强子椭圆流作为探测QCD临界点和相边界的信号以及选择长寿命高密相UU碰撞
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摘要
几千年以来,人类的好奇心驱使我们一直在问同样的问题：组成我们物质世界的最基本物质是什么?它们是通过什么样的方式组成我们的世界的?1883年,英国科学家道尔顿展了古希腊哲学家德谟克利特的朴素原子学说,提出物质是由原子组成的。随着近代科学技术的不断发展,人类对微观世界的探索不断深入,人们发现原子是由处于最中心的带正电的原子核和它外围的带负电的电子组成的,原子核内包括带正电的质子和不带电的中子。随后,通过电子打核子的深度非弹实验,人们认识到这些粒子可能有更基本的组成,这就是夸克和胶子。但是一直以来,人们并不曾观察到自由存在的孤立夸克。一种自然的解释是夸克通过相互作用被禁闭在强子里,这种相互作用称为强相互作用。描述强相互作用的理论-量子色动力学(QCD)指出所有参与强相互作用的基本粒子都是夸克q和反夸克(q)的束缚态。在通常条件下,我们所观察到的QCD物质都是以强子气体形式存在的。
有两种可能打破强子束缚的途径,一是：将强子物质压缩,即增大核子物质密度,使强子口袋相互重叠而破裂,这样夸克和胶子就能在较大的空间范围内自由运动；另一种方法将系统加热,使温度升高到足够高,真空中会产生很多正反夸克对,大量的夸克胶子发生激烈碰撞,从而可能形成夸克和胶子在较大范围内运动的状态。格点QCD理论预言,在极高的温度或者高重子数密度下,强子物质(夸克的禁闭相)会解禁闭形成其解禁闭相——夸克胶子等离子体(Quark Gluon Plasma—QGP)。进一步的数值计算显示,在临界温度Tc～160MeV(零密度)或物质密度5～10倍正常核物质密度(零温度)时会发生QGP相变。
夸克胶子等离子体可能存在于宇宙大爆炸早期阶段(很高的温度)以及中子星(重子数密度很高)内。然而这两种情况要么发生在过去,要么距离我们太遥远,没有办法去直接测量。在实验室,通过电磁场加速,我们让两束重离子以高速进行对撞,从而生成能量密度比较高并且寿命比较长的强相互作用物质,QGP有可能在这种情况下出现。重离子碰撞的主要目的之一就是希望在实验室条件下达到QGP相变所需要的高温高密环境,研究极端条件下的物质性质。从早期的伯克力(Berkeley/Bevalac)到欧洲核子中心(CERN)的超级质子同步加速器(SPS),以及布鲁克海文实验室(BNL)的交变梯度同步加速器(AGS)和相对论重离子对撞机(RHIC),人们已经对高温下的物质性质进行了很多卓有成效的研究。
实验上,我们只能测得末态的强子分布,希望通过末态强子说携带的信息寻找夸克胶子等离子体存在的信号。“集体行为”是指在一次碰撞事件中所观察到的多个粒子的共同性质,它是一种可能的信号。它源于中心快度区形成的火球从中心到边缘的密度梯度。我们称大量的粒子具有相同的运动方向和速度为“集体流”。按照方向的不同,流可分为“纵向流”和“横向流”。“横向流”又能分为“径向流”和“各向异性流”。各种流是整个集体流在不同物理图像方面的表现形式。
在非对心碰撞中(碰撞参数b≠0),“反应平面”被定义为碰撞参数和束流方向所决定的平面。系统初始坐标空间中的方位角各向异性在横向平面(垂直于反应平面)有一个类似椭圆的形状,密度梯度在椭圆短轴上比长轴上大,碰撞重叠区域粒子频繁的相互作用把密度梯度转化为压力梯度,导致压力梯度在椭圆短轴方向比长轴上大,表现为椭圆短轴上有较大的“径向流”。又由于这些粒子运动速度在两个不同方向的差别,椭圆长轴和短轴之间的压力梯度差将不断减小。因而,“各向异性流”产生并发展于系统的早期,反映了系统的早期性质。实验上,我们用末态粒子在横向平面上的方位角分布的傅立叶展开来描述动量空间的各向异性,傅立叶展开的系数就是“各向异性流”参数,第二谐波系数对应于椭圆的方位角分布,也称为“椭圆流”参数v2。
RHIC实验对各向异性椭圆流参数v2测量已经有了很多结果。在其最高质心能量(?)下,在低横动量区,实验上观测到了流体力学所预言的强子质量顺序性,它表明在AuAu碰撞中产生的物质是具有极低粘滞性的“液态”流体系统,并且已经形成了部分子层次的集体运动；在中间横动量区,实验上观测到强子的组分夸克数目标度性,它表明系统达到了解禁闭态。另外,多重奇异粒子横动量分布和椭圆流的结果暗示了系统可能已经达到了部分子层次的热化。值得注意的是,在RHIC能区,关于系统动力学热化的讨论直到现在还没有明确的结论。流体力学计算结果假设系统是理想流体,并成功的重复了RHIC能区的部分实验结果。此外,基于对带电粒子(?)的讨论暗示了,在RHIC能区系统可能在中心碰撞中达到了热化。
在不改变束流碰撞能量的情况下,用UU代替AuAu碰撞是验证系统是否热化的一种有效手段。理论上,在完全对心的条件下,UU比AuAu碰撞系统沉积的能量要高30%。这说明在UU碰撞里有更频繁更激烈的相互作用,系统更容易达到热化。这对研究部分子物质的热化性质是非常有帮助的。RHIC已经计划2012年实施部分UU碰撞。德国GSI/SIS300也在计划实施30AGeV的UU碰撞实验,中国兰州的HIFRL-CSR主环外靶实验也在计划实施束流能量Eb/A=0.52GeV的UU碰撞。虽然早些年,就有人提出要在高能重离子碰撞实验上实施UU碰撞,但是相应的实验一直没有展开实施。考虑到,238U核是自然界最大的稳定形变核,其在初始坐标空间具有长椭球几何外形。当沿着不同的轴向发生UU碰撞时,相互作用的强度和时间是非常不一样的,特别是在两种极端方位——头头和体体碰撞下。在CSR能区,前者在碰撞过程中产生的高密物质寿命大约是后者的两倍。如此长时间的相互作用,有利于热平衡的实现。因此,我们最希望能在头头UU碰撞里面看到明显的热化性质。
