RHIC能区D介子触发方位角关联的实验测量及各向异性流涨落的唯象研究

发布时间：2018-06-02 19:02

本文选题：重味夸克 + 方位角关联　；参考：《中国科学院研究生院(上海应用物理研究所)》2017年博士论文

【摘要】：现有宇宙理论认为宇宙“大爆炸”后的极短瞬间内形成的超高能量密度能够使得一种称为“夸克—胶子等离子体”(QGP)的物质在极短时间内产生。格点QCD理论计算预言在极高温度或者极高能量密度下,核物质能够发生强子态向夸克物质态的转变,部分子能够从禁闭的强子相中解禁出来形成夸克-胶子等离子体。为了研究这种新型物质形态,物理学家建造了相对论重离子对撞机(RHIC),试图在实验室中再现宇宙早期那种高温高密的状态。研究表明RHIC能区金核撞击产生的温度比太阳表面温度高出3亿多倍。这样的极端环境为探索强相互作用夸克-胶子等离子体和研究相变现象提供了有利条件。在相对论重离子碰撞中,重味夸克是QGP物质性质的敏感探针。重味夸克由于质量大、产生早,经历了整个源介质的演化过程,携带了大量初始时刻的特征信息。在RHIC能区,重味夸克一般通过初始硬过程成对产生。一种研究重夸克在QGP介质中能量损失机制的有效手段是测量重味夸克触发的方位角关联。理论研究表明重味关联有助于揭示重味夸克-QGP介质相互作用动力学特征,区分重夸克在介质中的能量损失机制。同时,在高能质子-质子碰撞中测量重味夸克关联则可以用于检验微扰QCD理论计算并比较重夸克喷注碎裂强子化机制与轻夸克的差别。本论文主要研究了实验测量质心系能量(?)=500 GeV下p+p碰撞中心快度区的D*介子与带电强子方位角关联和D*+-D*-方位角关联。利用STAR 2011年运行期间采集的p+p碰撞实验数据,我们分析研究了触发横动量6pT20 GeV/c区间D*介子与带电强子的方位角关联,比较了D*-强子(D*-h)和双强子(h-h)的关联信号。研究表明D*-h关联和h-h关联在near-side(0△(?)π)区间具有显著差别,主要表现为h-h关联产额系统性地高于D*-h,关联宽度小于D*-h。论文同时比较了实验测量结果和基于pQCD理论框架的PYTHIA模型模拟计算结果,发现基于pQCD理论框架的PYTHIA模型能够很好的描述实验测量结果。在此基础上,我们首次测量了p+p 500 GeV碰撞中D*+-D*-的方位角关联,并比较了PYTHIA的理论计算结果。目前在重离子碰撞中直接通过强子衰变道重建D介子并测量D介子触发关联信号具有很大挑战性。传统探测器的粒子径迹动量投影分辨率较低,在不变质量重建中组合背景的贡献极大,信号的显著度很低。为了精确测量重味夸克,STAR实验组在2014年升级运行重味径迹探测器-Heavy Flavor Tracker(HFT)。HFT是一个高分辨率的硅像素探测器,可以实现粒子径迹的高分辨率的测量,能够精确地测量次级衰变顶点的位置,显著减小组合背景贡献,极大地提高D介子测量的信噪比。HFT在2014-2016年运行期间采集了大量数据,有助于实现对重味强子总产生截面,重味夸克能量损失,重味夸克流和重味夸克触发方位角关联方面的精确测量。本论文工作还同时包括了重离子碰撞中各向异性流和初始偏心率涨落的唯象研究。利用AMPT(A Multi-Phase Transport Model)多相输运模型,我们系统地研究了质心系能量为200 GeV的金核-金核碰撞中各向异性流和各向异性流的涨落。通过研究流的涨落跟碰撞的中心度关系,横动量关系以及赝快度关系,具体地分析了流的涨落特征。我们发现椭圆流(v2)在中心碰撞情形下主要由涨落贡献,相对涨落在非中心碰撞时具有赝快度依赖较大,而在中心碰撞时较小。同时还研究了三阶流(v3)和四阶流(v4)的相对涨落,发现v3完全由涨落贡献且具有很小的中心度依赖性及横动量依赖性。进一步考虑部分子散射和强子再散射的影响,发现部分子相互作用对于流的涨落影响很大,强子散射对于流的涨落影响很小,说明流的涨落主要起源于部分子阶段。通过提取AMPT模型初始部分子阶段的信息,我们系统性地研究了质心系能量200GeV下金核-金核碰撞中的初始部分子偏心率和偏心率的涨落。部分子偏心率反映了高能核-核碰撞早期部分子空间的几何形态特征,对于碰撞系统早期演化有着重要的影响。通过系统性地比较参与部分子偏心率εn{part}和多粒子累积矩偏心率εn{m}(m=2,4,6),发现Q-cumulant偏心率略小于常规累积矩偏心率,高阶偏心率的涨落同对应阶流系数的涨落特征相似。我们同时还研究了高阶流和偏心率的比例关系,并研究了转变系数vn/εn的中心度、横动量以及赝快度依赖性,发现在中快度区转变系数vn/εn高于前后向快度区。研究横动量关系比较了低横动量强子和高横动量强子的转变效率的差别,发现在中低横动量区间转变效率随着横动量单调递增。研究初始几何涨落将有助于深入理解相对论重离子碰撞中QGP的演化图像。
[Abstract]:The existing universe theory holds that the ultra high energy density formed in the very short instant after the "Big Bang" can produce a substance called "quark gluon plasma" (QGP) in a very short period of time. The lattice QCD theory predicts that at extremely high temperature or high energy density, the nuclear matter can occur to the quark in the hadronic state. In order to study the form of the new material, the physicist built the Relativistic Heavy Ion Collider (RHIC), trying to reproduce the high temperature and high density of the early universe in the laboratory in order to study the new form of the material. The study showed that the RHIC energy zone was produced by the collision of gold nuclei. The temperature of the birth is more than 300 million times higher than the surface temperature of the sun. This extreme environment provides a favorable condition for the exploration of the strong interaction quark gluon plasma and the study of the phase transition. In the relativistic heavy ion collisions, heavy quark is a sensitive probe for the properties of QGP. Heavy quark has been produced early, and has experienced the whole source. The process of qualitative evolution carries the characteristic information of a large number of initial moments. In the RHIC energy region, heavy quarks are usually produced by the initial hard process. An effective means to study the energy loss mechanism of heavy quarks in the QGP medium is to measure the azimuth relation of heavy quark triggered. The kinetic characteristics of the interaction of the gram -QGP medium can distinguish the energy loss mechanism of heavy quark in the medium. At the same time, the measurement of the heavy quark Association in the high energy proton proton collisions can be used to test the perturbation QCD theory and compare the difference between the heavy quark shotcrete and the light quark. This paper mainly studies the experimental measurement. The D* meson of the center of mass energy (?) =500 GeV is associated with the azimuth relation of the charged hadron azimuth and the azimuth of the charged hadron in the p+p collision center. Using the p+p collision experiment data collected during the operation of STAR in 2011, we analyze and study the azimuth relation between the D* meson of the trigger transverse 6pT20 GeV/c interval and the charged hadron, and compare the D*- hadron (D*-h). ) correlation signal with double hadron (H-H). The study shows that D*-h Association and H-H association are significantly different in the near-side (0) (?) PI interval, which mainly shows