LHC/ALICE实验2.76TeV能区p+p对撞下π~0的产额及Pb+Pb对撞下π~0v_2的测量
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摘要
千百年来,在好奇心的驱动下人类追求对物质世界的理解从未停息,尤其是对构成物质世界的最基本单元的追问尤为突出。从古希腊的哲学家德谟克利特的朴素的的原子论,再到英国物理学家道尔顿提出的近代原子论,人类在思辨上持续地进行着尝试.从1897年汤姆逊通过测量阴极射线的荷质比而发现电子,到1911年卢瑟福的α粒子散射实验证实了原子的有核模型,从实验上明确了原子是由其中心的带正电的原子核和围绕核运动的带负电的电子构成。从1912年莫塞来分析元素X射线建立原子序数概念,到1919卢瑟福通过α粒子撞击氮原子核发现质子,再到1932年查得威客利用α粒子撞击铍原子核发现中子,基于实验基础人类对微观世界的认识已经逐渐深入到亚核尺度。然而随着轻子与核子碰撞的深度非弹实验,人类意识到核子应该还具有更小尺度的结构,并且理论上定义构成核子的更基本的物质单元为夸克(quark)和胶子(gluon)。由此建立起了一套描述强,弱,电磁相互作用力以及组成所有物质的物质世界的基元的标准模型(SM).在这个模型中夸克间相互作用被认为是强相互作用,并且定义描述这种强相互作用的理论为量子色动力学(Quantum Chromodynamic, QCD)量子色动力学指出强相互作用的一个重要性质是渐进自由,即在通常条件下参与强相互作用的夸克与反夸克(q)都被束缚在强子内,以致我们在通常条件下不能观察到孤立夸克的自由存在。然而格点量子色动力学(LQCD)理论及数值计算表明在极高的重子数密度(处于5-10倍零温下正常物质密度)或者在极高温度下(大于零密度下临界温度Tc～160MeV)强子的束缚态会解紧闭形成夸克自由存在的夸克胶子等离子体(Quark Gluon Plasma, QGP)。
从实验探测角度看,天体物理学中不仅大爆炸理论指出在久远的宇宙大爆炸的早期(高温)可能存在夸克胶子等离子体,同样也推测在遥远的中子星(高密)内部也有可能有夸克胶子等离子体的存在。但是,随着极长时间的宇宙的演化,宇宙大爆炸早期的信息已经很难提取,同样对于距离遥远的中子星的探测也充满挑战。因此,在上世纪八十年代,著名的物理学家李政道先生就提议在实验室中通过可控的相对论重离子的碰撞来产生高温或者高密的物质。即在实验室中通过加速两束重离子束流到接近光速然后让其对撞,由此期望能在实验室中创造出满足QGP相变所需的高温或高密环境,并且能实现对极端条件下产生的物质的性质进行研究。位于美国纽约长岛布的相对论重离子对撞机(The Relativistic Heavy Ion Collider, RHIC)最大能量能够到达(?)SNN=200GeV。基于这个实验的一系列结果和相关理论计算,人们认为RHIC能区的重离子碰撞已经形成QGP,但是并未有最终的结论。但是所发表的大量的实验数据和结果为正在运行的位于瑞士内瓦的欧洲核子中心(The European Organization for Nuclear Research, CERN)的大型强子对撞机(Large Hadron Collider, LHC)上的重离子研究提供了重要的启发。大型重离子实验探测器(A Large Ion Collider Experiment)是LHC上的四个重要的探测器之一,主要致力于相对论重离子碰撞的研究。设计的最高能量为强子对撞(?)=14TeV和重离子对撞(?)SNN=5.5TeV,相当于RHIC能区的约14倍。由此,碰撞所形成的更加热密的QCD物质将相对持续更长的时间.这为我们探寻夸克胶子等离子体存在的信号,理解并解释从强子相到夸克胶子等离子体相的相变的演化过程,和研究夸克胶子等离子体的性质提供了更加优越的条件.然而限于当今实验技术条件我们仍然不能直接探测到QGP，实验室中只能探测到演化后的末态的强子的信息.通过末态强子信息我们已经发展了众多的间接探测途径.例如：部分子的喷注淬火现象,高横动量强子谱及其压低,夸克偶数产额的压低,粒子的方位角关联,粒子的集体行为特性等.在众多的观测量及其可探测的兴趣粒子中由于光子在碰撞演化的各个时期都有生成并且只参与电磁相互.从而早期产生的光子与致密的QGP的相互作用截面会比较小,在QGP中产生的光子与末态强子气相互作用截面同样比较小,所以在碰撞的各个阶段产生的光子在探测时任然携带了其产生时的系统的信息.由此与光子相关的物理观测量被认为是比较理想的探针.