对于,激化靶实验,我们可以直接选择激化的方向来实现特定方位的UU碰撞；但是对于非激化U核,射弹和靶在反应平面具有随机的碰撞方位。如何通过选取末态的某些课测量物理量或可观察量,判选出我们所希望的头头UU碰撞事件,将是研究头头UU碰撞下高密物质性质的关键。在这篇论文里,我们模拟并研究了在CSR能区形变核UU碰撞,重点学习了碰撞方位对重子物质密度的影响,通过试探,找到了两个实验上的可测量物理量-前向中子数和核阻止本领,分别用作快速在线分析和离线分析,从而最终得到所期望的包含大部分的头-头UU碰撞事件的子样本。我们也希望,有关UU碰撞方位对末态测量量的影响的相关学习和研究,能为将来在高能下实施UU碰撞实验,提供有益的参考。
最近,有限温度格点规范理论给出的QCD相图是：在低温高重子化学势区,QGP相和强子相之间是一级相变；随着温度的升高和重子化学势的降低,一级相变曲线在临界点终止；在更高温度和更低化学势条件下,QGP相和强子相之间平滑过渡(crossover)。理论估计,在RHIC(以及将来的LHC)高能区,净重子化学势低,从强子物质到QGP的转变是平滑过渡。为学习相图,在2010年4月,RHIC将实施RHIC/STAR低能扫描(BES)实验,其目标就是为了寻找QCD相图上的一级相变相边界和临界终止点。其能量扫描范围大约是(?)=5～30GeV。简单的测试(?)已经在2008年秋实施,实验上仅仅得到约3,000个可利用的AuAu最小无偏事件。粒子产额、谱以及强子流等的分析,显示了与早先的AGS能区相似的结果。
关于在临界终止点高密物质的性质,目前还没有理论能给出明确的预言。有两个最有可能用于判断是否出现临界点和相边界的信号—净重子起伏和强子椭圆流。前者认为,在临界终止点,可能会遭遇非常强烈的动力学起伏；后者认为,在RHIC能区看到的强子椭圆流的组分夸克标度性是部分子特有的行为,对于仅仅只有强子物质相互作用的系统,这种标度行为将不会出现。特别是多重奇异粒子Φ,其组分是ss。由于s夸克与轻夸克较小的相互作用界面,这些Φ将直接从碰撞早期产生,并且不参与或者不完全参与轻夸克的集体行为,其强子椭圆流将比普通强子要小,甚至为0。因此在低能一级相转变区域,强子椭圆流的标度性将不成立。
在这篇论文里,我们通过研究(?)下强子椭圆流参数,提出强子椭圆流的组分夸克标度性可以用来作为寻找QCD一级相变相边界和临界终止点的有效探针,特别是多重奇异粒子Φ。如果实验上,做一个从低能到高能的逐点扫描,在部分子相为主的能量区域,我们将可以看到各种末态强子的椭圆流组分夸克刻度性；在强子相为主的能量区域,我们会看到破坏的椭圆流组分夸克刻度性,甚至可能看到多重奇异强子Φ的椭圆流明显比其他强子小或接近零。这将为将来实施低能能量扫描实验,寻找QCD一级相变相边界和临界终止点具有重要的指导和参考作用。
For thousands of years, the curiosity of mankind drive us to ask the same ques-tion:what and how forms our world? Now, the standpoint is that quarks and gluons build up the world of matter. And Quantum ChromoDynamics (QCD) established in 1970s is believed to be a very successful theory to describe the strong force be-tween the quarks and gluons with color charge carriers. QCD has two basal features asymptotic freedom and quarks confinement, which lead to the fact quarks and gluons are confined in hadrons and no free quarks and gluons are observed.
There are two ways by compressing/heating the hadrons system to increase the the matter density/temperature to smash the bag of hadrons. Lattice QCD predicts the phase transition at high temperature or high density from the normal hadron gas state to a partonic state with deconfined hadrons--quarks and gluons, which are named as the Quark Gluon Plasma (QGP). And the farther calculation indicates the phase transition will occur at the critical temperature Tc～160MeV or 10 time the density of normal matter.
