【作者】 张一飞；

【导师】 张子平； 许怒；

【作者基本信息】 中国科学技术大学 ， 核与粒子物理， 2007， 博士

【摘要】 自然界四大基本作用力之一的强相互作用具有禁闭特性：夸克禁闭在强子中，实验尚未观测到自由夸克。量子色动力学（QCD）被认为是一种正确的用来描述强相互作用的基本规范场理论。格点QCD计算预言在高能核核碰撞中可能会产生从强子气到一种新物质：夸克—胶子等离子体（QGP）的相变。在这种新的物质形态下，夸克会解禁闭从而达到局部热平衡状态。寻找QCp物质并研究其特性是重离子对撞实验最终目标之一。位于美国布鲁克海汶国家实验室（BNL）的相对论重离子对撞机（RHIC）通过质心能量高达200 GeV的核核碰撞来提供用于产生和寻找QGP物质的高能量密度的环境。近期在RHIC运行的实验研究表明200 GeV金金碰撞产生的核物质具有相当强的集体运动效应和对末态强子的不透明性。主要表现在低横动量区域的流体动力学行为和高横动量区域的粒子产额压低。粲夸克被认为是研究RHIC能区相对论重离子碰撞中产生的部分子物质的理想探针。由于它的质量比较大，约1.3 GeV／c²，理论预言粲夸克通过胶子辐射损失的能量要比轻夸克少。因此测量粲夸克强子半轻子衰变的单电子的核修正因子，并与轻强子作比较，对于构造完整的喷柱淬灭的物理图像以及帮助我们理解RHIC能区金金碰撞中部分子能损机制都有着非常重要的意义。此外，粲夸克和碰撞产生的物质之间的相互作用可能会形成粲夸克的径向流和椭圆流，从而改变粲夸克横动量谱的分布形状。由于粲夸克的质量较大，在与周围的高密度物质经过足够多的相互作用后，可能会获得集体运动流效应。实验上测量粲夸克强子的流效应和冻结温度等特性，对检验早期重离子碰撞中轻味夸克热化和部分子密度具有相当重要的意义。粲夸克被认为通过碰撞初始的胶子聚合产生，其产生截面可以用微扰QCD理论计算。因此研究不同碰撞系统下粲夸克产生截面及其对于两两碰撞数的标度不变特性对于检验理论和检验粲夸克是否是高能重离子碰撞中产生的部分子物质的理想探针都非常重要。粲夸克产生截面的测量是电子测量中分离底夸克的贡献，以及对试图解释RHIC和SPS观测到的J／Ψ压低现象的理论模型的不可或缺的重要依据。这篇论文里，我们将介绍低横动量区间（p_T≤2 GeV／c）D⁰→Kπ强子衰变道和粲夸克半轻子衰变的单电子谱（0.9≤p_T≤5 GeV／c）。另外，我们还提出一种新的方法来鉴别低横动量区间粲夸克半轻子衰变的μ子。联合以上三种独立的测量，可以较为精确的给出RHIC能区金金碰撞中中心快度区的粲夸克产生截面，并且可以很好的描述粲夸克强子的横动量谱形状，以及研究可能的粲夸克径向流。在最小偏置金金碰撞中，通过D⁰→Kπ强子衰变道（分支比为3.83％）来重建D⁰介子的质量峰。在K，π不变质量峰减除了混和事例背景之后，用线性或二次函数加上一个高斯函数来拟合D⁰介子的质量峰，从而得到D⁰介子的产额。在最小偏置金金碰撞中和中心金金碰撞中，通过联合STAR探测器上时间投影室（TPC）测得的电离能损（dE／dx）和飞行时间谱仪（TOF）测得的粒子质量谱来鉴别低横动量区间（0.17≤p_T≤0.25 GeV／c）的μ子。在μ子质量谱中，减除了π的污染。主要的背景是来自K，π弱衰变的μ子，通过背景μ子和粲夸克衰变的单μ子相对于初始对撞顶点的最接近距离（DCA）分布的不同来去除背景。通过联合TPC测得的电离能损和TOF测得的粒子速度（β），电子可以被鉴别到p_T=5 GeV／c。电磁过程产生的电子本底通过重建标记电子（正电子）和所有伴随正电子（电子）的不变质量来在统计上减除。为了减少随机组合造成的背景，我们采用了二维重建不变质量的方法。伴随径迹的重建效率从STAR探测器模拟和蒙特卡罗样本分析中得到。这种方法可以减除约95％的电磁过程本底（光子转换和中性π介子达利兹衰变）。于是可以得到中心金金碰撞，最小偏置金金碰撞及其他中心度下的非电磁产生的电子横动量谱。另外，在本论文里我们也将讨论获取非电磁产生的电子的椭圆流（v₂）的方法。联合以上三个独立的测量：RHIC能区金金碰撞中的D⁰→Kπ强子衰变道，粲夸克衰变的单μ子和单电子谱，我们观测到中心金金碰撞中的非电磁产生的电子横动量谱相对于氘金碰撞在横动量为0.9≤p_T≤5 GeV／c区间有很强的压低。在电子横动量约大于2 GeV／c区间，其相应的粲夸克强子横动量约大于4 GeV／c，电子谱的压低程度与轻夸克重子和介子相似。基于blast wave模型，拟合横动量在2 GeV／c以下的电子谱给出的结果说明粲夸克强子与介质的相互作用和耦合可能不同于多奇异夸克强子和轻夸克强子。将来探测器升级后对粲夸克强子的直接重建将会给出更准确的答案。联合这三种独立的测量，可以可靠的给出粲夸克中心快度区的产生截面。这种方法覆盖了约90％的粲夸克运动学区间。200 GeV中心金金碰撞中平均每核子对碰撞的粲夸克总产生截面的测量结果为：1.40±0.11（stat．）±0.39（sys．）mb。200 GeV最小偏置金金碰撞中平均每核子对碰撞的粲夸克总产生截面的测量结果为：1.29±0.12±0.36 mb。测量结果表明粲夸克产生截面相对于两两碰撞数具有标度不变性。这说明硬过程对于初始核子中的两两相互作用具有标度性。而且证明粲夸克可以用来作为研究碰撞早期动力学机制的探针。在以上的测量中，我们忽略了非电磁产生的电子谱里来自底夸克衰变的贡献。目前在实验得到的非电磁产生的电子谱中分离底夸克和粲夸克的贡献是相当困难的。理论模型预言的高能核碰撞中粲夸克和底夸克的产生的不确定性也很大。所以要更好的理解粲夸克物理，分离非电磁产生的电子谱中底夸克的贡献就相当的重要。在本论文的讨论部分，我们将介绍通过拟合非电磁产生的电子谱来估计底夸克的贡献。并且我们把模拟的电磁产生的电子椭圆流v₂与实验数据比较来估计可能的粲介子椭圆流v₂。更多还原

【Abstract】 The strong interaction, one of the four fundamental forces of nature, is confining: there is no single quark as a color-triplet state observed experimentally. Quantum Chromody-namics （QCD） is believed to be a correct basic gauge field theory of strong interactions. Lattice QCD calculations predict a phase transition from hadronic gas to a new matter, Quark- Gluon Plasma （QGP）, at high energy nuclear-nuclear collisions. In this new form of matter, quarks are deconfined and approach local thermalization. One of the ultimate goals of the heavy ion collision experiments is to search for the QGP matter and study its properties. The Relativistic Heavy Ion Collider （RHIC） located at Brookhaven National Laboratory （BNL） provides a high energy density environment to create