强子产额及其关联的能量依赖
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摘要
格点QCD理论预言,在温度足够高或密度足够大的极端条件下,禁闭的强子物质将会经过相变形成解禁闭的夸克物质——夸克胶子等离子体(QGP)。极端相对论重离子碰撞实验有可能提供产生这种特殊物质形态的条件。美国布鲁克海文国家实验室的相对论重离子对撞机(RHIC)其动机之一就是在实验室中产生QGP物质并研究其特殊性质。自2000年该实验开动以来,积累了海量的实验数据,各种信号显示解禁闭的热密夸克物质可能已经产生。欧洲核子研究中心的NA49实验组在top SPS能量下Pb+Pb碰撞中观测到的末态强子的某些特征性质,如强子的椭圆流,与RHIC实验在Au+Au碰撞中测量的结果有惊人的相似之处。这可能意味着,如果RHIC实验的Au+Au碰撞中产生了解禁闭的热密夸克物质,那么在top SPS能量下Pb+Pb碰撞中也应该有解禁闭的夸克物质产生。在更低的SPS能量下解禁闭的夸克物质有没有产生?究竟在哪个碰撞能量下解禁闭的夸克物质首次产生?这成为近几年来高能重离子碰撞物理所关注的热点问题。NA49实验组综合分析了各种可观测量反映的信息,初步给出——解禁闭的夸克胶子等离子体物质可能首次出现在碰撞束能Ebeam=30AGeV附近。目前,STAR实验组也正在进行能量扫描工作,期待对这一问题提供进一步的实验证据。
在极端相对论重离子碰撞中,解禁闭的热密夸克物质一旦形成,经过强子化之后形成的热强子的各种可观测量,比如：产额、横动量等,就会有某些来自早期夸克自由度的关联。最有力的证据之一就是RHIC实验观测到的强子椭圆流v2的组分夸克数目的scaling现象,即把不同强子的v2和pT都除以强子的组分夸克数,这些重新标度后的不同强子的v2在中等横动量区几乎重合。在夸克组合的框架下,强子重新标度后的v2正是组分夸克的v2,这是对夸克组合强子化机制的一个最有力的支持。在夸克组合图像下,非束缚的夸克和反夸克可以自由地组成各种强子,这些来自早期夸克自由度的强子之间的关联就很自然地形成了。反之,如果重离子碰撞中根本没有解禁闭的夸克物质产生,就没有自由的夸克和反夸克,更谈不上夸克自由组合,这些所谓的“夸克级”关联就可能消失。因此,我们可以通过研究不同强子之间的关联来研究高能重离子碰撞中解禁闭的夸克物质是否产生、它的各种性质及其强子化机制等。本文利用夸克组合模型就高能重离子碰撞中产生的热强子做了以下两方面的研究：
(一)研究不同碰撞能量下的各种强子在中间快度区的产额密度。强子产额是一个非常基本而又重要的可观测量,它反映了相对论重离子碰撞中产生的热密物质的整体性质。本文用夸克组合模型系统研究从RHIC能量到SPS能量Ebeam=158,80,40,30,20AGeV下核核中心碰撞中各种强子在中间快度区的产额密度。结果显示,在Ebeam=30AGeV到这一大的能量区间内,我们的计算结果与实验数据符合得很好,这一能量区间正好是NA49实验组给出的可能有解禁闭的夸克物质产生的能区；而在Ebeam=20AGeV时,夸克组合模型不能很好地描述实验,这可能是因为夸克自由度在热强子产生中已经不起决定性作用。进一步,我们预言了更高能量的LHC实验在质心系能量时Pb+Pb中心碰撞中各种强子在中间快度区的产额密度及产额比。
(二)研究不同碰撞能量下各种强子的产额之间的关联。本文定义了两个对夸克自由度敏感的关联量和在夸克组合的图像下,直生强子对应的A、B的值不依赖具体的模型,都应该等于1.0。因此在高能重离子碰撞中直生强子的关联量A、B的值是否偏离1.0可以看作是否有解禁闭的夸克物质产生的一个信号。本文利用强子产额的实验数据系统计算了各个能量下A、B的实验值。又利用夸克组合模型修正了共振态衰变的影响,得到直生强子对应的A、B的值。结果显示,当碰撞能量大于等于30AGeV时,直生强子对应的A、B的值都约等于1.0,而在20AGeV时,A的值严重地偏离了1.0,这可能意味着在该能量附近基于夸克自由度的夸克组合模型已不再适用。我们的这一发现与NA49实验组给出的解禁闭的夸克物质可能首次出现在Ebeam=30AGeV附近的结果是一致的。
According to lattice QCD, people predict that at extremely high temperature or high energy density, the confined hadronic matter will undergo a phase transition to a new state of deconfined matter—quark gluon plasma (QGP). Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Lab provides an environment to create QGP. A variety of experimental phenomena at RHIC have indicated that QGP has been probably produced. The experimental data about Pb+Pb collisions at top Super Proton Synchrotron (SPS) energy from NA49 Collaboration show similar properties to those of RHIC energies. This implies that QGP has been produced at top SPS, if it has been produced at RHIC energies. Has QGP been produced in A+A collisions at lower SPS energies? Which energy point does QGP first appear at? The Beam Energy Scan programme of NA49 experiment at CERN-SPS has suggested a preliminary answer—around 30AGeV. The ongoing Beam Energy Scan programme of STAR Collaboration at Brookhaven National Lab provides an opportunity to study it in more detail.