QGP maybe exit in the early universe in the first few seconds after the Big Bang or the neutron star, but can't be measured directly as a result of too early and too far to us. In the 1970s of last century, T.D. Lee et al. predicted that through high energy heavy ion collisions it is possible to form a high energy and/or high density environment in space so that a new state of matter - quark gluon plasma consisting of a large amount of deconfined quarks, antiquarks and gluons will be produced. One of the goal to perform the heavy ion collision in the laboratory is to create and study the property of the partonic matter with high temperature and/or high density. In so extreme cases, QGP maybe appear.
The Relativistic Heavy Ion Collider (RHIC) located at Brookhaven National Laboratory (BNL) was designed to collide high energy heavy ions to create such a high temperature and high density matter and to search for Quark-Gluon Plasma (QGP) and study its properties. After several years of measurements accumulation, the matter created RHIC has been proved to be more like a medium most resemble of properties of a perfect liquid of strongly interacting quark gluon plasma. One of the pillars for this discovery are the observed strong elliptic flow of partonic matter. From the data in RHIC top energy (?)200GeV AuAu collisions, the second harmonic azimuthal anisotropy, elliptic flowν2 of identified particles established hydro-like mass hierarchy at low pr, which demonstrates the development of partonic collectivity. Further, the Number-of-Constituent-Quark (NCQ) scaling observed at intermediate pT suggest that the system has been in the deconfined state prior to hadronization. On the other hand, charged particle v2 scaled by the eccentricity suggests possible thermalization only in the most central collisions at top RHIC energy.
An effective ways to verify whether or not the medium is thermalized is using UU instead of AuAu collisions. Theoretical calculations show that the energy density in (?)= 200GeV central UU collisions will exceed 30% than AuAu collisions. This hints that some visible sign about the thermalization of medium may be seen in central UU collisions. Some plans about UU collisions, (?)= 200GeV UU collision in RHIC, Eb= 30AGeV in GSI/SIS300 and Eb/A=0.52GeV in HIFRL-CSR, will be perfomed in the further.
