as an ideal gas of the massive quarks and gluons. From the statistical mechanics, we can calculate the thermodynamical quantities without any inconsistency, and then obtain a good fit to the lattice QCD data.
We also investigate the sensitivity of the hadron yields on the use of the different quark and hadron wave functions. The results show that the meson production rates strongly depend on the gluon mass at fix temperature (here T = 180 MeV). Considering the temperature dependence, the rates are very much insensitive on this parameter at fix gluon mass (mg= 800 MeV). With the ratioΦ/K*=0.60±0.15 measured at RHIC in central Au+Au collisions at (?)= 200 GeV, one could get that the hadron rations are insensitive on the wave function setups. These results prove most strikingly why the coalescence models yield very good agreement during data reconstruction. Besides, the problem of forward production of hadrons in heavy-ion collision at RHIC is revisited with modification of the theoretical treatment on the one hand and with the use of new data on the other. The basic formalism for hadronization remains the same as before, namely, recombination, but the details of momentum degradation and quark regeneration are improved. Recent data on the p/πand (?)/p ratios are used to constrain the value of the degradation parameter. The transverse momentum pT spectrum of the average charged particles is well reproduced. A prediction on the PT dependence of the p/p ratio atη= 3.2 is made.
With the careful consideration of the initial geometry, We have given a unified description of all the azimuthal dependencies of all observables onπproduction at low pT (pT＜2 GeV/c) in heavy-ion collisions. They are:the nuclear modification factor RAA(φ,Np), elliptic flow coefficient v2(Np) and ridge yield YR(φs, Np) as functions of Np, the number of participants. The main physics input is that the semihard scattering near the surface drives the azimuthal anisotropy on the one hand and the production of ridge particles on the other with or without trigger. The geometrical factor, S(φ, b), that makes precise the bridge between the two aspects of the problem follows from a study of the correlation between theφdirections of the trigger and ridge particles. The main understanding achieved in this picture is that the single-particle distribution dNAAπ/pTdpTdφhas two components:one is theφ-independent bulk B(pT,Np), different from the conventional bulk that isφ-dependent, and the other is the ridge component R(pT,φ,Np) that carries all theφdependence.
Finally, With the experimental data from STAR, PHENIX and BRAHMS programs on the centrality, rapidity and energy of transverse momentum pT spectrum in Au+Au and d+Au colli-sions, we show that there exists scaling distributions for pion, proton and antiproton. The difference between the scaling functions for protons and antiprotons is quite small, but they differ a lot from that of pions. From the scaling functionsΨ(u), one can see that the only parameter characterizing the normalized distribution is the average transverse momentum which depends on the cen-trality, rapidity and the colliding energy and system. Once is known, both the soft part with low pT and the hard part with high pT are determined byΨ(u). Using the obtained scaling func-tions of pions and protons, we could describe the charged particle pT spectra at different charged particle multiplicities in p+p collisions as a superposition of those from pion and proton. Different from momentum, kinetic energy is a scalar and is directly associated with the temperature of the hot medium created in the collisions. For different species of particles, the same momentum corre-sponds to different kinetic energy because of mass effect. Thus the distributions of kinetic energy of particles produced in ultra-relativistic heavy ion collisions are more effective in revealing the thermal properties of the system. So we pursue to investigate the scaling properties of transverse kinetic energy ET distributions of pions and protons, and find the agreement of the scaling ET distribution is better at low and high region. Besides, in the two frameworks:string fragmentation, cluster formation and decay, the universal transverse energy distributions for pion and proton can be described separately but not simultaneously. This fact indicates that they are not the universal mechanisms for the production of final state pions, protons, and other particles in high energy collisions. Obviously more detailed studies, both theoretically and experimentally, are needed.
