【作者】 朱祥荣；

【导师】 周代翠； Constantinos Loizides； 殷中宝；

【作者基本信息】 华中师范大学 ， 粒子物理与原子核物理， 2014， 博士

【摘要】 自古以来,物质微观结构一直都是人类感兴趣并探寻的对象。从古希腊哲学家德谟克利特(Democritus)猜想的原子不可分论到1803年英国物理学家约翰·道尔顿(John Dalton)依据“倍比定律”提出的近代原子论,再到1897年英国物理学家约瑟夫·约翰·汤姆逊(Joseph John Thomson)通过测量阴极射线的荷质比发现电子以及1911年英国物理学家欧内斯特·卢瑟福(Ernest Rutherford)通过α粒子散射实验提出的原子有核模型,再到之后通过各种实验陆续发现的质子、中子、夸克……。所有这些才使人们逐渐清晰地认识到：原子是由原子核和核外电子组成,原子核又是由质子和中子组成,而质子和中子又是由一定相互作用禁闭在其内的夸克与胶子组成等等。目前普遍的观点认为,构成物质的基本粒子分为三代夸克(quark)及其反夸克(anti-quark)和三代轻子(lepton),且这些基本粒子间通过传递胶子(gluon)、W±和Z0玻色子(W±Z0boson)及光子发生对应的强、弱和电磁相互作用。近代物理建立了一套描述这些基本粒子及它们间的基本相互作用的理论一标准模型(Standard Model)。该模型对诸多实验结果都给出了合理的解释,同时它预言的一种解释基本粒子质量之源的粒子—希格斯玻色子(Higgs boson),也已于2012年在欧洲核子研究组织(European Organization for Nuclear Research, CERN)的大型强子对撞机(Large Hadron Collider, LHC)实验中发现。在这个标准模型中,夸克胶子(部分子)间发生的强相互作用通过规范学理论—量子色动力学(Quantum ChromoDynamics, QCD)给予描述。量子色动力学存在三个显著的基本特性：1)禁闭(confinement),在普通环境中,因部分子间交换的横动量小,强相互作用很强(耦合常数(αs)大),因此,夸克与胶子都被禁闭在强子内部；2)渐近自由(asymptotic freedom),当交换的横动量越大或夸克间距离越小时,强相互作用越弱(耦合常数(αs)越小)。在渐近自由状态下,部分子间的相互作用是微扰的。3)手征对称性恢复(chiral symmetry restoration),希望在QGP物质中手征对称性是恢复的,这意味着在极端环境中夸克质量趋近于零。基于量子色动力学,采用格点方法发展的另一理论—格点量子色动力学(Lattice QCD)预言在极端高温和／或高重子数密度的条件下,禁闭在普通强子内的部分子将发生退禁闭形成一种由处于渐近自由状态的夸克和胶子组成的新物质相—夸克-胶子等离子体(Quark-Gluon Plasma, QGP).众多研究结果表明,宇宙大爆炸早期因形成极高温的环境而可能产生这种新物质相,或在中子星内因其具有极高的重子数密度而可能存在该物质相。基于此,寻找QGP形成的特征信号并研究其相关特性对理解强相互作用和宇宙早期形成及演化具有重要意义。但是,随着宇宙大爆炸后极长时间的演化,其早期信息已很难被提取,且探测遥远中子星内部亦充满巨大的挑战,因此如何在现有实验条件下通过实验产生QGP这种新物质相一直是科学家们思考的问题。高能重离子碰撞被认为是在现有实验条件下实现从强子相退禁闭到QGP相的一种可行途径。在高能重离子碰撞实验中,两重离子束流被加速到接近光速,然后实现对撞并将大部分能量沉积在碰撞区域,这使得在碰撞区域内产生能够发生退禁闭相变所需的超高能量密度,进而在碰撞区域内产生由渐近自由夸克和胶子组成的QGP热密物质。随着时间推移,产生的QGP物质系统迅速膨胀并通过强子化形成最后实验上能够观测到的各种粒子。自上世纪60年代,科学家们相继建造了一系列重离子加速器,如交变梯度同步加速器(Alternating Gradient Synchrotron, AGS)、超质子同步加速度器(Super Proton Synchrotron, SPS)和相对论重离子对撞机(Relativistic Heavy Ion Collider, RHIC),并在QGP特征信号的寻找及相关性质的研究方面取得了丰硕成果。欧洲核子研究组织建造了目前世界上最大的大型强子对撞机,并于2009年底正式运行。作为LHC上四大实验之一的大型重离子对撞实验(A Large Ion Collider Experiment, ALICE),其中各子探测器的设计和性能都为重离子碰撞研究提供了前所未有的优越条件。在2010年运行的铅—铅碰撞中,其质心系能量达到该能量相当于RHIC最高碰撞能量的～14倍,因此碰撞所形成的QGP物质相比于RHIC能区将持续时间更长且体积更大,这为全面而深入地研究QGP物质的特性提供了更加优越的条件。重离子碰撞中形成的QGP只存在于碰撞后的瞬间,然后就碎裂成末态强子,因此几乎不可能从实验上直接观察到QGP物质相,而只能通过对末态粒子的各种观察量来判断QGP形成并研究其相关性质。到目前为止,被认为在重离子碰撞中可能形成了QGP的特征信号主要有：奇异粒子的增加(Strangeness enhancement)、J/Ψ的产额压低、直接光子与热双轻子、喷注淬火(Jet quenching)、集体流(Collective flow)等等。末态两粒子间的方位角关联被认为是研究热密物质效应的成功方法之一。该测量方法通常选取两类粒子,一类具有高横动量的粒子称作“触发粒子”,其被认为是来自于一个喷注中的领头粒子,另一类具有较低的横动量粒子称作“伴随粒子”,它可能来自于喷注中的其他粒子,也可能来自于其他信号,如集体流等。在RHIC和LHC上,相比于质子—质子碰撞中的关联分布,核—核中心碰撞中高横动量区粒子间关联在远端(away side)存在压低甚至消失,以及低横动量区关联远端出现增宽,双峰结构和近端(near side)的“脊”。这些测量结果都可解释为热密物质的相互作用,并间接表明核—核碰撞中形成了夸克—胶子等离子体这种新物质相。当触发粒子选取的是直接光子时,这种两粒子关联分析到了碰撞中硬散射过程形成的光子—喷注事件。这类事件中,领头阶的直接光子通过夸克胶子的康普顿散射或正反夸克湮灭产生于碰撞初期的硬散射过程,这使得直接光子与其背对部分子在初始动量大小上近似平衡。同时光子的平均自由程很大且当它穿越碰撞形成的QGP相时与相中其他粒子只发生电磁相互作用,由此直接光子携带了碰撞初期的相关信息。当测量背对部分子碎裂的末态强子与直接关子的关联时,又可以获取部分子穿越密度物质后的相关信息,如介质效应作用下的部分子碎裂函数,进而研究密度物质的相关性质。本文基于LHC/ALICE的电磁量能器(EMCal)和中心桶部径迹探测系统,利用中性粒子(中性π介子和光子)与强子关联,分析研究热密物质的介质效应,同时测量部分子碎裂函数。在中性π介子(π0)—强子关联分析中,基于质心系能量的质子—质子与铅—铅碰撞数据,通过构建电磁量能器测量的π0与径迹探测系统测量的带电强子在方位角上的关联,测量π0—带电强子方位角分布函数和铅—铅中心碰撞中每π0触发的带电强子产额在关联近端与远端的修正因子IAA=YPbPb/Ypp。