238U is the most deformed (δ≈0.27) stable nucleus. As a homogeneous e1-lipsoid, the ratio of the long-axis over short-axis is as large as 1.3. As a result of the deformation, UU collisions at the same beam energy and impact parameter but different orientations are expected to form dense matter with different compressions and lifetimes. It is expected that the tip-tip collisions can form a higher densities of nuclear matter with longer duration than body-body or the spherical nuclei collisions and easier to reach thermal equilibrium.
For the non-polarized UU collision, target and projectile have random orienta-tions in the initial coordinate space. Ideal tip-tip and body-body events are quite unusual. So, how to select the central tip-tip like events in experiment is the key to perform UU collisions.
In this paper,we have test and developed two measurements allowing us to se-lect those high-density events in CSR. The forward neutron multiplicity and nuclear stopping power are suitable for fast on-line and off-line physics analysis, respectively. We also expect these results is useful for performing UU collisions in RHIC and GSI in the further.
The newest QCD phase diagram predicted by lattice gauge theory(LGT) is the following:in the region with high baryon chemical potential and low temperature the transition between normal hadronic matter and QGP is a first order phase transition. As the increase of temperature and decrease of baryon chemical potential, the first order phase-transition line ends at the critical point and at even higher temperature and lower baryon chemical potential, there is a crossover. It is estimated that at the RHIC and the coming LHC energy regions, a high temperature and low baryon chemical potential environment will be created, and thus falls in the crossover region.
For studying QCD phase diagram, RHIC/STAR beam energy scan(BES) pro-gram focus on drawing the QCD phase boundary and locating the critical point(CP). The range of beam energy is (?)=5～30GeV. At the critical point of phase transition, there should be some observables undergoing dramatic change. There are two candidates as the signs for this search, the fluctuation of net-baryon multiplicity and the elliptical flow of hardons. This paper only focus on the latter. The NCQ scaling of hardons v2 in RHIC is a particular feature about the medium with partonic freedom degree. So, if the interactions only occur on hardons, this NQ scaling will be broken.
By employing the AMPT model, it's clearly seen that the NCQ scaling will be preserved in a partonic phase scenario (string melting) while be broken in a hadronic phase scenario (default version). The possible deviation of the NCQ scaling for identified particles may signal a transition from partonic phase to hadronic phase.