本文首先简单回顾了相对论重离子碰撞的实验和理论的现状。然后介绍计算中所需的基本物理概念和唯象模型,包括描述核-核碰撞中,核几何形状的Glauber模型；重组合模型(Recombination/Coalescence(ReCo) Model),特别是Oregon小组(Hwa/Yang)模型的基本思想；以及Peter Levai小组的ALgebraic COalescence Rehadronization模型(ALCOR)。
首先利用统计力学方法探讨了高能重离子碰撞中产生的QGP的热力学性质。在高温下,由于渐近自由,部分子之间的相互作用很弱,所以可以把夸克和胶子当作自由气体来处理。但是,在临界温度(Tc)附近,由于偶合常数比较大,所以很难再利用解析的方法。如果从第一原理出发,利用格点QCD计算,又需要大量时间和计算机资源。因此,在本文第二章中,将会用一个自洽的准粒子唯象模型研究QGP的热化性质。假设部分子间的相互作用可以全部包含在其质量之中,这样就可以把系统当作理想气态来处理,然后再利用统计力学,计算各种热力学量。在T>Tc区域,可以很好的拟合格点QCD数据。
基于ALCOR模型讨论奇异粒子在不同波函数组合下的产生率。结果表明强子产生率依赖于胶子质量,但对温度并不敏感；而产生率之比几乎与胶子质量无关。再选择能量为s~(1/2)=200AGeV, Au+Au中心碰撞中的粒子比Φ／K*=0.60±0.15作为出发点,计算在不同波函数组合的强子比,结果表明粒子之比对波函数的选择不敏感。另外,本文还利用最新的实验数据p／π和p/p,在夸克重组合模型(Recombination Model)的框架下,重新探讨了高能重离子碰撞中,在向前快度区域(η=3.2)末态强子的产生。重新考虑了系统中部分子动量衰减和夸克再生效应后,得到与实验数据相符的带电强子谱和较大的质子与π介子之比,而且还预言了反质子与质子之比。
在深入研究核-核碰撞的重叠区域几何结构基础上,统一描述了在低横动量区域(pT<2GeV／c),π0介子所有与轴向方位角有关的观测量：核修正因子RAA(φ,Np),椭圆流v2(Np)和ridge产量YR(φs,Np),并与实验数据符合的很好。两个基本的出发点是：系统表面的半硬散射导致轴向异性和ridge粒子的产生与触发粒子的选择无关。虽然RAA是单粒子分布的量度,而YR是触发粒子和其伴随粒子关联的量度,但两者之间存在着紧密的联系。在整个物理图像中,最关键一点就是把单粒子分布dNAAπ／pTdptdφ分为两部分：不依赖于方位角φ的bulk组分(B(pT,Np))和依赖于φ的ridge组分(R(pT,φ,Np))。
最后,本文利用RHIC实验数据,深入研究不同的中心度,(赝)快度和碰撞系统下,π介子,质子和反质子横动量谱的标度行为。结果表明：π介子的标度行为不依赖与中心度、(赝)快度、碰撞系统和质心能量而存在。而对于质子和反质子,在SNN~(1/2)=200 GeV下,Au+Au碰撞中也存在不依赖于中心度和快度的标度行为。但三者的标度行为存在着差别,这与粒子的夸克组成有着紧密的联系。在这些过程中,表征这些粒子标度行为的参数只有一个：粒子的平均横动量〈pT〉,其依赖于中心度、快度、碰撞能量及其碰撞系统。一旦知道了〈pT〉,那么低pT区的软过程部分和高pT区的硬过程部分就可以由粒子标度函数决定了。然后再利用π介子和质子标度行为,研究p+p碰撞中带电强子在不同多重数下横动量分布的标度行为。发现不同多重数下的分布谱仍具有标度行为,并可以用π介子和质子标度函数的线性叠加来表示。与动量不同,动能是一个标量,并直接与热密物质的温度相关。由于质量效应,对于不同的粒子,相同的动量对应着不同的动能。这样,在超相对论重离子碰撞中,动能的分布更能有效地揭示系统的热化性质。因此,第七章还进一步研究质子和π介子横能分布的标度行为,并与其横动量的标度行为进行了比较,结果表明在低和高动量(横能)区域,横能的标度性更好。标度律可以作为探寻粒子产生的机制的一个重要信息。通过分析,发现弦碎裂和团碎裂机制可以分别描述π介子和质子的标度行为,但是不能同时得到这两种粒子标度行为,因此这两种机制并不是普适的粒子产生机制,需要寻找新的机制来解释末态粒子的产生。
At extremely high energy densities, Quantum Chromodynamics (QCD) predicts a new form of matter, consisting of an extended volume of interacting quarks, antiquarks and gluons. This is quark-gluon plasma (QGP). The relativistic heavy ion collision is a significant experimental tool for deep understanding of this new form of matter, through which scientists expect to realize the transition from nuclear matter to quark matter. The research of final observables in nucleus-nucleus collision, such as:the particle distribution, correlation and fluctuation, etc, is greatly important for understanding the evolution of the high energy heavy ion collision and the particle production mechanisms. This dissertation mainly focuss on the particle production mechanisms and properties of particle distributions, with the data from the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory.
