200GeV金金碰撞中两粒子方位角关联对反应平面的依赖性
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摘要
夸克和胶子被认为是物质最基本的组成单元之一。量子色动力学(QCD)则是描述夸克和胶子之间强相互作用的一个成功的规范理论。QCD有两个显著的基本特征:1)渐进自由:横动量交换越大或夸克之间的距离越小,夸克之间的相互作用越弱;2)夸克禁闭:夸克只能禁闭在强子物质内,目前还没有观测到孤立的夸克。格点QCD的计算表明在高温度或高密度的环境里,普通的强子气体会转变成为另外一种夸克胶子解禁闭的QCD物质相——夸克胶子等离子体(QGP)。位于美国布鲁克海汶国家实验室的相对论重离子对撞机(RHIC)通过对撞两束接近光速的金离子来期望产生这种高能量密度的物质,以模拟宇宙大爆炸后微秒量级左右的物质组成。经过几年的运行和实验数据分析,目前为止观察到的实验现象——强椭圆流和喷注焠火——预示着RHIC产生的物质性质极类似于一种由强相互作用夸克胶子等离子体组成的理想流体。在研究RHIC重离子碰撞所产生的这种物质的性质中,从喷注碎裂产生的高横动量(p_T)粒子是一种理想的探针。通常人们通过对粒子的单举分布产额以及这些粒子的强子对关联的研究来探测RHIC所产生的物质的性质。然而,在这两种方法中,由于单举粒子产额的研究对RHIC所产生的物质核心部分不很敏感,而强子对关联的测量则由于多引入了一个粒子的信息,将为我们研究产生物质的特性提供更为丰富更有价值的信息。
RHIC上重要的有关强子对关联的测量包括:1)对两个高横动量粒子的方位角关联的测量。其中一个高横动量的粒子称作“触发粒子”(trigger particle),它被认为是来自于喷注中的一个横动量很高的粒子,因此它的方向可以认为就是喷注轴的方向。另外的一个横动量相对低一些的粒子称作“伴随粒子”(associated particles),它们可能来自于喷注中的其他粒子(信号),也可能来自于各向异性流或者共振态衰变(均为背景)。通过测量触发粒子和伴随粒子的方位角关联并扣除背景粒子的贡献,就可以得到来自于喷注的关联结果。这一测量发现:在金金的中心碰撞中,高横动量粒子在背对背方向的关联消失了(伴随粒子产额压低)。而这一压低现象在基本的质子质子对撞中却没有发现。这说明,双喷注中背向(与触发粒子背对背的方向)的喷注在金金的中心对撞中消失了。这可以用喷注淬火理论来解释:背向的喷注在经过金金中心碰撞所产生的高温高密物质时,由于胶子辐射损失了大量的能量,因此不能被观测到。这一现象被认为是喷注淬火理论的一个重要的证据。2)对较低横动量伴随粒子与一个高横动量触发粒子的角关联的研究。这一研究发现在金金中心碰撞中,背向的低横动量粒子的分布比在质子质子碰撞中要变宽很多。这一结果与预期的背向喷注在介质中的能量损失是一致的:来自于喷注的背对背方向的高横动量粒子,在穿过RHIC金金碰撞产生的高温高密物质时,损失了能量,并且这些损失的能量“传递”给了低横动量的粒子,因此使得背对背方向上较低横动量强子对关联的产额分布变宽。3)RHIC实验的STAR实验组第一次测量了强子对关联相对于反应平面的依赖。并发现背向高横动量粒子产额在垂直于反应平面的方向上有较强的压低,而在靠近反应平面的方向上却没有观测到强压低现象。这一结果似乎暗示了喷注的能量损失可能与路径长度有关:在靠近反应平面的方向,背对背方向的喷注穿过物质时所经过路径比较短,能量损失较少;而在垂直于反应平面的方向上穿过的路径则要长很多,因此能量损失也大。4)“脊”的发现:最近STAR实验组在研究二维的△φ×△η(触发粒子和伴随粒子方位角夹角以及赝快度间隔)的强子对关联时发现了一个新现象:在质子质子碰撞中,触发粒子的近端,小的△φ和△η区间上有关联,从图像上则表现为在触发粒子近端的一个峰。它是来自于喷注的。而在金金碰撞中,这一关联在△η方向上的很大范围内都存在,从图像上看则表现为在小的△φ,大的△η区间有一个很宽的平台。而来自于喷注的峰则位于这一平台之上。这一二维图像很像一个大的山脉,一个山峰立于一个平坦的山脊之上。因此,人们把在△η方向上的这一长程关联称为“脊”(ridge)。对于脊的产生机制目前尚不清楚仍在探索之中。
在这篇论文中,通过分析RHIC实验上STAR探测器获取的大样本的质心系能量为200GeV金金最小无偏碰撞和中心碰撞数据,我们测量了强子对关联并研究它随高横动量触发粒子相对于反应平面方位角的变化。同时我们也分析了最小无偏的200GeV氘金碰撞数据中的强子对关联,其结果将和金金碰撞的结果进行比较。
我们根据触发粒子相对于反应平面的方位角方向(φ_s=|φ~(trig)-Ψ_(EP)|)把横向的φ平面均分成6个小区间,然后分别研究强子对关联在触发粒子落入每个小区间时的分布,从而更细致地研究强子对关联对介质路径长度的依赖性。反应平面通过STAR标准的重建方法得到,但是落在与关联粒子横动量p_T~(assoc)相同p_T区间的粒子将不被用做反应平面的重建以避免自相关。高阶的各向异性流v_4的贡献在以前的强子对关联的背景研究中都没有考虑。而这里我们通过研究表明它的贡献在相对于反应平面强子对关联的背景研究中不可忽略。因此,我们修正了流本底的计算公式,把v_4的贡献考虑进去。对于流参数的选取,我们采用了STAR通过反应平面方法和4粒子累积关联方法得到的结果的平均值。两者之间的差别是系统误差的主要来源。流本底通过改进的Zero-Yield-At-Minumum(ZYAM)方法来归一化。在每个p_T~(assoc)和φ_s区间分别计算流本底,然后从原始的强子关联分布中扣除流的贡献,从而得到我们感兴趣的与喷注相关的强子对关联分布。我们分别研究了20-60%中心度和0-5%中心度的金金碰撞数据,以及最小无偏的氘金碰撞的数据。对于触发粒子近端关联分布的研究,我们分别计算了在大的和小的△η区间(被认为分别来自于脊的贡献以及脊和喷注共同贡献)中强子对关联的分布,从而来区分在近端关联中喷注和脊的贡献,并研究它们随φ_s的变化趋势。在这里,我们把离触发粒子比较近的方向(△φ～0°)称为近端(near-side),把背对着触发粒子的方向(△φ～180°)称为背端(away-side)。