分析结果显示在近端区这个修正因子存在～1.2倍的增强,而在远端区则存在～0.6倍的压低。这个结果可以用于研究热密物质的喷注淬火机制和部分子碎裂函数的改变。其中近端区的增强反应了近端部分子受热密物质的作用,这种作用主要体现在以下三点：1)受热密物质作用,近部部分子的碎裂函数被改变；2)因对热密物质的不同耦合导致末态夸克与胶子喷注比例发生变化；3)因触发粒子π0的选择导致能量损失后的部分子谱存在偏离(bias).而远端区的压低则是因远端部分子穿越热密物质过程中损失部分能量导致末态粒子在高横动量区内的产额压低。在直接光子—强子关联分析中,基于质心系能量为(?)=7Tev的质子—质子碰撞数据,分别通过孤立分析技术和统计减除方法提取直接光子信号并与带电强子构建方位角关联,进而通过分析非平衡参数xE=-PTγ·PTh/|PTγ|2测量部分子碎裂函数。孤立分析技术是基于领头阶的直接光子周围没有或只有很少的粒子,采用该方法测量了横动量在8.0<pT<25.0GeV/c的部分子的碎裂函数,并与理论计算进行比较。而统计减除的分析方法则是基于全部光子由直接光子和来自强子的衰变光子组成,通过从全部光子中减除强子衰变的光子的贡献而获取直接光子信号。因目前(?)=7TeV质子—质子碰撞数据统计量的局限,很难利用统计减除方法提取到有意义的直接光子—强子关联分布,但本论文中的分析工作建立了统计减除方法在ALICE实验上的应用,当下次运行达到足够统计量时,我们可以很快采用该方法测量直接光子—强子关联分布并研究部分子碎裂函数和热密物质效应。本论文的章节安排如下：在第一章中,将简单介绍标准模型的相关内容,包括最新发现的解释质量之源的希格斯玻色子、量子色动力学,同时也给出了高能重离子碰撞中格点QCD对相变的预言和QCD相图的相关内容。而在第二章中,我们将介绍与重离子碰撞相关的物理内容,包括相对论重离子碰撞的时空演化,碰撞中软、硬过程中的粒子产生及影响。同时在这章中还总结了部分来自于SPS,RHIC和LHC能区表征重离子碰撞中形成了QGP热密物质相的特征信号。作为本论文工作的分析基础,我们将在第三章介绍ALICE实验探测器,ALICE实验数据获取与分析的在线和离线系统。同时还将讨论与本工作物理分析相关的数据分析软件框架。第四章里则阐述了两粒子关联的分析方法并总结了之前实验中测量到的强子—强子,中性π介子—强子以及直接光子—强子关联结果,这也是本论文工作物理分析的参考基础。而在接下的第五章到第七章,我们将对实验数据的选取,中性π介子—强子关联与直接光子—强子关联分析与处理过程以及测量结果进行详尽的讨论。其中在第五章中将介绍与本论文工作直接相关的数据事件、中心粒子团簇(cluster)及带电粒子径迹的选择标准。第六章中则详细讨论基于质心系能量的质子—质子和铅—铅碰撞数据的中性π介子鉴别及其触发的强子关联测量。利用孤立分析技术和统计减除方法提取质心系能量(?)=7TeV质子—质子碰撞数据中的直接光子—强子关联并测量部分子碎裂函数的分析细节将在第七章中给予详细的论述。最后第八章,我们将对本论文工作的分析方法与结果进行讨论和展望。更多还原

【Abstract】 Since a long time ago, the ultimate constituents of matter have always puzzled the mankind and been researching. It began with the theory of atomism (indestruc-tible atom) speculated by Democritus, a philosopher of ancient Greece, followed by the modern atomic theory proposed by John Dalton with the law of multiple proportions in1803. In the early20th century, the electron was discovered by J. J. Thomson through the measurement of mass to charge ratio in his explorations on the properties of cathode rays, and Ernest Rutherford theorized that atoms have their charge concentrated in a very small nucleus through his discovery and interpretation of Rutherford scattering with the gold foil experiment. With more experiments built for researching, more particles, such as proton, neutron and quark, are discovered in succession. All the discoveries let us clearly know that the atom consists of the nucleus and electrons, the nucleus is made up of protons and neutrons which are composed of fundamental particles, quarks and gluons.Currently, it is widely known that the ultimate constitutes of the matter are three generation quarks, their anti-quarks and three leptons. These fundamental particles have strong, weak and electromagnetic interactions by mediating gluons, W±and Z0bosons, and photons. A famous theory, Standard Model(SM), estab-lished to describe the electromagnetic, weak, and strong nuclear interactions and the fundamental particles, is success in explaining a wide variety of experimental results. The strong force of quarks and gluons is described by a theory of Quantum Chromodynamics (QCD). Three significant features, confinement, asymptotic free-dom and chiral symmetry restoration, reveal