结合理论背景的分析和实验的合作组合作分工情况与实验的进展,博士期间主要的的工作集中于研究实验上与光子相关的课题,尤其致力于ALICE实验数据的分析.其中主要包括(1)利用2011年收集的在(?)SNN=2.76TeV能量下的p+p碰撞的数据进行π0产额的研究。(2)利用2011年收集的在(?)SNN=2.76TeV能量下的Pb+Pb碰撞的数据进行π0v2的研究。
首先,有关π0产额的分析工作是基于ALICE实验处于探测器刚开始取数的早期,对探测器的校正和理解还在进行中,所以这部分工作通过对最基本的粒子谱的测量来帮助我们对探测器的校正和加深对探测器性能的理解.在具体的分析中,我们以ALICE探测器的子探测器之一的电磁量能器(Electromagnetic Calorimeter, EMCal)为主,从探测器采集的原始重建数据出发开始进行测量.首先对采集的数据进行了质量的验证(QA),剔除非物理对撞的事件,以保证进入分析的事件为我们所期望的真实对撞.然后我们对沉积在EMCal中的簇射(Cluster)进行挑选与甄别,依据簇射的形状,能量大小,最高能量的子通道占总能量大小,簇射所在位置等,在挑选过程中加深对探测器的性能的理解,并对探测器相关的参数进行了研究和确定,最终挑选出被认为是光子的候选的簇射。然后通过将两个候选簇射进行不变质量组合,我们可以得到在不同横动量区域的不变质量分布.通过构建不变质量模型并将其应用于拟合不变质量分布,一方面我们可以得到候选π0的质量峰的位置和质量峰的峰宽.由于探测器不可能达到完美,所以我们必须通过模特卡罗模拟(MC)研究探测器的探测效率来校正探测器迷失的未探测到的部分.那么质量峰的位置和峰宽就成为指导我们优化模型的模拟的重要参数.另一方面通过计算不变质量分布拟合中信号部分,我们可以得到π0在特定横动量区间的产额并归一到单事件。通过分析全部有效横动量区间我们就可以得出粗谱,再经过探测效率和接受度以及衰变分子比等等的校正就可以得到仅包含统计误差的单事件的不变质量谱。通过考虑拟合模型,簇射的不同选择参数,快度(Rapidity)区间的选择,光子在探测器前转换,奇异簇射等的系统误差因素,我们得到了粒子的单事件含系统误差的不变质量谱,为我们理解探测器并对其校正提供了信息,同时为核核碰撞环境下研究核修正因子(RAA)提供了一个基准.这部分工作的结果作为墙报报告的主要部分于2011年的国际夸克物质大会(Quark Matter2011)上进行了报告,而且在分析的过程中在合作组内进行若干次分析进展报告.