and search for the QGP matter by colliding ions like Au at energies up to (S_NN)^1/2=200 GeV. Recent experimental studies at RHIC have given strong evidences that the nuclear matter created in Au+Au collisions at(S_NN)^1/2=200 GeV has surprisingly large collectivity and opacity as reflected by its hydrodynamic behavior at low p_T and its particle suppression behavior at high p_T.Charm quarks provide a unique tool to probe the partonic matter created in relativistic heavy-ion collisions at RHIC energies. Due to their large quark mass （（?）1.3 GeV/c²）, charm quarks are predicted to lose less energy than light quarks via only gluon radiation. A measurement of the nuclear modification factor for the charmed hadrons semileptonic decayed single electrons compared to light hadrons is valuably important to complete the picture of the observed jet-quenching phenomenon and help us better understand the energy-loss mechanisms at parton stage in Au+Au collisions at RHIC.Furthermore, the interactions between charm quarks and the medium could boost the radial and elliptic flow resulting in a different charm p_T spectrum shape. Due to its large mass, a charm quark could acquire flow from the sufficient interactions with the surrounding partons in the dense medium. The measurement of charm flow and freeze-out properties is vital to test light flavor thermalization and the partonic density in the early stage of heavy ion collisions.Charm quarks are believed to be produced only at early stages via initial gluon fusions and its cross section can be evaluated by perturbative QCD calculations. Thus study of the binary collision (N_bin) scaling properties for the total charm cross-section from different collision systems can test the theoretical assumptions and determine if charm quarks are indeed good probes to the partonic matter created in high energy heavy ion collisions. Charm total cross-section measurement is also essential for the separation of bottom contribution in non-photonic electron measurement, and for the model calculations, which tries to explain the observed similar suppression pattern of J/ψat RHIC and SPS.In this thesis, we present the measurements of D⁰→Kπat low p_T （p_T≤2 GeV/c） and non-photonic electron spectra （0.9≤P_T≤5 GeV/c） from D⁰ semi-leptonic decay. In addition, we use a newly proposed technique to identify muons from charm decays at low p_T The combination of all these three measurements stringently constrains the total charm production cross-section at mid-rapidity at RHIC. They also allow the extraction of the charmed hadron spectral shape and a study of possible charm radial flow in Au+Au collisions.D⁰ mesons were reconstructed from hadronic decay D⁰→K^-π⁺ （（D|-）⁰→K⁺π^-） with a branching ratio of 3.83% in minbias Au+Au collisions. The D⁰ yields were obtained from fitting a Gaussian plus a linear （or a second-order polynomial function） for residual background to the invariant mass distributions of kaon-pion pairs after mixed-event combinatorial background subtraction. The inclusive muons at 0.17≤p_T≤0.25 GeV/c in minbias Au+Au and central Au+Au collisions were analyzed by combining the ionization energy loss （dE/dx） measured in the Time Projection Chamber （TPC） and the mass calculated