More specially, due to OZI rule,Φmesons can only be formed via the coalescence of strange quarks. In the hadronic environment of hadronic interactions, there is no partonic collectivity, in addition to the lack of quark-scaling, the value of v2 of theΦmesons must be close to zero or very small.
More predict the property of critical point and the phase boundary is worth further study.
有两种可能打破强子束缚的途径,一是：将强子物质压缩,即增大核子物质密度,使强子口袋相互重叠而破裂,这样夸克和胶子就能在较大的空间范围内自由运动；另一种方法将系统加热,使温度升高到足够高,真空中会产生很多正反夸克对,大量的夸克胶子发生激烈碰撞,从而可能形成夸克和胶子在较大范围内运动的状态。格点QCD理论预言,在极高的温度或者高重子数密度下,强子物质(夸克的禁闭相)会解禁闭形成其解禁闭相——夸克胶子等离子体(Quark Gluon Plasma—QGP)。进一步的数值计算显示,在临界温度Tc～160MeV(零密度)或物质密度5～10倍正常核物质密度(零温度)时会发生QGP相变。
夸克胶子等离子体可能存在于宇宙大爆炸早期阶段(很高的温度)以及中子星(重子数密度很高)内。然而这两种情况要么发生在过去,要么距离我们太遥远,没有办法去直接测量。在实验室,通过电磁场加速,我们让两束重离子以高速进行对撞,从而生成能量密度比较高并且寿命比较长的强相互作用物质,QGP有可能在这种情况下出现。重离子碰撞的主要目的之一就是希望在实验室条件下达到QGP相变所需要的高温高密环境,研究极端条件下的物质性质。从早期的伯克力(Berkeley/Bevalac)到欧洲核子中心(CERN)的超级质子同步加速器(SPS),以及布鲁克海文实验室(BNL)的交变梯度同步加速器(AGS)和相对论重离子对撞机(RHIC),人们已经对高温下的物质性质进行了很多卓有成效的研究。
实验上,我们只能测得末态的强子分布,希望通过末态强子说携带的信息寻找夸克胶子等离子体存在的信号。“集体行为”是指在一次碰撞事件中所观察到的多个粒子的共同性质,它是一种可能的信号。它源于中心快度区形成的火球从中心到边缘的密度梯度。我们称大量的粒子具有相同的运动方向和速度为“集体流”。按照方向的不同,流可分为“纵向流”和“横向流”。“横向流”又能分为“径向流”和“各向异性流”。各种流是整个集体流在不同物理图像方面的表现形式。
在非对心碰撞中(碰撞参数b≠0),“反应平面”被定义为碰撞参数和束流方向所决定的平面。系统初始坐标空间中的方位角各向异性在横向平面(垂直于反应平面)有一个类似椭圆的形状,密度梯度在椭圆短轴上比长轴上大,碰撞重叠区域粒子频繁的相互作用把密度梯度转化为压力梯度,导致压力梯度在椭圆短轴方向比长轴上大,表现为椭圆短轴上有较大的“径向流”。又由于这些粒子运动速度在两个不同方向的差别,椭圆长轴和短轴之间的压力梯度差将不断减小。因而,“各向异性流”产生并发展于系统的早期,反映了系统的早期性质。实验上,我们用末态粒子在横向平面上的方位角分布的傅立叶展开来描述动量空间的各向异性,傅立叶展开的系数就是“各向异性流”参数,第二谐波系数对应于椭圆的方位角分布,也称为“椭圆流”参数v2。
RHIC实验对各向异性椭圆流参数v2测量已经有了很多结果。在其最高质心能量(?)下,在低横动量区,实验上观测到了流体力学所预言的强子质量顺序性,它表明在AuAu碰撞中产生的物质是具有极低粘滞性的“液态”流体系统,并且已经形成了部分子层次的集体运动；在中间横动量区,实验上观测到强子的组分夸克数目标度性,它表明系统达到了解禁闭态。另外,多重奇异粒子横动量分布和椭圆流的结果暗示了系统可能已经达到了部分子层次的热化。值得注意的是,在RHIC能区,关于系统动力学热化的讨论直到现在还没有明确的结论。流体力学计算结果假设系统是理想流体,并成功的重复了RHIC能区的部分实验结果。此外,基于对带电粒子(?)的讨论暗示了,在RHIC能区系统可能在中心碰撞中达到了热化。
在不改变束流碰撞能量的情况下,用UU代替AuAu碰撞是验证系统是否热化的一种有效手段。理论上,在完全对心的条件下,UU比AuAu碰撞系统沉积的能量要高30%。这说明在UU碰撞里有更频繁更激烈的相互作用,系统更容易达到热化。这对研究部分子物质的热化性质是非常有帮助的。RHIC已经计划2012年实施部分UU碰撞。德国GSI/SIS300也在计划实施30AGeV的UU碰撞实验,中国兰州的HIFRL-CSR主环外靶实验也在计划实施束流能量Eb/A=0.52GeV的UU碰撞。虽然早些年,就有人提出要在高能重离子碰撞实验上实施UU碰撞,但是相应的实验一直没有展开实施。考虑到,238U核是自然界最大的稳定形变核,其在初始坐标空间具有长椭球几何外形。当沿着不同的轴向发生UU碰撞时,相互作用的强度和时间是非常不一样的,特别是在两种极端方位——头头和体体碰撞下。在CSR能区,前者在碰撞过程中产生的高密物质寿命大约是后者的两倍。如此长时间的相互作用,有利于热平衡的实现。因此,我们最希望能在头头UU碰撞里面看到明显的热化性质。
对于,激化靶实验,我们可以直接选择激化的方向来实现特定方位的UU碰撞；但是对于非激化U核,射弹和靶在反应平面具有随机的碰撞方位。如何通过选取末态的某些课测量物理量或可观察量,判选出我们所希望的头头UU碰撞事件,将是研究头头UU碰撞下高密物质性质的关键。在这篇论文里,我们模拟并研究了在CSR能区形变核UU碰撞,重点学习了碰撞方位对重子物质密度的影响,通过试探,找到了两个实验上的可测量物理量-前向中子数和核阻止本领,分别用作快速在线分析和离线分析,从而最终得到所期望的包含大部分的头-头UU碰撞事件的子样本。我们也希望,有关UU碰撞方位对末态测量量的影响的相关学习和研究,能为将来在高能下实施UU碰撞实验,提供有益的参考。