the main characteristics of QCD. The strong interaction increases or the coupling constant as describing the strong inter-action strength becomes larger with the momentum transfer decreasing. Therefore, the quarks and gluons are confined in the hadrons in the normal world with low mo-mentum transfers, known as confinement. The second feature is called asymptotic freedom. According to the asymptotic freedom, the coupling constant αs becomes smaller and the interaction is perturbative with the momentum transfer increas-ing (equivalently at short distances). The third characteristic associated with QCD is chiral symmetry restoration. The chiral symmetry exists as an exact symmetry only when the mass parameter of a quark is strictly zero. At low energy region, Lattice QCD is a well-established non-perturbative approach to solving the quan-tum chromodynamics theory of quarks and gluons. According to the Lattice QCD prediction, a new matter, Quark-Gluon Plasma (QGP), which consists of decon- fined quarks and gluons, is excepted to be created at extremely high temperature and/or high baryons density. More results demonstrate that the QGP matter may be created in the universe after a microsecond of the Big Bang due to the formed extremely high temperature or in the interior of the neutron stars with high baryons density. Therefore, it is great significant to search for the characteristic signatures of the QGP and research its properties for understanding the evolution and forma-tion of the early stages of the universe. However, it is impossible to extract directly the signatures of the early stages of the universe due to its long time evolution and explore the interior of the neutron stars. So how to create the QGP matter under the normal laboratory conditions is a great challenge.Ultra-relativistic heavy-ion collision experiment is considered as an available ap-proach to producing the QGP phase. In the heavy-ion collisions, two Lorentz con-tracted nuclei approach to each other with velocities nearly equal to the velocity of light and have colliding. In the colliding instant, both contracted nuclei pass through each other in the region of geometrical overlap. Many processes of parton-parton hard scatterings occur in the overlap region, which result in depositing a large amount of energy in a limit volume. The energy density is so high that a new matter state consisting of defined quarks and gluons is created. Since1960’s, a series of heavy ion accelerators, such as Alternating Gradient Synchrotron (AGS), Super Proton Synchrotron (SPS) and Relativistic Heavy Ion Collider (RHIC), have been built to search for the QGP signatures and research its properties.In European Organization for Nuclear Research (CERN), the current biggest accelerator, Large Hadron Collider (LHC), was designed at1998and run successfully at the end of2009. As one of four experiments, A Large Ion Collider Experiment (ALICE), whose aim is to study the physics of the strongly interacting matter at extreme energy densities. The centre-of-mass energy of (?)