其次,对于π0v2的研究工作是有关于在核-核碰撞环境下研究热密物质的性质.因为光子在碰撞过程的各个演化阶段都有产生,所以对集体流(Elliptic Flow)尤其是与光子物理相关的集体流与系统早期的演化有着密切的联系.对集体流的研究不仅是判定是否存在QGP相提供信息,同时也可以通过集体流的研究来研究QGP的性质.在RHIC实验中通过集体流的研究发现：在低横动量区间,观察到的椭圆流具有质量顺序性与流体力学计算预言的相一致,表明了在RHIC能区能够形成部分子的集体运动,也表明了产生的热密物质具有极低的粘滞性的“液态”的理想流体。而在中等横动量区域,实验所观察到的末态的强子物质的组分夸克数的标度性,极强地暗示产生了解禁闭物质。作为集体流的信号之一的椭圆流(v2)定义为末态出射的要研究的感兴趣的粒子(Particle of Interest, POI)相对于事件的反应平面(Event Plane, EP)的傅里叶展开的第二谐波系数。它是有关于碰撞核的参与者(Participant)在X-Y平面上的方位角分布的情况,通过对其研究我们可以了解到相同于反应平面和垂直于反应平面的粒子的分布情况,在本课题中我们选择π0为POI是基于：首先,通过对这个强子的研究可以了解到特定强子的椭圆流的信息.其次,其衰变后的主要产物衰变光子与碰撞演化直接产生光子的椭圆流有本质区别,作为研究碰撞产生光子椭圆流的首要条件是计算并剔除掉衰变而来的光子的信息,尤其是主要来源之一的π0衰变而来的光子.根据不同的横动量区域粒子的产生源我们可以反推得到粒子出射的阶段的热密物质的位形分布等信息。在实验数据的分析过程中,主要可分为重建特定感兴趣粒子的方位角分布并把其与重建的反应平面相关联,然后取傅里叶展开的第二谐波系数(v2)。
在分析中,我们首先采用前向的探测器V0来重建反应平面,较大的快度区间(2.8> ηu0A>-3.7<ηu0C<1.7)能够尽可能地消除非流效应(Non-flow)的影响。采用位于中心快度区的电磁量能器(EMCal)重建π0及其方位角。再将两者的方位角信息进行相关联来研究椭圆流。在对流信号提取过程中,我们采用了(1)标准的通过拟合相对角分布方法提取。(2)通过拟合不变质量和相对分布的余弦值的二维分布来提去流信号。并把他们之间的差别作为系统误差的一部分,并且加上不同探测器重建反应平面,不同的不变质量拟合模型等系统误差源。最终给出微分的分不同横动量区间的π0v2的结果。这部分结果在国际夸克物质会议(Quark Matter2012)上作墙报报告,作为主要部分在LHC高横动量会议(8th International Workshop on high pT Physics at LHC2012)上作口头报告,而且作为分析进展,在合作组内进行若干次进展报告。
通过以上工作的开展,加深相关的理论背景知识,也提高了相关的实验技巧,加入到了国际重离子物理实验的领域.然而人类对物质世界的理解永不停息,相关的重离子实验也不会停止.
For over thousand years people have thought about the fundamental particles from which all matter is made, starting with the gradual development of atomic theory. It began as a philosophi-cal concept in ancient Greece (Democritus), followed by the modern atomic theory proposed by John Dalton. It then entered the scientific mainstream in the early19th century by J. J. Thomson who discovered the electron by measuring the Mass-to-charge ratio of electron itself, People confirmed the model used to describe atom is called Rutherford model and in this model Ernest Rutherford demonstrated atom has a tiny but heavy nucleus. Besides the famous experiment by James Chadwick in finding the neutron and by Rutherford in α scattering experiment in con-firming the proton, more and more experiment results were announced on the deeper structure study the history of searching for structure of elementary particle getting started to set up on the experiment results. Then more and more experiment facilities set up on studying various aspect of elementary particle structure, In the deep inelastic collision experiment, we realized that nucleus should also has its sub-structure which now know it as quark and gluon. Hence one series of theory was build to describe the structure of nucleus or even much deeper structure. Among those theories, the most famous one is called Standard Model(SM) which summarized on electromagnetic, weak, and strong nuclear interactions. As part of standard model the strong force is one of the four fundamental interactions in particle physics. It describes the interaction between partons (quarks and gluons) which make up hadrons. The theory of the strong force is called quantum chromodynamics (QCD), which is a quantum field theory of the color charged partons. The force between color charges does not diminish while they are separated. This property causes the color charges to be confined in to hadrons, in ordinary matter. Quark-Gluon Plasma (QGP) is one phase of the QCD matter at extremely high temperature and/or pressure, where the partons are asymptotically free. In astronomy, the big bang theory not only suggests that the Big Bang in the early universe (high temperature) may create Quark-gluon plasma, but also expects that in the neutron star (high density) QGP may also exist. However, with the uni-verse going through long timescale evolution, the information of such created plasma has been fell off hence difficult to extract. On the other hand neutron Star is so far away from us that its detection is also challenging. Therefore, in the80s of last century, the Nobel Prize Winner T.D.Lee proposed idea that one could create high temperature and(or) density state of material in the laboratory by heavy ion collisions.