研究发现,相比于来自最小无偏的氘金数据的关联结果,金金碰撞中强子对关联的近端和背端分布都受到很强的来自于RHIC所产生的介质的修正。这种修正依赖于触发粒子相对于反应平面的方向以及关联粒子的横动量p_T~(assoc)。强子对关联分布随φ_s的变化趋势在5%的中心碰撞和20-60%非中心碰撞中比较相似,但在20-60%中心度碰撞中变化比较得快。
从触发粒子在反应平面方向到它垂直于反应平面方向,强子对关联的背向分布逐渐变宽,同时它也随p_T~(assoc)的增加而变宽。在20-60%中心度碰撞中,当触发粒子沿着反应平面的方向时,背向关联的分布呈现一个单峰结构,随着触发粒子偏向垂直于反应平面方向时,单峰结构逐渐变成越来越明显的双峰结构。在0-5%中心碰撞中,即使在触发粒子沿着反应平面的方向,背向关联的分布已经呈现出双峰结构,而且双峰结构也随着触发粒子偏向垂直于反应平面方向时而变得越来越明显。这种背向结构的修正趋势反映出路径长度对于硬散射出来的背向部分子在穿过介质时的修正的重要性。这在性质上与喷注淬火理论的预言一致。
近端关联分布随着触发粒子沿着反应平面变化到垂直于反应平面时,它的关联幅度会降低。而通过把近端关联分成脊和喷注的关联来分别研究时,发现这种降低几乎全是来自于脊的产额的随着φ_s的增加而减少。而喷注的产额则基本不随φ_s变化(或者可以认为略微随着φ_s增加而增加)。在20-60%中心度中,脊的产额在垂直于反应平面方向上几乎降为0,而在0-5%中心碰撞中,从反应平面内变化到垂直于反应平面的各个角度,脊的产额都比较的显著。在20-60%中心度中,从反应平面内变化到垂直于反应平面时,喷注的产额有略微的增加。这些现象似乎表明如下一个脊产生图像:近端的喷注,在沿着反应平面的方向上与介质发生较强的相互作用,损失了一定的能量,从而使得观测到的喷注产额减少,并且生成了大△η范围内的的脊;而在垂直于反应平面的方向上,喷注受到的介质影响最小,从而产生的脊也较少。对于0-5%中心碰撞,喷注和脊的产额随φ_s的变化趋势基本相似,只是由于初始碰撞时的几何结构更接近圆形,于是在各个φ_s角度上都有一定的脊的贡献,喷注和脊的幅度在各个φ_s角度上的变化也小很多。
这篇论文中的主要的结果是选择触发粒子横动量3＜p_T~(trig)＜4 GeV/c。在将来更多统计量时,选择更高横动量的触发粒子来研究是必需的,因为它们有更多的可能来自于喷注的碎裂。在不久的将来,STAR探测器将升级TPC的数据获取系统至DAQ1000。届时,我们将可以采集至少10倍于这篇论文所有的2004年运行采集的数据,从而使得我们可以利用高p_T触发粒子来进一步研究粒子关联。
Quarks and gluons are the basic building blocks of the matter.And Quantum ChromoDynamics (QCD)is believed to be a successful theory to describe the strong force between the color charge carriers- quarks and gluons.According to asymptotic freedom - one of key features of QCD,quarks and gluons are confined in hadrons and no free quarks and gluons are observed.Lattice QCD predicts a phase transition at high temperature or high density from the normal hadron gas state to a state with quarks and gluons which are deconfined from hadrons-the Quark Gluon Plasma (QGP).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,simulating the evolution of the early universe in the first few seconds after the Big Bang.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.The two pillars for this discovery are the observed strong elliptic flow and jet quenching.To quantify the property of the medium created at RHIC,high transverse momentum(PT) particles,which are presumably from jet fragmentations,are ideal penetrating probes. Usually people use high PT single hadron yields and di-hadron correlations with a high PT trigger particle to study the medium created at RHIC.While suppression of high PT single hadron yields has limited sensitivity to the medium core,di-hadron correlation measurements will provide richer and more valuable information about the properties of the created medium.