最近,有限温度格点规范理论给出的QCD相图是：在低温高重子化学势区,QGP相和强子相之间是一级相变；随着温度的升高和重子化学势的降低,一级相变曲线在临界点终止；在更高温度和更低化学势条件下,QGP相和强子相之间平滑过渡(crossover)。理论估计,在RHIC(以及将来的LHC)高能区,净重子化学势低,从强子物质到QGP的转变是平滑过渡。为学习相图,在2010年4月,RHIC将实施RHIC/STAR低能扫描(BES)实验,其目标就是为了寻找QCD相图上的一级相变相边界和临界终止点。其能量扫描范围大约是(?)=5～30GeV。简单的测试(?)已经在2008年秋实施,实验上仅仅得到约3,000个可利用的AuAu最小无偏事件。粒子产额、谱以及强子流等的分析,显示了与早先的AGS能区相似的结果。
关于在临界终止点高密物质的性质,目前还没有理论能给出明确的预言。有两个最有可能用于判断是否出现临界点和相边界的信号—净重子起伏和强子椭圆流。前者认为,在临界终止点,可能会遭遇非常强烈的动力学起伏；后者认为,在RHIC能区看到的强子椭圆流的组分夸克标度性是部分子特有的行为,对于仅仅只有强子物质相互作用的系统,这种标度行为将不会出现。特别是多重奇异粒子Φ,其组分是ss。由于s夸克与轻夸克较小的相互作用界面,这些Φ将直接从碰撞早期产生,并且不参与或者不完全参与轻夸克的集体行为,其强子椭圆流将比普通强子要小,甚至为0。因此在低能一级相转变区域,强子椭圆流的标度性将不成立。
在这篇论文里,我们通过研究(?)下强子椭圆流参数,提出强子椭圆流的组分夸克标度性可以用来作为寻找QCD一级相变相边界和临界终止点的有效探针,特别是多重奇异粒子Φ。如果实验上,做一个从低能到高能的逐点扫描,在部分子相为主的能量区域,我们将可以看到各种末态强子的椭圆流组分夸克刻度性；在强子相为主的能量区域,我们会看到破坏的椭圆流组分夸克刻度性,甚至可能看到多重奇异强子Φ的椭圆流明显比其他强子小或接近零。这将为将来实施低能能量扫描实验,寻找QCD一级相变相边界和临界终止点具有重要的指导和参考作用。
For thousands of years, the curiosity of mankind drive us to ask the same ques-tion:what and how forms our world? Now, the standpoint is that quarks and gluons build up the world of matter. And Quantum ChromoDynamics (QCD) established in 1970s is believed to be a very successful theory to describe the strong force be-tween the quarks and gluons with color charge carriers. QCD has two basal features asymptotic freedom and quarks confinement, which lead to the fact quarks and gluons are confined in hadrons and no free quarks and gluons are observed.
There are two ways by compressing/heating the hadrons system to increase the the matter density/temperature to smash the bag of hadrons. Lattice QCD predicts the phase transition at high temperature or high density from the normal hadron gas state to a partonic state with deconfined hadrons--quarks and gluons, which are named as the Quark Gluon Plasma (QGP). And the farther calculation indicates the phase transition will occur at the critical temperature Tc～160MeV or 10 time the density of normal matter.
QGP maybe exit in the early universe in the first few seconds after the Big Bang or the neutron star, but can't be measured directly as a result of too early and too far to us. In the 1970s of last century, T.D. Lee et al. predicted that through high energy heavy ion collisions it is possible to form a high energy and/or high density environment in space so that a new state of matter - quark gluon plasma consisting of a large amount of deconfined quarks, antiquarks and gluons will be produced. One of the goal to perform the heavy ion collision in the laboratory is to create and study the property of the partonic matter with high temperature and/or high density. In so extreme cases, QGP maybe appear.