By accelerating two heavy ion beams in opposite direction and then let them collided, one expects to realize the extreme high temperature and density matter in the laboratory. A great of results have been obtained at the Relativistic Heavy Ion Collider (RHIC) which was build at Brookhaven National Laboratory with a top center-of-mass energy200GeV. The results found at RHIC is a important baseline for the experiments take out at Large Hadron Collider (LHC) at CERN since90s of last century. A Large Ion Collider Experiment (ALICE) is one of the four main detectors at the LHC, which is dedicated in the heavy-ion collisions studied. The design top energy of it is up to (?)SNN=5.5TeV in Pb+Pb collision while (?)SNN=14TeV at p+p collision which is roughly14times larger than RHIC. Thus, relatively dense QCD matter will be created and last longer than in RHIC. It also provided us a better opportunity to study the properties of the QGP. Even We still can not measure QGP directly due to the limitation of the experiment facilities, several measurement method was also developed depending on the final measurable decay particle. Such as Jet quenching, High pT particle production, suppression of quarkonium, particle azimuthal correlation, collective phenomena etc. In the various final state particle, photon is treated as the golden particle who keeping the collision information in each stage of collision because of it generated in the whole stage of collision. It is also involved only in the electromagnetic interaction hence with lower interaction probability with others particle.
With the theory background as explain above, during Ph.D study period my work is fo-cusing on the data analysis in ALICE experiment especially on the photon relative physics via EMCal detector. The thesis mainly includes the two topics:(1) Measurement of π0production in (?)SNN=2.76TeV proton+proton collisions data collected at2011. and (2) Measurement of π0u2in (?)SNN=2.76TeV Pb+Pb collision collected at2011.
Before starting the data analysis, it is important to understand the experimental facilities we have. Hence in the chapter2, the experiment facilities of ALICE is introduced with start from an overview of ALICE detector. The sub-detector which involved in our analysis is presented from inner layer to out layer depending on their physical location. The first sub-detector need to be introduced is Inner Tracker System(ITS). It is a3layers silicon pixel technical base detector which dedicated in measuring the position of the charged tracks. By analysis the distribution of the tracks which deposited information in the ITS, the interaction vertex in the collision was fixed. The forward rapidity detector used to reconstruction the reaction event plane is introduced afterward. Finally but most important part of this chapter is the explained the detail composition of the π0detector which is Electromagnetic Calorimeter(EMCal), not only the material of its but also the performance of detector was introduced.
The first work on analysis in the real data is on the data collected in2010proton+proton (?)SNN=2.76TeV collision. The analysis is presented by separated it to several analysis steps. After introducing the motivation of this part of work in the beginning, the event selection and cluster selection criteria are listed. Since detector can not100percent correctly reconstruct out what come across it. By comparing the different of the π0yield on the generated level to the one after go through the detector reconstruction, the π0reconstruction efficiency in EMCal is studied with the embedding technical. In the same time the raw yield extraction and Invariant yield calculation is also studied by employing different extraction method. By testing on dif-ferent combination of real data reconstruction procedure and efficiency calculation method, the invariant yield which count as central point for later systematic study is fixed. The systematic uncertainty in this analysis is studied by variant the different reconstruction procedure and cuts while keep the other stable, Hence the different between central point and yield after variant is treated as one source of systematic uncertainty. We hence present the minimum bias data sam-ple result with adding the systematic error on it. In order to extend the measurement of yield to higher transverse momentum range in the limitation of statistic, we analysis the EMCal trigger data sample which by selecting the particular event by online setting several criteria to accept particular events. With the same procedure the invariant yield of trigger data and systematic uncertainty are estimated. Finally the results from different transverse momentum are combined by scaling down the trigger yield to match the minimum bias yield with trigger rate calculation.