=2.76TeV of Pb+Pb collisions running in2010is about14higher than the highest energy of RHIC. Hence, it is excepted that the QGP created at LHC has longer lifetime and larger volume than at RHIC. This provides much better conditions for searching for the QGP and studying its properties. In the heavy-ion collisions, the formed QGP only exists in a short time and then fragment into a great variety of final hadrons. In this case, we can only study the QGP phase by different measurements from the final particles. Up to now, some main measurements which are considered as the signatures of the formation of the QGP are strangeness enhancement, suppression of J/Ψ production, direct photons and thermal di-leptons, jet quenching, collective flow, and so on.Two-particles correlation is considered as a powerful probe for understanding the properties of the strongly interacting hot and dense medium. In such an analysis, a particle is chosen from higher pT region and called the trigger particle, which is presumably from jet fragmentations. The so called associated particles from lower PT region are always from the other fragmentation of the jet, or another production, such as collective flow. At RHIC and LHC, the measurements of the azimuthal angle distribution from two-particle correlations in A+A collisions show a strong suppression even disappeared at the high pT and enhancement with double-peak at the low pT on the away side, and "ridge" structure in pseudo-rapidity direction at the low pT on the near side compared to pp collisions. All the measurements can be explained as the effects of the hot and dense medium, and imply the Quark-Gluon Plasma is indeed formed in the heavy-ion collisions. When the direct photon is selected as the trigger particle, the correlations probably tag the γ-jet events produced from the QCD Compton scattering process, q+g→q+γ and q+q→g+γ annihilation process. In these processes, the photons momenta in the center-of-mass frame are approximately balanced by that of the recoil partons. The photons do not occur energy loss when going through the medium due to only electromagnetic interactions happen between photons and other particles because of the large mean free path of photons. The fragments of the recoil partons have rich information, such as the parton fragmentation function with the medium effects, due to the interactions of the recoil partons and medium.In this thesis, the medium effects and the parton fragmentation function are measured by π0-hadron correlations and direct photon-hadron correlations, where the π0and photons are detected by the electromagnetic calorimeters (EMCal) and the charged hadrons are reconstructed by the central barrel detector system. In the π0-hadron correlations, the azimuthal angle distribution of the correlations and the per-trigger yield modification factor,IAA=YPbPb/Ypp, on the near side and away side are measured in pp and Pb+Pb collisions at (?)