The disappearance of back-to-back jet from the first high PT di-hadron correlation measurement at RHIC has been recognized as an evidence of the jet quenching discovery. The following analysis by correlating low PT particles associated with the high PT trigger particle shows the broadening in the away side,which is consistent with the jet energy dissipation in the medium.The first di-hadron analysis with respect to the reaction plane shows the away-side suppression happens out-of-plane while it is not significant in-plane.This indicates the possible path length effect in the jet energy loss.Recently, a new phenomenon of the long range△ηcorrelation——so called "Ridge" has been observed in the near side di-hadron correlation,but the origin of ridge phenomenon is still not understood yet[Arm04,Vol06,Maj04,Won07,Hwa05].
In this thesis,we present STAR results of di-hadron azimuthal correlations with a high PT trigger particle relative to reaction plane and a lower pT particle associated with the trigger particle in(?)= 200 GeV Au + Au collisions at RHIC.The large data sample(which is about 10 times more than previous analysis):Au + Au minimum bias events and Au + Au central events collected during year 2004 run were used in this analysis.The minimum bias d + Au collision data collected in year 2003 were used for the baseline comparison.
The di-hadron azimuthal angle correlation was calculated according to the azimuthal angle of the trigger particle relative to the reaction plane(φ_s=|φ~(trig)-ψ_(EP)|).Reaction plane was reconstructed using the standard event plane method.Particles falling in the PT bin of associated particles(p_T~(assoc))were eliminated in the event plane reconstruction to avoid the auto-correlation.The v_4 contribution in the flow background,which had not been taken into account,was found to be not negligible in the di-hadron correlation analysis relative to the reaction plane.We re-evaluated the flow background formula to include the v_4 contribution.The flow parameters were taken from the averaged STAR measurements using the standard event plane method and the four particle cumulant method.The difference between two methods is the most significant systematic error contribution in the results.The normalization of the flow background was calculated using an improved method based on the Zero-Yield-At-Minimum(ZYAM).The flow background was calculated in each p_T~(assoc)bin andφ_s bin,and it was subtracted from the raw di-hadron correlation distributions to extract the correlation function we are interested.Both the mid-central 20-60%and the top 5%central Au + Au collisions were investigated,and the minimum bias d + Au collision data were presented for baseline comparison.The correlation functions are also obtained from small and large |△ηregions separately,in attempt to isolate the jet and ridge contributions to the nearside correlation strength and study their behavior inφ_s.