Once the deconfined hot and dense quark matter is produced in heavy ion collisions, the observables of various thermal hadrons after hadronization, e.g. yields and momentum spectra etc, have some correlations originated from early quark degrees of freedom. One of the most typical examples is the elliptic flow v2 of hadrons measured at RHIC energies. As both v2 and transverse momentum PT are divided by the constituent quark number of hadron, the rescaled v2 of various baryons and mesons, which are just that of constituent quarks, almost coincide with each other in the intermediate PT range. If the hot and dense quark matter is hadronized by quark combination, as is commonly accepted, these correlations of hadrons can be beautifully explained. In quark combination scenario, quarks and anti-quarks are available in unbound state before hadronization and they can coalesce freely into various hadrons, and thereby these correlations from early quark degrees of freedom among different hadron species are naturally formed. On the other hand, if the deconfined quark matter is not produced at all in collisions, there is no free quarks and anti-quarks (much less their subsequent combination) and these so-called "quark-level" correlations of hadrons maybe disappear or contort. Therefore, we can study whether the deconfinement is achieved by investigating these correlations among various hadrons produced in heavy ion collisions. The work contains two aspects about whether there is deconfined quark matter and the hadronization mechanism in heavy ion collisions as follows: (Ⅰ)We study the yield densities of various hadrons at different energies. Hadron yield is one of the most fundamental and significant observables from which one can obtain a lot of important information on the hot nuclear matter produced at the early stage of relativistic heavy ion collisions. We systematically study the yield densities of various hadrons at mid-rapidity from RHIC energies SNN~(1/2)= 200,130,62.4GeV to SPS energies Ebeam=158,80,40,30,20AGeV in heavy ion collisions. It is shown that the quark combination model can describe the experimental data well from Ebeam=30AGeV to SNN~(1/2)=200GeV and this energy region is just the region claimed by NA49 Collaboration where QGP maybe has been produced. At20AGeV, the quark combination model fails. This suggests that the constituent quark degrees of freedom do not represent a decisive factor in thermal hadron production. Furthermore, we predict hadron yields as well as particle ratios at mid-rapidity in the most central Pb+Pb collisions at SNN~(1/2)=5.5TeV. (Ⅱ)We also investigate the hadron yield correlations at different energies. In this work, we lefine two correlation quantities A=(ΛK-p)/ΛK+p and B=(ΛK-Ξ+)/(ΛK+Ξ-), sensitive to quark degrees of freedom. The values of A and B for directly produced hadrons are equal to 1.0 in the framework of quark combination, independent of special models. The deviation of A and B from 1.0 or not can be regarded as a possible signal of deconfinement in heavy ion collisions. Following the experimental data at different collision energies, we evaluate the values of A and B. In order to explore the decay effect, we use the quark combination model to compute the values of A and B for the directly produced hadrons, and find that as the collision energy is greater than or equal to 30AGeV the values of A and B are nearly equal to 1.0, while at 20AGeV, it deviates from 1.0. This supports the conclusion to some degree drown by NA49 Collaboration that the onset of deconfinement may be located around 30 AGeV.