=2.76TeV. In central Pb+Pb collisions, an away side suppression from in-medium energy loss is observed (IAA≈0.6), which is from the effects of partons energy loss. Moreover, there is an enhancement above unity of (IAA≈1.2) on the near side which has not been observed with any significance at lower collision energies. The significant near side enhancement of IAA in the pT region observed shows that the near side parton is also subject to medium effects.IAA is sensitive to (ⅰ) a change of the fragmentation function,(ⅱ) a possible change of the quark/gluon jet ratio in the final state due to the different coupling to the medium, and (ⅲ) a bias on the parton pT spectrum after energy loss due to the trigger particle selection. In the direct photon-hadron correlations, both isolation technique and statistical subtraction method are used to extract, the direct photons and measure the azimmthal angle distribution of the correlations and the parton fragmentation function in pp at (?)=7TeV. The isolation technique used for the analysis is based on the physics that there is no particle or only a few particles around the leading order direct photons. The parton fragmentation function is measured and compared to the theory calculations at8.0<pTiso,γ<25.0GeV/c. The statistical subtraction method is based on the fact that all photons consist of direct photons and decay photons from hadrons decay. Since there is no enough statistics of pp collisions at (?)=7TeV, it is impossible to extract significant results with the statistical subtraction method. But the work in this thesis develops the method in ALICE data analysis, which can be used quickly for measuring the parton fragmentation function and studying the medium effects in the next running.This thesis is organized as follows:Chapter1presents the Standard Model of the particle physics including the description of the Higgs boson and the Quantum Chromodynamics, the Lattice QCD predication and the QCD phase diagram. The space-time evolution of the heavy-ion collisions and some significant measurements for searching for the QGP phase from SPS, RHIC and LHC are summarized in Chapter2. Chapter3gives an overview of the ALICE experiment and a description of the ALICE online and offline systems. The analysis framework for measuring the correlations is also presented shortly in this chapter. In Chapter4, the analysis method of two-particle correlations is introduced as well as the measurements of the correlations with the triggers as charged hadrons, neutral pions and direct photons from RHIC and LHC. From Chapter5to7, the selection criteria of the analysis data, analysis details of neutral pion-hadron correlations and direct photon-hadron correlations are discussed. Chapter5summarizes the selection criteria of data, clusters and tracks. The π0identification at EMCal and its trigger correlations are presented in Chapter6. Chapter7deals with the analysis of direct photon-hadron correlations extracted from the pp collisions at (?)=7TeV with the methods of the isolation and the statistical subtraction. At last, the discussion and outlook to the work in this thesis are addressed in Chapter8.更多还原