The di-hadron correlations are strongly modified in Au + Au collisions with respect to minimum bias d + Au collisions.The modification strongly depend on the trigger particle orientation relative to the event plane and evolve with associated p_T~(assoc).The qualitative trend of the correlation function withφ_s appears similar in central and midcentral collisions;quantitatively,theφ_s dependence of the correlation function is stronger in the middle central collisions.The p_T~(assoc)dependences of the correlation function are similar in the two centrality selections.
The away-side correlation broadens from in-plane to out-of-plane,and broadens with increasing associated p_T~(assoc)for mostφ_s slices.For 20-60%Au + Au collisions,the awayside correlation starts as a single peak in the reaction plane,and becomes a stronger and stronger double-peak structure as the trigger particle moves from in-plane to out-of-plane. For top 5%central Au + Au,the away-side correlation already shows a doublepeak structure in the reaction plane,and the double-peak becomes stronger and stronger from in-plane to out-of-plane.The trends of the away-side modification underscore the importance of the path-length that the away-side parton transverses in the medium. The away-side medium path-length in the reaction plane direction in 20-60%Au + Au collisions is quite modest and not enough to generate significant modification to jet correlation,while that in the top 5%collisions is long enough to cause significant jet modification.The strongest modification is found for trigger particles perpendicular to the reaction plane where the away-side medium path-length is the longest,and this path-length appears to be not very different in 20-60%and top 5%Au + Au collisions.
The near-side correlation amplitude decreases from in-plane to out-of-plane.The decrease was found to entirely come from the decrease in the long range△ηcorrelation (ridge).The ridge yield decreases to～0 at out-of-plane in the 20-60%centrality,while significant ridge yields persist from in-plane to out-of-plane in top 5%collisions.The jet contribution to the near-side correlation is extracted from the difference of small and large△ηcorrelations,subject to small experimental systematic uncertainties.The jet contribution in the 20-60%centrality appears to somehow increase from in-plane to out-of-plane.The near-side jet parallel to the reaction plane appears to have suffered significant interactions with the medium,which reduce the real jet correlated multiplicity and produce a long range△ηridge containing a large number of hadrons.The near-side jet perpendicular to the reaction plane,on the other hand,appears to suffer minimal medium modification,generating small amount of ridge.The top 5%results are qualitatively similar,but the significant ridge contribution persists over allφ_s,and the variations of the jet and ridge magnitudes inφ_s is significantly smaller,consistent with the more spherical collision geometry.