The second work is on the analysis of Pb+Pb collisions data collected at2011. The motiva-tion of this part of work comes from the perspective of theory are introduced. In the introduction we explain the physics information expect to be reveals from this analysis in the three different transverse momentum ranges which is low, medium and high pT regions. Afterward the two main methods used to extract v2in the experiment are introduced. The first one which also assumed as common one is called dN/dΦ method, which this method after reconstruction π0in different transverse and its relatively azimuthal location with respect to event plane, the v2information thus can be extracted by fitting those relatively production distributions with certain models. The second method is called invariant mass method, In this method by calculating the different of background to total and signal to total yield ratio as the fitting model parameter, one fitted the cos(2△φ) as function of transverse momentum distribution to extract raw u2in-formation. The final step of extraction in both method1and method2are the raw u2extracted from above fitting divided the event plane resolution to get final result. The event plane count the different of the reconstructed event plane compared to the collision one which can not be measured. Since the event plane reconstruction in this analysis is a crucial item, the definition of it and how to reconstruction it is emphasized in the second part of this section. In the assumption of not perfect reconstruction of event plane, not only the event plane resolution which defined in the theory that should be studied but also the additional bias found in analysis due to cen- trality trigger bias should also be investigated. On other hand π0reconstruction strategy is also stressed in this chapter. After explaining the detail of event plane and π0reconstruction, the raw result thus come out by computing the correlation of this two parts in the azimuthal different. The sources of systematic uncertainty we take into count in this analysis are the:(1) the extract method difference, method1or method2as explain before.(2) The π0fitting models when extract its yield, the different is background description with Polynomial2or Polynomial1.(3) The detector used to reconstruct event plane, using VOA or VOC. By comparing the different of the result of u2calculated from different combination of three sources of systematic uncertainty as considered above, the final estimation of systematic uncertainty is calculated by combing the different source linearity in the assumption that they should be independent on each other. Final-ly the results of π0u2are presented as function of transverse momentum in6different centrality bins.
从实验探测角度看,天体物理学中不仅大爆炸理论指出在久远的宇宙大爆炸的早期(高温)可能存在夸克胶子等离子体,同样也推测在遥远的中子星(高密)内部也有可能有夸克胶子等离子体的存在。但是,随着极长时间的宇宙的演化,宇宙大爆炸早期的信息已经很难提取,同样对于距离遥远的中子星的探测也充满挑战。因此,在上世纪八十年代,著名的物理学家李政道先生就提议在实验室中通过可控的相对论重离子的碰撞来产生高温或者高密的物质。即在实验室中通过加速两束重离子束流到接近光速然后让其对撞,由此期望能在实验室中创造出满足QGP相变所需的高温或高密环境,并且能实现对极端条件下产生的物质的性质进行研究。位于美国纽约长岛布的相对论重离子对撞机(The Relativistic Heavy Ion Collider, RHIC)最大能量能够到达(?)SNN=200GeV。基于这个实验的一系列结果和相关理论计算,人们认为RHIC能区的重离子碰撞已经形成QGP,但是并未有最终的结论。但是所发表的大量的实验数据和结果为正在运行的位于瑞士内瓦的欧洲核子中心(The European Organization for Nuclear Research, CERN)的大型强子对撞机(Large Hadron Collider, LHC)上的重离子研究提供了重要的启发。大型重离子实验探测器(A Large Ion Collider Experiment)是LHC上的四个重要的探测器之一,主要致力于相对论重离子碰撞的研究。设计的最高能量为强子对撞(?)=14TeV和重离子对撞(?)SNN=5.5TeV,相当于RHIC能区的约14倍。由此,碰撞所形成的更加热密的QCD物质将相对持续更长的时间.这为我们探寻夸克胶子等离子体存在的信号,理解并解释从强子相到夸克胶子等离子体相的相变的演化过程,和研究夸克胶子等离子体的性质提供了更加优越的条件.然而限于当今实验技术条件我们仍然不能直接探测到QGP，实验室中只能探测到演化后的末态的强子的信息.通过末态强子信息我们已经发展了众多的间接探测途径.例如：部分子的喷注淬火现象,高横动量强子谱及其压低,夸克偶数产额的压低,粒子的方位角关联,粒子的集体行为特性等.在众多的观测量及其可探测的兴趣粒子中由于光子在碰撞演化的各个时期都有生成并且只参与电磁相互.从而早期产生的光子与致密的QGP的相互作用截面会比较小,在QGP中产生的光子与末态强子气相互作用截面同样比较小,所以在碰撞的各个阶段产生的光子在探测时任然携带了其产生时的系统的信息.由此与光子相关的物理观测量被认为是比较理想的探针.
