RHIC和SPS能区末态强子的快度分布
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摘要
夸克胶子等离子体(QGP)的产生及其性质的研究是相对论重离子碰撞物理的热点课题。近年来,许多QGP的特征信号被提出来,并进行了实验观测。RHIC和SPS能区的重离子碰撞实验发现了一系列独特的强子产生的新现象,这些实验结果从不同方面极大促进了人们对QGP的产生、性质及其强子化机制的深入理解。其中有一类实验现象特别有意思,即中等横动量区重子和介子的产生存在明显差异。RHIC的实验数据显示：在该区间,重子介子的椭圆流呈现组分夸克数标度(Constituent Quark Number Scaling)现象；重子的产生相对介子有大的增强,即所谓的大重子介子比；重子介子的核效应因子RAA和RCP也有明显差异。这些实验现象揭示了重离子碰撞中强子产生机制的新特征。
在夸克组合框架下,强子由夸克组合而成,即一个夸克和反夸克对组合成一个介子,三个夸克组合成一个重子。组分夸克数的不同造成了重子和介子产生的差异。研究结果表明,这种夸克计数可以成功解释RHIC能区所观测到的重子和介子产生差异。考虑到组分夸克分布的味道差别,夸克组合机制即可自洽地描述各种介子和重子的产生。这种介子和重子间的夸克计数关联是建立在组分夸克自由度存在的基础上,与相对论重离子碰撞中热密部分子物质的产生紧密关联。
上述RHIC实验的这些亮点主要集中在中等横动量区的强子的产生,这也是夸克组合图像最闪光的地方。事实上,强子的纵向快度分布同样是检验强子产生机制的有力工具。强子的快度密度主要取决于低横动量区(PT)的强子产生。小横动量区的夸克和反夸克比中间PT区更丰富,夸克之间的组合应该能够更自然的发生。与PT直接进行矢量相加不同,组合过程中快度的变化可能不是那么明显,但是通过组合而来的各种强子的快度分布可以明显区分。在夸克组合图像下,某种特定强子的快度分布是其组分夸克的快度分析与组合几率函数的卷积；由于不同味道夸克的快度分布不同,尤其是新生夸克和净夸克的快度分布差异更大,不同组分夸克组成的强子的快度分布一般并不相同,它们之间通过组分夸克相互关联。因此可以通过强子快度谱的实验数据识别这种内在的夸克级关联来检验夸克组合机制的适用性,或通过研究各种末态强子的快度分布能否用一组夸克谱自洽描述来检验。本文用山东夸克组合模型系统研究了从RHIC能区到SPS能区核核对心碰撞中各种末态强子的快度分布。具体研究内容如下：
(一)系统研究了从RHIC (?)=200,62AGeV到SPS Ebeam=158,80,40,30,20AGeV能量下组分夸克的快度分布。结果显示,在该能量区间,组分夸克的快度分布有明显的变化,净夸克的快度分布随着碰撞能量降低双峰结构逐渐消失,新生夸克快度分布的宽度也逐渐变小。在解禁闭开始的闽值能区30AGeV和20AGeV,奇异夸克的快度谱宽开始比轻夸克窄,这与更高能量下的结果相反。
(二)基于上面得到的组分夸克的纵向分布,用山东夸克组合模型,对RHIC(?)=62.4GeV和SPS Eveam=80,40,30,20AGeV能量下的重离子碰撞中末态强子的快度分布进行了系统研究,并和实验结果进行了比较。研究结果表明,从RHIC(?)=200,62.4GeV到SPS Ebeam=158,80,40,30,20AGeV这么宽的能量区间内,各种强子的快度谱都能够被夸克组合机制很好地描述。
The production of quark gluon plasma (QGP) and its properties are hot topics in relativistic heavy ion collisions. A huge number of possible QGP signals were proposed and measured by experiments, and many unexpected novel phenomena were observed at RHIC and SPS. These experimental data greatly contribute to the identification of QGP and the understanding of its properties and hadronization mechanism from different aspects. Especially, there are a class of phenomena that are of particular interest, i.e. the big difference between meson and baryon production in the intermediate PT range. The experimental data from RHIC have shown that, in this range, the elliptic flow of baryons is much greater than that of mesons, and correlated with that of mesons by the constituent quark number scaling; furthermore, the production of baryons significantly enhances relative to the meson production, i.e. the well-known high baryon/meson ratio; and baryons and mesons also differ greatly in their nuclear modification factors RAA and Rcp. This is a unique characteristic of relativistic heavy ion collisions which reveals the mechanism of hadron production.
In quark (re-)combination/coalescence scenario, hadrons are combined from quarks and antiquarks, that is, a quark-antiquark pair merges into a meson and three quarks into a baryon. The difference of production between baryon and meson mainly results from their different constituent quark numbers. It is shown that such a quark number counting can successfully explain the experimentally observed difference between baryon and meson at RHIC energies. Considering the flavor difference of light and strange quarks, the mechanism can self-consistently reproduce the data of various light, strange mesons and baryons. Of course, such production correlation between meson and baryon via quark number counting is based on the existence of constituent quark degrees of freedom in relativistic heavy ion collisions, which is closely related to the creation of partonic bulk matter.