The main results presented in this thesis are with 3＜p_T~(trig)＜4 GeV/c.The results from trigger particles with higher PT is necessary to check the results since they are more probably coming from jet fragmentation.In the future RHIC run with the TPC DAQ1000 upgrade,we are able to accumulate more than ten times statistics of the data sample in RunⅣ,used in this thesis.It provides us the possibility of investigating the correlations with high PT trigger particles in the future.
RHIC上重要的有关强子对关联的测量包括:1)对两个高横动量粒子的方位角关联的测量。其中一个高横动量的粒子称作“触发粒子”(trigger particle),它被认为是来自于喷注中的一个横动量很高的粒子,因此它的方向可以认为就是喷注轴的方向。另外的一个横动量相对低一些的粒子称作“伴随粒子”(associated particles),它们可能来自于喷注中的其他粒子(信号),也可能来自于各向异性流或者共振态衰变(均为背景)。通过测量触发粒子和伴随粒子的方位角关联并扣除背景粒子的贡献,就可以得到来自于喷注的关联结果。这一测量发现:在金金的中心碰撞中,高横动量粒子在背对背方向的关联消失了(伴随粒子产额压低)。而这一压低现象在基本的质子质子对撞中却没有发现。这说明,双喷注中背向(与触发粒子背对背的方向)的喷注在金金的中心对撞中消失了。这可以用喷注淬火理论来解释:背向的喷注在经过金金中心碰撞所产生的高温高密物质时,由于胶子辐射损失了大量的能量,因此不能被观测到。这一现象被认为是喷注淬火理论的一个重要的证据。2)对较低横动量伴随粒子与一个高横动量触发粒子的角关联的研究。这一研究发现在金金中心碰撞中,背向的低横动量粒子的分布比在质子质子碰撞中要变宽很多。这一结果与预期的背向喷注在介质中的能量损失是一致的:来自于喷注的背对背方向的高横动量粒子,在穿过RHIC金金碰撞产生的高温高密物质时,损失了能量,并且这些损失的能量“传递”给了低横动量的粒子,因此使得背对背方向上较低横动量强子对关联的产额分布变宽。3)RHIC实验的STAR实验组第一次测量了强子对关联相对于反应平面的依赖。并发现背向高横动量粒子产额在垂直于反应平面的方向上有较强的压低,而在靠近反应平面的方向上却没有观测到强压低现象。这一结果似乎暗示了喷注的能量损失可能与路径长度有关:在靠近反应平面的方向,背对背方向的喷注穿过物质时所经过路径比较短,能量损失较少;而在垂直于反应平面的方向上穿过的路径则要长很多,因此能量损失也大。4)“脊”的发现:最近STAR实验组在研究二维的△φ×△η(触发粒子和伴随粒子方位角夹角以及赝快度间隔)的强子对关联时发现了一个新现象:在质子质子碰撞中,触发粒子的近端,小的△φ和△η区间上有关联,从图像上则表现为在触发粒子近端的一个峰。它是来自于喷注的。而在金金碰撞中,这一关联在△η方向上的很大范围内都存在,从图像上看则表现为在小的△φ,大的△η区间有一个很宽的平台。而来自于喷注的峰则位于这一平台之上。这一二维图像很像一个大的山脉,一个山峰立于一个平坦的山脊之上。因此,人们把在△η方向上的这一长程关联称为“脊”(ridge)。对于脊的产生机制目前尚不清楚仍在探索之中。
在这篇论文中,通过分析RHIC实验上STAR探测器获取的大样本的质心系能量为200GeV金金最小无偏碰撞和中心碰撞数据,我们测量了强子对关联并研究它随高横动量触发粒子相对于反应平面方位角的变化。同时我们也分析了最小无偏的200GeV氘金碰撞数据中的强子对关联,其结果将和金金碰撞的结果进行比较。
我们根据触发粒子相对于反应平面的方位角方向(φ_s=|φ~(trig)-Ψ_(EP)|)把横向的φ平面均分成6个小区间,然后分别研究强子对关联在触发粒子落入每个小区间时的分布,从而更细致地研究强子对关联对介质路径长度的依赖性。反应平面通过STAR标准的重建方法得到,但是落在与关联粒子横动量p_T~(assoc)相同p_T区间的粒子将不被用做反应平面的重建以避免自相关。高阶的各向异性流v_4的贡献在以前的强子对关联的背景研究中都没有考虑。而这里我们通过研究表明它的贡献在相对于反应平面强子对关联的背景研究中不可忽略。因此,我们修正了流本底的计算公式,把v_4的贡献考虑进去。对于流参数的选取,我们采用了STAR通过反应平面方法和4粒子累积关联方法得到的结果的平均值。两者之间的差别是系统误差的主要来源。流本底通过改进的Zero-Yield-At-Minumum(ZYAM)方法来归一化。在每个p_T~(assoc)和φ_s区间分别计算流本底,然后从原始的强子关联分布中扣除流的贡献,从而得到我们感兴趣的与喷注相关的强子对关联分布。我们分别研究了20-60%中心度和0-5%中心度的金金碰撞数据,以及最小无偏的氘金碰撞的数据。对于触发粒子近端关联分布的研究,我们分别计算了在大的和小的△η区间(被认为分别来自于脊的贡献以及脊和喷注共同贡献)中强子对关联的分布,从而来区分在近端关联中喷注和脊的贡献,并研究它们随φ_s的变化趋势。在这里,我们把离触发粒子比较近的方向(△φ～0°)称为近端(near-side),把背对着触发粒子的方向(△φ～180°)称为背端(away-side)。
研究发现,相比于来自最小无偏的氘金数据的关联结果,金金碰撞中强子对关联的近端和背端分布都受到很强的来自于RHIC所产生的介质的修正。这种修正依赖于触发粒子相对于反应平面的方向以及关联粒子的横动量p_T~(assoc)。强子对关联分布随φ_s的变化趋势在5%的中心碰撞和20-60%非中心碰撞中比较相似,但在20-60%中心度碰撞中变化比较得快。
从触发粒子在反应平面方向到它垂直于反应平面方向,强子对关联的背向分布逐渐变宽,同时它也随p_T~(assoc)的增加而变宽。在20-60%中心度碰撞中,当触发粒子沿着反应平面的方向时,背向关联的分布呈现一个单峰结构,随着触发粒子偏向垂直于反应平面方向时,单峰结构逐渐变成越来越明显的双峰结构。在0-5%中心碰撞中,即使在触发粒子沿着反应平面的方向,背向关联的分布已经呈现出双峰结构,而且双峰结构也随着触发粒子偏向垂直于反应平面方向时而变得越来越明显。这种背向结构的修正趋势反映出路径长度对于硬散射出来的背向部分子在穿过介质时的修正的重要性。这在性质上与喷注淬火理论的预言一致。