结合理论背景的分析和实验的合作组合作分工情况与实验的进展,博士期间主要的的工作集中于研究实验上与光子相关的课题,尤其致力于ALICE实验数据的分析.其中主要包括(1)利用2011年收集的在(?)SNN=2.76TeV能量下的p+p碰撞的数据进行π0产额的研究。(2)利用2011年收集的在(?)SNN=2.76TeV能量下的Pb+Pb碰撞的数据进行π0v2的研究。
首先,有关π0产额的分析工作是基于ALICE实验处于探测器刚开始取数的早期,对探测器的校正和理解还在进行中,所以这部分工作通过对最基本的粒子谱的测量来帮助我们对探测器的校正和加深对探测器性能的理解.在具体的分析中,我们以ALICE探测器的子探测器之一的电磁量能器(Electromagnetic Calorimeter, EMCal)为主,从探测器采集的原始重建数据出发开始进行测量.首先对采集的数据进行了质量的验证(QA),剔除非物理对撞的事件,以保证进入分析的事件为我们所期望的真实对撞.然后我们对沉积在EMCal中的簇射(Cluster)进行挑选与甄别,依据簇射的形状,能量大小,最高能量的子通道占总能量大小,簇射所在位置等,在挑选过程中加深对探测器的性能的理解,并对探测器相关的参数进行了研究和确定,最终挑选出被认为是光子的候选的簇射。然后通过将两个候选簇射进行不变质量组合,我们可以得到在不同横动量区域的不变质量分布.通过构建不变质量模型并将其应用于拟合不变质量分布,一方面我们可以得到候选π0的质量峰的位置和质量峰的峰宽.由于探测器不可能达到完美,所以我们必须通过模特卡罗模拟(MC)研究探测器的探测效率来校正探测器迷失的未探测到的部分.那么质量峰的位置和峰宽就成为指导我们优化模型的模拟的重要参数.另一方面通过计算不变质量分布拟合中信号部分,我们可以得到π0在特定横动量区间的产额并归一到单事件。通过分析全部有效横动量区间我们就可以得出粗谱,再经过探测效率和接受度以及衰变分子比等等的校正就可以得到仅包含统计误差的单事件的不变质量谱。通过考虑拟合模型,簇射的不同选择参数,快度(Rapidity)区间的选择,光子在探测器前转换,奇异簇射等的系统误差因素,我们得到了粒子的单事件含系统误差的不变质量谱,为我们理解探测器并对其校正提供了信息,同时为核核碰撞环境下研究核修正因子(RAA)提供了一个基准.这部分工作的结果作为墙报报告的主要部分于2011年的国际夸克物质大会(Quark Matter2011)上进行了报告,而且在分析的过程中在合作组内进行若干次分析进展报告.
其次,对于π0v2的研究工作是有关于在核-核碰撞环境下研究热密物质的性质.因为光子在碰撞过程的各个演化阶段都有产生,所以对集体流(Elliptic Flow)尤其是与光子物理相关的集体流与系统早期的演化有着密切的联系.对集体流的研究不仅是判定是否存在QGP相提供信息,同时也可以通过集体流的研究来研究QGP的性质.在RHIC实验中通过集体流的研究发现：在低横动量区间,观察到的椭圆流具有质量顺序性与流体力学计算预言的相一致,表明了在RHIC能区能够形成部分子的集体运动,也表明了产生的热密物质具有极低的粘滞性的“液态”的理想流体。而在中等横动量区域,实验所观察到的末态的强子物质的组分夸克数的标度性,极强地暗示产生了解禁闭物质。作为集体流的信号之一的椭圆流(v2)定义为末态出射的要研究的感兴趣的粒子(Particle of Interest, POI)相对于事件的反应平面(Event Plane, EP)的傅里叶展开的第二谐波系数。它是有关于碰撞核的参与者(Participant)在X-Y平面上的方位角分布的情况,通过对其研究我们可以了解到相同于反应平面和垂直于反应平面的粒子的分布情况,在本课题中我们选择π0为POI是基于：首先,通过对这个强子的研究可以了解到特定强子的椭圆流的信息.其次,其衰变后的主要产物衰变光子与碰撞演化直接产生光子的椭圆流有本质区别,作为研究碰撞产生光子椭圆流的首要条件是计算并剔除掉衰变而来的光子的信息,尤其是主要来源之一的π0衰变而来的光子.根据不同的横动量区域粒子的产生源我们可以反推得到粒子出射的阶段的热密物质的位形分布等信息。在实验数据的分析过程中,主要可分为重建特定感兴趣粒子的方位角分布并把其与重建的反应平面相关联,然后取傅里叶展开的第二谐波系数(v2)。
在分析中,我们首先采用前向的探测器V0来重建反应平面,较大的快度区间(2.8> ηu0A>-3.7<ηu0C<1.7)能够尽可能地消除非流效应(Non-flow)的影响。采用位于中心快度区的电磁量能器(EMCal)重建π0及其方位角。再将两者的方位角信息进行相关联来研究椭圆流。在对流信号提取过程中,我们采用了(1)标准的通过拟合相对角分布方法提取。(2)通过拟合不变质量和相对分布的余弦值的二维分布来提去流信号。并把他们之间的差别作为系统误差的一部分,并且加上不同探测器重建反应平面,不同的不变质量拟合模型等系统误差源。最终给出微分的分不同横动量区间的π0v2的结果。这部分结果在国际夸克物质会议(Quark Matter2012)上作墙报报告,作为主要部分在LHC高横动量会议(8th International Workshop on high pT Physics at LHC2012)上作口头报告,而且作为分析进展,在合作组内进行若干次进展报告。
通过以上工作的开展,加深相关的理论背景知识,也提高了相关的实验技巧,加入到了国际重离子物理实验的领域.然而人类对物质世界的理解永不停息,相关的重离子实验也不会停止.