These highlights at RHIC stated above are mainly of hadron production in transverse direction where quark recombination scenario mostly flashes. In fact, the longitudinal rapidity distributions of hadrons are also good tools of testing the scenario. The rapidity density of hadrons is mainly contributed from the hadrons in low PT region. In this region quarks and antiquarks are more abundant in phase space, compared with those in the intermediate PT region, so the combination happens more easily and naturally and should dominate the observed rapidity distributions of hadrons in relativistic heavy ion collisions and can be tested by the data. Compared with the direct (vector) summation of PT in transverse combination, the change of rapidity during combination may be not so explicit but the rapidity distributions of various hadrons formed via quark recombination are definitely distinguished. The rapidity distribution of a specific hadron in the mechanism is the convolution of the rapidity spectra of their constituent quarks and combination probability function. Since the rapidity distributions for different flavors of quarks are different, in particular the difference between newborn quarks and net-quarks coming from colliding nuclei, the shapes of rapidity spectra of hadrons with different quark components are different and correlated with each other. Therefore, the applicability of the mechanism can be tested by identifying this quark-level correlation underlain in the experimental data of hadronic rapidity distributions, or equivalently by studying wether the data of various hadron species can be simultaneously explained by a set of quark rapidity distributions. In this paper, we use a quark combination model to systematically investigate the rapidity distributions of various identified hadrons in central nucleus nucleus collisions in a broad collision energy region (from top RHIC to SPS).
The work contains two aspects as follows:
(I) Combining the past work, we systematically study the rapidity spectra of constituent quarks in central A+A collisions from RHIC energies (?)=200,62.4GeV to SPS energies Ebeam=158,80,40,30,20AGeV. There are obvious changes in such a broad energy interval, i.e. The dual hump shape of rapidity spectrum of net quarks are disappearing with the collision energies descent, the Gaussian width of the rapidity distribution of newborn light quarks reduces gradually with the decreasing energies. Besides, we notice another interesting phenomenon that the Gaus-sian width of newborn ud quarks is larger than that of s quarks at 20 AGeV while at other energies are contrary.
(II) In this section, we systematically calculate the rapidity distributions of identified hadrons in central A+A collisions from RHIC (?)=62 AGeV to SPS Ebeam=80,40,30,20AGeV under the framework of the quark combination. It is shown that the quark combination model can describe the experimental data well in such a broad energy interval.
在夸克组合框架下,强子由夸克组合而成,即一个夸克和反夸克对组合成一个介子,三个夸克组合成一个重子。组分夸克数的不同造成了重子和介子产生的差异。研究结果表明,这种夸克计数可以成功解释RHIC能区所观测到的重子和介子产生差异。考虑到组分夸克分布的味道差别,夸克组合机制即可自洽地描述各种介子和重子的产生。这种介子和重子间的夸克计数关联是建立在组分夸克自由度存在的基础上,与相对论重离子碰撞中热密部分子物质的产生紧密关联。
上述RHIC实验的这些亮点主要集中在中等横动量区的强子的产生,这也是夸克组合图像最闪光的地方。事实上,强子的纵向快度分布同样是检验强子产生机制的有力工具。强子的快度密度主要取决于低横动量区(PT)的强子产生。小横动量区的夸克和反夸克比中间PT区更丰富,夸克之间的组合应该能够更自然的发生。与PT直接进行矢量相加不同,组合过程中快度的变化可能不是那么明显,但是通过组合而来的各种强子的快度分布可以明显区分。在夸克组合图像下,某种特定强子的快度分布是其组分夸克的快度分析与组合几率函数的卷积；由于不同味道夸克的快度分布不同,尤其是新生夸克和净夸克的快度分布差异更大,不同组分夸克组成的强子的快度分布一般并不相同,它们之间通过组分夸克相互关联。因此可以通过强子快度谱的实验数据识别这种内在的夸克级关联来检验夸克组合机制的适用性,或通过研究各种末态强子的快度分布能否用一组夸克谱自洽描述来检验。本文用山东夸克组合模型系统研究了从RHIC能区到SPS能区核核对心碰撞中各种末态强子的快度分布。具体研究内容如下：
(一)系统研究了从RHIC (?)=200,62AGeV到SPS Ebeam=158,80,40,30,20AGeV能量下组分夸克的快度分布。结果显示,在该能量区间,组分夸克的快度分布有明显的变化,净夸克的快度分布随着碰撞能量降低双峰结构逐渐消失,新生夸克快度分布的宽度也逐渐变小。在解禁闭开始的闽值能区30AGeV和20AGeV,奇异夸克的快度谱宽开始比轻夸克窄,这与更高能量下的结果相反。
(二)基于上面得到的组分夸克的纵向分布,用山东夸克组合模型,对RHIC(?)=62.4GeV和SPS Eveam=80,40,30,20AGeV能量下的重离子碰撞中末态强子的快度分布进行了系统研究,并和实验结果进行了比较。研究结果表明,从RHIC(?)=200,62.4GeV到SPS Ebeam=158,80,40,30,20AGeV这么宽的能量区间内,各种强子的快度谱都能够被夸克组合机制很好地描述。
The production of quark gluon plasma (QGP) and its properties are hot topics in relativistic heavy ion collisions. A huge number of possible QGP signals were proposed and measured by experiments, and many unexpected novel phenomena were observed at RHIC and SPS. These experimental data greatly contribute to the identification of QGP and the understanding of its properties and hadronization mechanism from different aspects. Especially, there are a class of phenomena that are of particular interest, i.e. the big difference between meson and baryon production in the intermediate PT range. The experimental data from RHIC have shown that, in this range, the elliptic flow of baryons is much greater than that of mesons, and correlated with that of mesons by the constituent quark number scaling; furthermore, the production of baryons significantly enhances relative to the meson production, i.e. the well-known high baryon/meson ratio; and baryons and mesons also differ greatly in their nuclear modification factors RAA and Rcp. This is a unique characteristic of relativistic heavy ion collisions which reveals the mechanism of hadron production.