近端关联分布随着触发粒子沿着反应平面变化到垂直于反应平面时,它的关联幅度会降低。而通过把近端关联分成脊和喷注的关联来分别研究时,发现这种降低几乎全是来自于脊的产额的随着φ_s的增加而减少。而喷注的产额则基本不随φ_s变化(或者可以认为略微随着φ_s增加而增加)。在20-60%中心度中,脊的产额在垂直于反应平面方向上几乎降为0,而在0-5%中心碰撞中,从反应平面内变化到垂直于反应平面的各个角度,脊的产额都比较的显著。在20-60%中心度中,从反应平面内变化到垂直于反应平面时,喷注的产额有略微的增加。这些现象似乎表明如下一个脊产生图像:近端的喷注,在沿着反应平面的方向上与介质发生较强的相互作用,损失了一定的能量,从而使得观测到的喷注产额减少,并且生成了大△η范围内的的脊;而在垂直于反应平面的方向上,喷注受到的介质影响最小,从而产生的脊也较少。对于0-5%中心碰撞,喷注和脊的产额随φ_s的变化趋势基本相似,只是由于初始碰撞时的几何结构更接近圆形,于是在各个φ_s角度上都有一定的脊的贡献,喷注和脊的幅度在各个φ_s角度上的变化也小很多。
这篇论文中的主要的结果是选择触发粒子横动量3＜p_T~(trig)＜4 GeV/c。在将来更多统计量时,选择更高横动量的触发粒子来研究是必需的,因为它们有更多的可能来自于喷注的碎裂。在不久的将来,STAR探测器将升级TPC的数据获取系统至DAQ1000。届时,我们将可以采集至少10倍于这篇论文所有的2004年运行采集的数据,从而使得我们可以利用高p_T触发粒子来进一步研究粒子关联。
Quarks and gluons are the basic building blocks of the matter.And Quantum ChromoDynamics (QCD)is believed to be a successful theory to describe the strong force between the color charge carriers- quarks and gluons.According to asymptotic freedom - one of key features of QCD,quarks and gluons are confined in hadrons and no free quarks and gluons are observed.Lattice QCD predicts a phase transition at high temperature or high density from the normal hadron gas state to a state with quarks and gluons which are deconfined from hadrons-the Quark Gluon Plasma (QGP).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,simulating the evolution of the early universe in the first few seconds after the Big Bang.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.The two pillars for this discovery are the observed strong elliptic flow and jet quenching.To quantify the property of the medium created at RHIC,high transverse momentum(PT) particles,which are presumably from jet fragmentations,are ideal penetrating probes. Usually people use high PT single hadron yields and di-hadron correlations with a high PT trigger particle to study the medium created at RHIC.While suppression of high PT single hadron yields has limited sensitivity to the medium core,di-hadron correlation measurements will provide richer and more valuable information about the properties of the created medium.
The disappearance of back-to-back jet from the first high PT di-hadron correlation measurement at RHIC has been recognized as an evidence of the jet quenching discovery. The following analysis by correlating low PT particles associated with the high PT trigger particle shows the broadening in the away side,which is consistent with the jet energy dissipation in the medium.The first di-hadron analysis with respect to the reaction plane shows the away-side suppression happens out-of-plane while it is not significant in-plane.This indicates the possible path length effect in the jet energy loss.Recently, a new phenomenon of the long range△ηcorrelation——so called "Ridge" has been observed in the near side di-hadron correlation,but the origin of ridge phenomenon is still not understood yet[Arm04,Vol06,Maj04,Won07,Hwa05].