In the first part, the status of heavy ion collision is briefly reviewed, both in experimental and theoretical aspects. Then some important definitions and phenomenological models are introduced, including the Glauber model, Recombination/Coalescence (ReCo) Model by the Oregon group and ALgebraic COalescence Rehadronization model (ALCOR).
At very high temperature, due to the asymptotic freedom, the interactions among the partons are so weak that one could approximately treat the system as an ideal gas of quarks and gluons. However, near the critical temperature Tc, it is very difficult to solve QCD by analytical methods, such as the perturbative theory, because the coupling constant is not small enough. Lattice QCD can be used to calculate, from the first principle, the properties of simple systems. But this method is time consuming and needs big computing capacity. Hence, a self-consistent quasiparticle model is presented to investigate the thermal properties of quark-gluon plasma above the critical temperature. All effects of the interactions among the partons are contained in the thermal mass of partons, so the system may be treated as an ideal gas of the massive quarks and gluons. From the statistical mechanics, we can calculate the thermodynamical quantities without any inconsistency, and then obtain a good fit to the lattice QCD data.
We also investigate the sensitivity of the hadron yields on the use of the different quark and hadron wave functions. The results show that the meson production rates strongly depend on the gluon mass at fix temperature (here T = 180 MeV). Considering the temperature dependence, the rates are very much insensitive on this parameter at fix gluon mass (mg= 800 MeV). With the ratioΦ/K*=0.60±0.15 measured at RHIC in central Au+Au collisions at (?)= 200 GeV, one could get that the hadron rations are insensitive on the wave function setups. These results prove most strikingly why the coalescence models yield very good agreement during data reconstruction. Besides, the problem of forward production of hadrons in heavy-ion collision at RHIC is revisited with modification of the theoretical treatment on the one hand and with the use of new data on the other. The basic formalism for hadronization remains the same as before, namely, recombination, but the details of momentum degradation and quark regeneration are improved. Recent data on the p/πand (?)/p ratios are used to constrain the value of the degradation parameter. The transverse momentum pT spectrum of the average charged particles is well reproduced. A prediction on the PT dependence of the p/p ratio atη= 3.2 is made.
With the careful consideration of the initial geometry, We have given a unified description of all the azimuthal dependencies of all observables onπproduction at low pT (pT＜2 GeV/c) in heavy-ion collisions. They are:the nuclear modification factor RAA(φ,Np), elliptic flow coefficient v2(Np) and ridge yield YR(φs, Np) as functions of Np, the number of participants. The main physics input is that the semihard scattering near the surface drives the azimuthal anisotropy on the one hand and the production of ridge particles on the other with or without trigger. The geometrical factor, S(φ, b), that makes precise the bridge between the two aspects of the problem follows from a study of the correlation between theφdirections of the trigger and ridge particles. The main understanding achieved in this picture is that the single-particle distribution dNAAπ/pTdpTdφhas two components:one is theφ-independent bulk B(pT,Np), different from the conventional bulk that isφ-dependent, and the other is the ridge component R(pT,φ,Np) that carries all theφdependence.
Finally, With the experimental data from STAR, PHENIX and BRAHMS programs on the centrality, rapidity and energy of transverse momentum pT spectrum in Au+Au and d+Au colli-sions, we show that there exists scaling distributions for pion, proton and antiproton. The difference between the scaling functions for protons and antiprotons is quite small, but they differ a lot from that of pions. From the scaling functionsΨ(u), one can see that the only parameter characterizing the normalized distribution is the average transverse momentum
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