In quark (re-)combination/coalescence scenario, hadrons are combined from quarks and antiquarks, that is, a quark-antiquark pair merges into a meson and three quarks into a baryon. The difference of production between baryon and meson mainly results from their different constituent quark numbers. It is shown that such a quark number counting can successfully explain the experimentally observed difference between baryon and meson at RHIC energies. Considering the flavor difference of light and strange quarks, the mechanism can self-consistently reproduce the data of various light, strange mesons and baryons. Of course, such production correlation between meson and baryon via quark number counting is based on the existence of constituent quark degrees of freedom in relativistic heavy ion collisions, which is closely related to the creation of partonic bulk matter.
These highlights at RHIC stated above are mainly of hadron production in transverse direction where quark recombination scenario mostly flashes. In fact, the longitudinal rapidity distributions of hadrons are also good tools of testing the scenario. The rapidity density of hadrons is mainly contributed from the hadrons in low PT region. In this region quarks and antiquarks are more abundant in phase space, compared with those in the intermediate PT region, so the combination happens more easily and naturally and should dominate the observed rapidity distributions of hadrons in relativistic heavy ion collisions and can be tested by the data. Compared with the direct (vector) summation of PT in transverse combination, the change of rapidity during combination may be not so explicit but the rapidity distributions of various hadrons formed via quark recombination are definitely distinguished. The rapidity distribution of a specific hadron in the mechanism is the convolution of the rapidity spectra of their constituent quarks and combination probability function. Since the rapidity distributions for different flavors of quarks are different, in particular the difference between newborn quarks and net-quarks coming from colliding nuclei, the shapes of rapidity spectra of hadrons with different quark components are different and correlated with each other. Therefore, the applicability of the mechanism can be tested by identifying this quark-level correlation underlain in the experimental data of hadronic rapidity distributions, or equivalently by studying wether the data of various hadron species can be simultaneously explained by a set of quark rapidity distributions. In this paper, we use a quark combination model to systematically investigate the rapidity distributions of various identified hadrons in central nucleus nucleus collisions in a broad collision energy region (from top RHIC to SPS).
The work contains two aspects as follows:
(I) Combining the past work, we systematically study the rapidity spectra of constituent quarks in central A+A collisions from RHIC energies (?)=200,62.4GeV to SPS energies Ebeam=158,80,40,30,20AGeV. There are obvious changes in such a broad energy interval, i.e. The dual hump shape of rapidity spectrum of net quarks are disappearing with the collision energies descent, the Gaussian width of the rapidity distribution of newborn light quarks reduces gradually with the decreasing energies. Besides, we notice another interesting phenomenon that the Gaus-sian width of newborn ud quarks is larger than that of s quarks at 20 AGeV while at other energies are contrary.
(II) In this section, we systematically calculate the rapidity distributions of identified hadrons in central A+A collisions from RHIC (?)=62 AGeV to SPS Ebeam=80,40,30,20AGeV under the framework of the quark combination. It is shown that the quark combination model can describe the experimental data well in such a broad energy interval.
引文
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[3]J. Adams, et al. (STAR Collaboration), Particle-Type Dependence of Azimuthal Anisotropy and Nuclear Modification of Particle Production in Au+Au Collisions at (?)=200GeV[J]. Phys. Rev. Lett.2004,92:052302.
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[5]R. C. Hwa, C. B. Yang, Scaling behavior at high PT and the p/pi ratio [J]. Phys. Rev. C 2003,67:034902.
[6]R. J. Fries, et al. Hadronization in Heavy-Ion Collisions: Recombination and Fragmentation of Partons [J]. Phys. Rev. Lett.2003,90:202303.
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