In this thesis,we present STAR results of di-hadron azimuthal correlations with a high PT trigger particle relative to reaction plane and a lower pT particle associated with the trigger particle in(?)= 200 GeV Au + Au collisions at RHIC.The large data sample(which is about 10 times more than previous analysis):Au + Au minimum bias events and Au + Au central events collected during year 2004 run were used in this analysis.The minimum bias d + Au collision data collected in year 2003 were used for the baseline comparison.
The di-hadron azimuthal angle correlation was calculated according to the azimuthal angle of the trigger particle relative to the reaction plane(φ_s=|φ~(trig)-ψ_(EP)|).Reaction plane was reconstructed using the standard event plane method.Particles falling in the PT bin of associated particles(p_T~(assoc))were eliminated in the event plane reconstruction to avoid the auto-correlation.The v_4 contribution in the flow background,which had not been taken into account,was found to be not negligible in the di-hadron correlation analysis relative to the reaction plane.We re-evaluated the flow background formula to include the v_4 contribution.The flow parameters were taken from the averaged STAR measurements using the standard event plane method and the four particle cumulant method.The difference between two methods is the most significant systematic error contribution in the results.The normalization of the flow background was calculated using an improved method based on the Zero-Yield-At-Minimum(ZYAM).The flow background was calculated in each p_T~(assoc)bin andφ_s bin,and it was subtracted from the raw di-hadron correlation distributions to extract the correlation function we are interested.Both the mid-central 20-60%and the top 5%central Au + Au collisions were investigated,and the minimum bias d + Au collision data were presented for baseline comparison.The correlation functions are also obtained from small and large |△ηregions separately,in attempt to isolate the jet and ridge contributions to the nearside correlation strength and study their behavior inφ_s.
The di-hadron correlations are strongly modified in Au + Au collisions with respect to minimum bias d + Au collisions.The modification strongly depend on the trigger particle orientation relative to the event plane and evolve with associated p_T~(assoc).The qualitative trend of the correlation function withφ_s appears similar in central and midcentral collisions;quantitatively,theφ_s dependence of the correlation function is stronger in the middle central collisions.The p_T~(assoc)dependences of the correlation function are similar in the two centrality selections.
The away-side correlation broadens from in-plane to out-of-plane,and broadens with increasing associated p_T~(assoc)for mostφ_s slices.For 20-60%Au + Au collisions,the awayside correlation starts as a single peak in the reaction plane,and becomes a stronger and stronger double-peak structure as the trigger particle moves from in-plane to out-of-plane. For top 5%central Au + Au,the away-side correlation already shows a doublepeak structure in the reaction plane,and the double-peak becomes stronger and stronger from in-plane to out-of-plane.The trends of the away-side modification underscore the importance of the path-length that the away-side parton transverses in the medium. The away-side medium path-length in the reaction plane direction in 20-60%Au + Au collisions is quite modest and not enough to generate significant modification to jet correlation,while that in the top 5%collisions is long enough to cause significant jet modification.The strongest modification is found for trigger particles perpendicular to the reaction plane where the away-side medium path-length is the longest,and this path-length appears to be not very different in 20-60%and top 5%Au + Au collisions.
The near-side correlation amplitude decreases from in-plane to out-of-plane.The decrease was found to entirely come from the decrease in the long range△ηcorrelation (ridge).The ridge yield decreases to～0 at out-of-plane in the 20-60%centrality,while significant ridge yields persist from in-plane to out-of-plane in top 5%collisions.The jet contribution to the near-side correlation is extracted from the difference of small and large△ηcorrelations,subject to small experimental systematic uncertainties.The jet contribution in the 20-60%centrality appears to somehow increase from in-plane to out-of-plane.The near-side jet parallel to the reaction plane appears to have suffered significant interactions with the medium,which reduce the real jet correlated multiplicity and produce a long range△ηridge containing a large number of hadrons.The near-side jet perpendicular to the reaction plane,on the other hand,appears to suffer minimal medium modification,generating small amount of ridge.The top 5%results are qualitatively similar,but the significant ridge contribution persists over allφ_s,and the variations of the jet and ridge magnitudes inφ_s is significantly smaller,consistent with the more spherical collision geometry.
The main results presented in this thesis are with 3＜p_T~(trig)＜4 GeV/c.The results from trigger particles with higher PT is necessary to check the results since they are more probably coming from jet fragmentation.In the future RHIC run with the TPC DAQ1000 upgrade,we are able to accumulate more than ten times statistics of the data sample in RunⅣ,used in this thesis.It provides us the possibility of investigating the correlations with high PT trigger particles in the future.
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