在极端相对论重离子碰撞中,解禁闭的热密夸克物质一旦形成,经过强子化之后形成的热强子的各种可观测量,比如：产额、横动量等,就会有某些来自早期夸克自由度的关联。最有力的证据之一就是RHIC实验观测到的强子椭圆流v2的组分夸克数目的scaling现象,即把不同强子的v2和pT都除以强子的组分夸克数,这些重新标度后的不同强子的v2在中等横动量区几乎重合。在夸克组合的框架下,强子重新标度后的v2正是组分夸克的v2,这是对夸克组合强子化机制的一个最有力的支持。在夸克组合图像下,非束缚的夸克和反夸克可以自由地组成各种强子,这些来自早期夸克自由度的强子之间的关联就很自然地形成了。反之,如果重离子碰撞中根本没有解禁闭的夸克物质产生,就没有自由的夸克和反夸克,更谈不上夸克自由组合,这些所谓的“夸克级”关联就可能消失。因此,我们可以通过研究不同强子之间的关联来研究高能重离子碰撞中解禁闭的夸克物质是否产生、它的各种性质及其强子化机制等。本文利用夸克组合模型就高能重离子碰撞中产生的热强子做了以下两方面的研究：
(一)研究不同碰撞能量下的各种强子在中间快度区的产额密度。强子产额是一个非常基本而又重要的可观测量,它反映了相对论重离子碰撞中产生的热密物质的整体性质。本文用夸克组合模型系统研究从RHIC能量到SPS能量Ebeam=158,80,40,30,20AGeV下核核中心碰撞中各种强子在中间快度区的产额密度。结果显示,在Ebeam=30AGeV到这一大的能量区间内,我们的计算结果与实验数据符合得很好,这一能量区间正好是NA49实验组给出的可能有解禁闭的夸克物质产生的能区；而在Ebeam=20AGeV时,夸克组合模型不能很好地描述实验,这可能是因为夸克自由度在热强子产生中已经不起决定性作用。进一步,我们预言了更高能量的LHC实验在质心系能量时Pb+Pb中心碰撞中各种强子在中间快度区的产额密度及产额比。
(二)研究不同碰撞能量下各种强子的产额之间的关联。本文定义了两个对夸克自由度敏感的关联量和在夸克组合的图像下,直生强子对应的A、B的值不依赖具体的模型,都应该等于1.0。因此在高能重离子碰撞中直生强子的关联量A、B的值是否偏离1.0可以看作是否有解禁闭的夸克物质产生的一个信号。本文利用强子产额的实验数据系统计算了各个能量下A、B的实验值。又利用夸克组合模型修正了共振态衰变的影响,得到直生强子对应的A、B的值。结果显示,当碰撞能量大于等于30AGeV时,直生强子对应的A、B的值都约等于1.0,而在20AGeV时,A的值严重地偏离了1.0,这可能意味着在该能量附近基于夸克自由度的夸克组合模型已不再适用。我们的这一发现与NA49实验组给出的解禁闭的夸克物质可能首次出现在Ebeam=30AGeV附近的结果是一致的。
According to lattice QCD, people predict that at extremely high temperature or high energy density, the confined hadronic matter will undergo a phase transition to a new state of deconfined matter—quark gluon plasma (QGP). Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Lab provides an environment to create QGP. A variety of experimental phenomena at RHIC have indicated that QGP has been probably produced. The experimental data about Pb+Pb collisions at top Super Proton Synchrotron (SPS) energy from NA49 Collaboration show similar properties to those of RHIC energies. This implies that QGP has been produced at top SPS, if it has been produced at RHIC energies. Has QGP been produced in A+A collisions at lower SPS energies? Which energy point does QGP first appear at? The Beam Energy Scan programme of NA49 experiment at CERN-SPS has suggested a preliminary answer—around 30AGeV. The ongoing Beam Energy Scan programme of STAR Collaboration at Brookhaven National Lab provides an opportunity to study it in more detail.