from the Time Of Flight （TOF） detector at STAR after the residual pion contamination subtracted. The dominant background muons from pion/kaon weak decays were statistically subtracted using the distribution of the distance of closest approach to the collision vertex （DCA）.Inclusive electrons up to p_T= 5 GeV/c are identified by using a combination of velocity （β） measurements from the TOF detector and dE/dx measured in the TPC. Photonic background electrons are subtracted statistically by reconstructing the invariant mass of the tagged e^±and every other partner candidate e^? using a 2-dimensional invariant mass method. Partner track finding efficiency is estimated from STAR Geant + Monte Carlo embedding data. More than～95% of photonic background （photon conversion andπ⁰ Dalitz decay） can be subtracted through this method. The p_T spectra for non-photonic electrons in central Au+Au minbias Au+Au collisions and its subdivided centralities will be presented. In addition, the method to extract the elliptic flow v₂ of non-photonic electron is also developed in the thesis.By combining these three independent measurements: D→Kπ, muons and electrons from charm semileptonic decays in minbias and central Au+Au collisions at RHIC, we observed that: The transverse momentum spectra from non-photonic electrons are strongly suppressed at 0.9≤P_T≤5 GeV/c in central Au+Au collisions relative to d+Au collisions. For electrons with p_T （?） 2 GeV/c, corresponding to charmed hadrons with P_T （?） 4 GeV/c, the suppression is similar to that of light baryons and mesons. The blast wave fit to the electron spectra with p_T （?） 2 GeV/c indicates that charmed hadrons may interact and decouple from the system differently from multi-strange hadrons and light hadrons. Future upgrades with a direct reconstruction of charmed hadrons are crucial for more quantitative answers. Charm differential cross-sections at mid-rapidity (dσ_cc|-^NN)/dy) are extracted from a combination of the three measurements covering～90% of the kinematics. The total charm cross-section per nucleon-nucleon collision (σ_cc|-^NN)) is reported as 1.40±0.11（stat.）±0.39（sys.） mb in 0-12% central Au+Au and 1.29±0.12±0.36 mb in minbias Au+Au collisions at (s_NN)^1/2=200 GeV. The charm production cross sections are found to follow the number of binary collisions scaling. This supports the assumption that hard processes scale with binary interactions among initial nucleons and charm quarks can be used as a probe sensitive to the early dynamical stage of the system.In the above measurements, the bottom contribution to the non-photonic electron spectrum is neglected. The separation of bottom and charm contributions in current non-photonic electron measurements is very difficult. There are large uncertainties in the model predictions for charm and bottom production in high-energy nuclear collisions. Thus identification of bottom from the non-photonic electron measurements is crucial to better understand charm physics. In the discussion section of this thesis, we will try a fit to non-photonic electron spectrum and estimate the bottom contributions. We will also compare the v₂ distribution from simulation to the experimental data and estimate the possible charm v₂.更多还原