For over thousand years people have thought about the fundamental particles from which all matter is made, starting with the gradual development of atomic theory. It began as a philosophi-cal concept in ancient Greece (Democritus), followed by the modern atomic theory proposed by John Dalton. It then entered the scientific mainstream in the early19th century by J. J. Thomson who discovered the electron by measuring the Mass-to-charge ratio of electron itself, People confirmed the model used to describe atom is called Rutherford model and in this model Ernest Rutherford demonstrated atom has a tiny but heavy nucleus. Besides the famous experiment by James Chadwick in finding the neutron and by Rutherford in α scattering experiment in con-firming the proton, more and more experiment results were announced on the deeper structure study the history of searching for structure of elementary particle getting started to set up on the experiment results. Then more and more experiment facilities set up on studying various aspect of elementary particle structure, In the deep inelastic collision experiment, we realized that nucleus should also has its sub-structure which now know it as quark and gluon. Hence one series of theory was build to describe the structure of nucleus or even much deeper structure. Among those theories, the most famous one is called Standard Model(SM) which summarized on electromagnetic, weak, and strong nuclear interactions. As part of standard model the strong force is one of the four fundamental interactions in particle physics. It describes the interaction between partons (quarks and gluons) which make up hadrons. The theory of the strong force is called quantum chromodynamics (QCD), which is a quantum field theory of the color charged partons. The force between color charges does not diminish while they are separated. This property causes the color charges to be confined in to hadrons, in ordinary matter. Quark-Gluon Plasma (QGP) is one phase of the QCD matter at extremely high temperature and/or pressure, where the partons are asymptotically free. In astronomy, the big bang theory not only suggests that the Big Bang in the early universe (high temperature) may create Quark-gluon plasma, but also expects that in the neutron star (high density) QGP may also exist. However, with the uni-verse going through long timescale evolution, the information of such created plasma has been fell off hence difficult to extract. On the other hand neutron Star is so far away from us that its detection is also challenging. Therefore, in the80s of last century, the Nobel Prize Winner T.D.Lee proposed idea that one could create high temperature and(or) density state of material in the laboratory by heavy ion collisions.
By accelerating two heavy ion beams in opposite direction and then let them collided, one expects to realize the extreme high temperature and density matter in the laboratory. A great of results have been obtained at the Relativistic Heavy Ion Collider (RHIC) which was build at Brookhaven National Laboratory with a top center-of-mass energy200GeV. The results found at RHIC is a important baseline for the experiments take out at Large Hadron Collider (LHC) at CERN since90s of last century. A Large Ion Collider Experiment (ALICE) is one of the four main detectors at the LHC, which is dedicated in the heavy-ion collisions studied. The design top energy of it is up to (?)SNN=5.5TeV in Pb+Pb collision while (?)SNN=14TeV at p+p collision which is roughly14times larger than RHIC. Thus, relatively dense QCD matter will be created and last longer than in RHIC. It also provided us a better opportunity to study the properties of the QGP. Even We still can not measure QGP directly due to the limitation of the experiment facilities, several measurement method was also developed depending on the final measurable decay particle. Such as Jet quenching, High pT particle production, suppression of quarkonium, particle azimuthal correlation, collective phenomena etc. In the various final state particle, photon is treated as the golden particle who keeping the collision information in each stage of collision because of it generated in the whole stage of collision. It is also involved only in the electromagnetic interaction hence with lower interaction probability with others particle.