that the H-H associated yield is systematically higher than D*-h, the correlation width is less than D*-h., and the experimental measurement results and the PYTHIA model simulation results based on the pQCD theoretical framework are compared. The PYTHIA model of the pQCD theoretical framework can describe the experimental results very well. On this basis, we first measure the azimuth relation of D*+-D*- in the p+p 500 GeV collision and compare the theoretical calculation results of PYTHIA. At present, the D meson is rebuilt by the hadron decay path in the heavy ion collisions and the D meson triggers the associated signal. It is very challenging. The resolution of the particle track momentum projection of the traditional detector is low, the contribution of the combined background in the constant mass reconstruction is great, the signal's saliency is very low. In order to measure the heavy flavor quark accurately, the STAR experimental group upgraded the heavy track detector -Heavy Flavor Tracker (HFT).HFT in 2014 as a high resolution. The silicon pixel detector can measure the high resolution of the particle track. It can accurately measure the position of the secondary decay vertex, significantly reduce the combined background contribution, and greatly improve the signal to noise ratio of the D meson measurement.HFT during the 2014-2016 year operation, which helps to realize the total cross section and heavy quark of the heavy hadron. Energy loss, heavy quark flow and heavy quark triggered azimuth correlation are accurately measured. This paper also includes the phenomenological study of the anisotropy flow and the initial eccentricity fluctuations in heavy ion collisions. Using AMPT (A Multi-Phase Transport Model) multiphase transport mode, we systematically studied the mass of the mass of 200 G The fluctuation of anisotropic and anisotropic flows in the gold nucleus collisions of eV. By studying the relationship between the fluctuation of the flow and the centrality of the collisions, the relation of the transverse momentum and the pseudopotential, the fluctuation characteristics of the flow are analyzed. We find that the elliptic flow (V2) is mainly contributed by the fluctuation in the central collision, and the relative fluctuation is in the non central collision. At the same time, the relative fluctuations of the three order flow (V3) and the four order flow (V4) are also studied. It is found that the V3 is fully contributed by the fluctuation and has a very small centrality dependence and transverse momentum dependence. The effect of partial scattering and hadronic scattering is further considered, and the interaction of partial subgroups is found. The fluctuation of the flow has a great influence. The scattering of hadrons has little influence on the fluctuation of the flow. It shows that the fluctuation of the flow mainly originated from the partial substage. By extracting the information of the initial partial substage of the AMPT model, we systematically studied the initial partial eccentricity and the eccentricity of the gold nucleus and gold nucleus collisions under the mass of the mass 200GeV. The molecular eccentricity reflects the geometric shape characteristics of the early part of the high energy nuclear nucleus collisions, and has an important influence on the early evolution of the collision system. By systematically comparing the partial eccentricity rate epsilon n{part} and the eccentricity of the multi particle cumulative moment of epsilon n{m} (m=2,4,6), it is found that the eccentricity of the Q-cumulant is slightly less than the conventional cumulative moment eccentricity. The fluctuation of the higher order eccentricity is similar to the fluctuation of the corresponding order flow coefficient. At the same time, we also study the ratio relation between the high order flow and the eccentricity, and study the center degree, the transverse momentum and the pseudopotential dependence of the transition coefficient vn/ e n. It is found that the transition coefficient vn/ n in the middle speed region is higher than the front and back region. The difference between the transition efficiency of the low transverse momentum and the high transverse momentum hadrons shows that the transition efficiency of the middle and low transverse momentum increases monotonically with the transverse momentum. The study of the initial geometric fluctuation will help to understand the evolution image of the QGP in the relativistic heavy ion collisions.
【学位授予单位】：中国科学院研究生院(上海应用物理研究所)
【学位级别】：博士
【学位授予年份】：2017
【分类号】：O572.33;O572.214

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