Once the deconfined hot and dense quark matter is produced in heavy ion collisions, the observables of various thermal hadrons after hadronization, e.g. yields and momentum spectra etc, have some correlations originated from early quark degrees of freedom. One of the most typical examples is the elliptic flow v2 of hadrons measured at RHIC energies. As both v2 and transverse momentum PT are divided by the constituent quark number of hadron, the rescaled v2 of various baryons and mesons, which are just that of constituent quarks, almost coincide with each other in the intermediate PT range. If the hot and dense quark matter is hadronized by quark combination, as is commonly accepted, these correlations of hadrons can be beautifully explained. In quark combination scenario, quarks and anti-quarks are available in unbound state before hadronization and they can coalesce freely into various hadrons, and thereby these correlations from early quark degrees of freedom among different hadron species are naturally formed. On the other hand, if the deconfined quark matter is not produced at all in collisions, there is no free quarks and anti-quarks (much less their subsequent combination) and these so-called "quark-level" correlations of hadrons maybe disappear or contort. Therefore, we can study whether the deconfinement is achieved by investigating these correlations among various hadrons produced in heavy ion collisions. The work contains two aspects about whether there is deconfined quark matter and the hadronization mechanism in heavy ion collisions as follows: (Ⅰ)We study the yield densities of various hadrons at different energies. Hadron yield is one of the most fundamental and significant observables from which one can obtain a lot of important information on the hot nuclear matter produced at the early stage of relativistic heavy ion collisions. We systematically study the yield densities of various hadrons at mid-rapidity from RHIC energies SNN~(1/2)= 200,130,62.4GeV to SPS energies Ebeam=158,80,40,30,20AGeV in heavy ion collisions. It is shown that the quark combination model can describe the experimental data well from Ebeam=30AGeV to SNN~(1/2)=200GeV and this energy region is just the region claimed by NA49 Collaboration where QGP maybe has been produced. At20AGeV, the quark combination model fails. This suggests that the constituent quark degrees of freedom do not represent a decisive factor in thermal hadron production. Furthermore, we predict hadron yields as well as particle ratios at mid-rapidity in the most central Pb+Pb collisions at SNN~(1/2)=5.5TeV. (Ⅱ)We also investigate the hadron yield correlations at different energies. In this work, we lefine two correlation quantities A=(ΛK-p)/ΛK+p and B=(ΛK-Ξ+)/(ΛK+Ξ-), sensitive to quark degrees of freedom. The values of A and B for directly produced hadrons are equal to 1.0 in the framework of quark combination, independent of special models. The deviation of A and B from 1.0 or not can be regarded as a possible signal of deconfinement in heavy ion collisions. Following the experimental data at different collision energies, we evaluate the values of A and B. In order to explore the decay effect, we use the quark combination model to compute the values of A and B for the directly produced hadrons, and find that as the collision energy is greater than or equal to 30AGeV the values of A and B are nearly equal to 1.0, while at 20AGeV, it deviates from 1.0. This supports the conclusion to some degree drown by NA49 Collaboration that the onset of deconfinement may be located around 30 AGeV.
引文
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[14]F. L. Shao, T. Yao, Q. B. Xie, Phys. Rev. C 75 (2007) 034904.
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[16]D. M. Wei, F. L. Shao, J. Song, Y. F. Wang, Int. J. Mod. Phys. A 23 (2008) 5217.
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