The Relativistic Heavy Ion Collider (RHIC) located at Brookhaven National Laboratory (BNL) was designed to collide high energy heavy ions to create such a high temperature and high density matter and to search for Quark-Gluon Plasma (QGP) and study its properties. After several years of measurements accumulation, the matter created RHIC has been proved to be more like a medium most resemble of properties of a perfect liquid of strongly interacting quark gluon plasma. One of the pillars for this discovery are the observed strong elliptic flow of partonic matter. From the data in RHIC top energy (?)200GeV AuAu collisions, the second harmonic azimuthal anisotropy, elliptic flowν2 of identified particles established hydro-like mass hierarchy at low pr, which demonstrates the development of partonic collectivity. Further, the Number-of-Constituent-Quark (NCQ) scaling observed at intermediate pT suggest that the system has been in the deconfined state prior to hadronization. On the other hand, charged particle v2 scaled by the eccentricity suggests possible thermalization only in the most central collisions at top RHIC energy.
An effective ways to verify whether or not the medium is thermalized is using UU instead of AuAu collisions. Theoretical calculations show that the energy density in (?)= 200GeV central UU collisions will exceed 30% than AuAu collisions. This hints that some visible sign about the thermalization of medium may be seen in central UU collisions. Some plans about UU collisions, (?)= 200GeV UU collision in RHIC, Eb= 30AGeV in GSI/SIS300 and Eb/A=0.52GeV in HIFRL-CSR, will be perfomed in the further.
238U is the most deformed (δ≈0.27) stable nucleus. As a homogeneous e1-lipsoid, the ratio of the long-axis over short-axis is as large as 1.3. As a result of the deformation, UU collisions at the same beam energy and impact parameter but different orientations are expected to form dense matter with different compressions and lifetimes. It is expected that the tip-tip collisions can form a higher densities of nuclear matter with longer duration than body-body or the spherical nuclei collisions and easier to reach thermal equilibrium.
For the non-polarized UU collision, target and projectile have random orienta-tions in the initial coordinate space. Ideal tip-tip and body-body events are quite unusual. So, how to select the central tip-tip like events in experiment is the key to perform UU collisions.
In this paper,we have test and developed two measurements allowing us to se-lect those high-density events in CSR. The forward neutron multiplicity and nuclear stopping power are suitable for fast on-line and off-line physics analysis, respectively. We also expect these results is useful for performing UU collisions in RHIC and GSI in the further.
The newest QCD phase diagram predicted by lattice gauge theory(LGT) is the following:in the region with high baryon chemical potential and low temperature the transition between normal hadronic matter and QGP is a first order phase transition. As the increase of temperature and decrease of baryon chemical potential, the first order phase-transition line ends at the critical point and at even higher temperature and lower baryon chemical potential, there is a crossover. It is estimated that at the RHIC and the coming LHC energy regions, a high temperature and low baryon chemical potential environment will be created, and thus falls in the crossover region.
For studying QCD phase diagram, RHIC/STAR beam energy scan(BES) pro-gram focus on drawing the QCD phase boundary and locating the critical point(CP). The range of beam energy is (?)=5～30GeV. At the critical point of phase transition, there should be some observables undergoing dramatic change. There are two candidates as the signs for this search, the fluctuation of net-baryon multiplicity and the elliptical flow of hardons. This paper only focus on the latter. The NCQ scaling of hardons v2 in RHIC is a particular feature about the medium with partonic freedom degree. So, if the interactions only occur on hardons, this NQ scaling will be broken.
By employing the AMPT model, it's clearly seen that the NCQ scaling will be preserved in a partonic phase scenario (string melting) while be broken in a hadronic phase scenario (default version). The possible deviation of the NCQ scaling for identified particles may signal a transition from partonic phase to hadronic phase.
More specially, due to OZI rule,Φmesons can only be formed via the coalescence of strange quarks. In the hadronic environment of hadronic interactions, there is no partonic collectivity, in addition to the lack of quark-scaling, the value of v2 of theΦmesons must be close to zero or very small.
More predict the property of critical point and the phase boundary is worth further study.
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