With the theory background as explain above, during Ph.D study period my work is fo-cusing on the data analysis in ALICE experiment especially on the photon relative physics via EMCal detector. The thesis mainly includes the two topics:(1) Measurement of π0production in (?)SNN=2.76TeV proton+proton collisions data collected at2011. and (2) Measurement of π0u2in (?)SNN=2.76TeV Pb+Pb collision collected at2011.
Before starting the data analysis, it is important to understand the experimental facilities we have. Hence in the chapter2, the experiment facilities of ALICE is introduced with start from an overview of ALICE detector. The sub-detector which involved in our analysis is presented from inner layer to out layer depending on their physical location. The first sub-detector need to be introduced is Inner Tracker System(ITS). It is a3layers silicon pixel technical base detector which dedicated in measuring the position of the charged tracks. By analysis the distribution of the tracks which deposited information in the ITS, the interaction vertex in the collision was fixed. The forward rapidity detector used to reconstruction the reaction event plane is introduced afterward. Finally but most important part of this chapter is the explained the detail composition of the π0detector which is Electromagnetic Calorimeter(EMCal), not only the material of its but also the performance of detector was introduced.
The first work on analysis in the real data is on the data collected in2010proton+proton (?)SNN=2.76TeV collision. The analysis is presented by separated it to several analysis steps. After introducing the motivation of this part of work in the beginning, the event selection and cluster selection criteria are listed. Since detector can not100percent correctly reconstruct out what come across it. By comparing the different of the π0yield on the generated level to the one after go through the detector reconstruction, the π0reconstruction efficiency in EMCal is studied with the embedding technical. In the same time the raw yield extraction and Invariant yield calculation is also studied by employing different extraction method. By testing on dif-ferent combination of real data reconstruction procedure and efficiency calculation method, the invariant yield which count as central point for later systematic study is fixed. The systematic uncertainty in this analysis is studied by variant the different reconstruction procedure and cuts while keep the other stable, Hence the different between central point and yield after variant is treated as one source of systematic uncertainty. We hence present the minimum bias data sam-ple result with adding the systematic error on it. In order to extend the measurement of yield to higher transverse momentum range in the limitation of statistic, we analysis the EMCal trigger data sample which by selecting the particular event by online setting several criteria to accept particular events. With the same procedure the invariant yield of trigger data and systematic uncertainty are estimated. Finally the results from different transverse momentum are combined by scaling down the trigger yield to match the minimum bias yield with trigger rate calculation.
The second work is on the analysis of Pb+Pb collisions data collected at2011. The motiva-tion of this part of work comes from the perspective of theory are introduced. In the introduction we explain the physics information expect to be reveals from this analysis in the three different transverse momentum ranges which is low, medium and high pT regions. Afterward the two main methods used to extract v2in the experiment are introduced. The first one which also assumed as common one is called dN/dΦ method, which this method after reconstruction π0in different transverse and its relatively azimuthal location with respect to event plane, the v2information thus can be extracted by fitting those relatively production distributions with certain models. The second method is called invariant mass method, In this method by calculating the different of background to total and signal to total yield ratio as the fitting model parameter, one fitted the cos(2△φ) as function of transverse momentum distribution to extract raw u2in-formation. The final step of extraction in both method1and method2are the raw u2extracted from above fitting divided the event plane resolution to get final result. The event plane count the different of the reconstructed event plane compared to the collision one which can not be measured. Since the event plane reconstruction in this analysis is a crucial item, the definition of it and how to reconstruction it is emphasized in the second part of this section. In the assumption of not perfect reconstruction of event plane, not only the event plane resolution which defined in the theory that should be studied but also the additional bias found in analysis due to cen- trality trigger bias should also be investigated. On other hand π0reconstruction strategy is also stressed in this chapter. After explaining the detail of event plane and π0reconstruction, the raw result thus come out by computing the correlation of this two parts in the azimuthal different. The sources of systematic uncertainty we take into count in this analysis are the:(1) the extract method difference, method1or method2as explain before.(2) The π0fitting models when extract its yield, the different is background description with Polynomial2or Polynomial1.(3) The detector used to reconstruct event plane, using VOA or VOC. By comparing the different of the result of u2calculated from different combination of three sources of systematic uncertainty as considered above, the final estimation of systematic uncertainty is calculated by combing the different source linearity in the assumption that they should be independent on each other. Final-ly the results of π0u2are presented as function of transverse momentum in6different centrality bins.
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