LHC RUN-I对SUSY的限制以及ATLAS Z-PEAK超出迹象的研究

发布时间：2018-06-06 18:09

本文选题：大型强子对撞机 + 超对称模型　；参考：《河南师范大学》2016年博士论文

【摘要】：在粒子物理学中标准模型是迄今为止最成功的模型,许多实验观测数据都与它的理论预言相吻合。并且在历史上,通过蕴含在其中的对称性预言的顶夸克、希格斯粒子等也相继被实验发现。但是,标准模型并不是终极理论,它自身仍然存在着一些问题。例如,126GeV附近类标准模型希格斯粒子的发现,完善了标准模型的理论框架,解释了质量起源：通过希格斯场自发对称性破缺使得粒子获得质量。但是,随之而来的问题是,标准模型自身不能解释为什么这个粒子的质量在126GeV附近。除此之外,标准模型不能把几种基本的相互作用统一到一起,不能为占据宇宙组分约21%的暗物质提供合适的候选者。为了解决上述问题,物理学家提出了各种各样的新物理模型,超对称模型(SUSY)是目前被研究最为广泛的模型。物理学家迫切地希望通过实验可以验证超对称的存在,即观测到超对称粒子。自从大型正负电子对撞机LEP开始运行,物理学家开始寻找超对称粒子。但是目前实验上没有观测到超对称粒子,于是给超对称模型带来了实验限制,比如LEP的实验结果要求Chargino粒子的质量必须大于O(100GeV)。随着2010年大型强子对撞机LHC成功运行、取数,人们期望能够通过数年的运行找到新物理。在LHC运行的第一期阶段(LHC Run-I)已经有重大成果,即发现质量在126GeV附近的类标准模型希格斯粒子。但是,除此之外并没有发现任何超出标准模型的物理存在。实验上没有发现超粒子,给超对称理论带来了很强的限制。LHC Run-I实验结果已经把胶微子Gluino的质量排除到了O(1TeV),Stop和Sbottom的质量排除到了O(600GeV),电弱超对称粒子的质量排除到了O(300GeV)。在大型强子对撞机LHC上寻找超对称粒子信号的常用动力学变量包括HT和meff'它们用于区分超对称信号和标准模型背景。其中,HT指的是所有观测到的粒子的横向动量的标量和,meff=HT+ETmiss,ETmiss代表丢失能量。HT通常表示末态母粒子的质量,变量meff对于超对称信号主要分布在超对称标度,而对于标准模型背景则分布在较低的标度,所以可以利用HT和meff区分超对称信号和标准模型背景。除此之外,可以要求信号中包含轻子或者光子,进一步压低标准模型多jets背景。在过去几年,一些复合型变量被提出和发展,它们与重超对称粒子(质量在1TeV附近)衰变过程的运动学息息相关,增加了对重超对称粒子产生过程的敏感性,进一步压低了标准模型多jets背景,这些变量包括αT,razor,mT2,mT2W和mCT等。尽管LHC Run-Ⅰ阶段没有发现新粒子(指新物理信号超出标准模型背景在5σ或以上),但是仍然发现了一些新物理的迹象存在(指新物理信号超出标准模型背景在3σ左右),比如ATLAS实验组在Z玻色子质量附近发现的超出迹象。对此,我们利用次最小超对称模型,通过前两代Squark对的产生解释了ATLAS实验组观测到的Z-peak超出迹象,要求Squark的级联衰变q→qX2-0→qZX1-0中分支比分别接近100%。接近100%的分支比要求x10以singlino为主,x20以bino为主。除此之外要求质量谱比较紧致,从而满足LHC Run-Ⅰ阶段其他实验结果。目前LHC Run-Ⅱ正在运行,它的质心能已经达到13TeV,于是超对称粒子的产生截面会被大幅抬高。因此,即使在运行初期积分亮度不太高的情况下,也会有比LHC Run-Ⅰ更显著的现象出现,我们期待着在LHC Run-Ⅱ阶段有令人振奋的结果出现。
[Abstract]:In particle physics, the standard model is the most successful model so far. Many experimental observations coincide with its theoretical predictions. And in history, Higgs particles, such as the top quark contained in the symmetry prophecy in it, have been experimentally discovered. However, the standard model is not the ultimate theory, and it still remains. In some cases, for example, the discovery of the standard model Higgs particles near 126GeV has perfected the theoretical framework of the standard model and explained the origin of the mass: the mass of the particles through the spontaneous symmetry breaking of the Higgs field. However, the following problem is that the standard model itself cannot explain why the mass of the particle is 1 In addition to this, the standard model can not integrate several basic interactions together, and can not provide a suitable candidate for the dark matter occupying about 21% of the universe. In order to solve the problem, physicists have proposed a variety of new physical models, and the supersymmetric model (SUSY) is the most widely studied model at present. Physicists are eager to verify the existence of supersymmetry through experiments, that is, supersymmetric particles. Since the large positive and negative electron Collider LEP started running, physicists began to look for supersymmetric particles. But there is no observation of supersymmetric particles in the experiment, so the supersymmetric model is limited, such as LEP. The experimental results require that the mass of Chargino particles must be greater than O (100GeV). With the successful operation of the Large Hadron Collider LHC in 2010, people expect to be able to find new physics through several years' operation. In the first phase of LHC operation (LHC Run-I), there have been significant results, that is, the quality of the class standard model Higgs particles near 126GeV is found. But, in addition, there is no physical existence beyond the standard model. No superparticles have been found in the experiment. The supersymmetry theory is strongly restricted by the.LHC Run-I experiment. The mass of the colloid Gluino is excluded to O (1TeV), the mass of Stop and Sbottom is excluded to O (600GeV), and the quality of the electrically weak supersymmetric particles The quantity is excluded to O (300GeV). The common dynamic variables for finding supersymmetric particle signals on the Large Hadron Collider LHC include HT and meff'which are used to distinguish the supersymmetric signal and the standard model background. In this, HT refers to the scalar of the transverse momentum of all observed particles and meff=HT+ETmiss, ETmiss represents the loss of energy.HT usually table. The mass of the last state particle, the variable MEFF is mainly distributed in the supersymmetric scale for the supersymmetric signal, and the standard model background is distributed at a lower scale. So the supersymmetric signal and the standard model background can be distinguished by HT and MEFF. In addition, the lepton or photon can be included in the signal to further reduce the standard mode. Type multi jets background. In the past few years, some complex variables have been proposed and developed. They are closely related to the kinematics of heavy supersymmetric particles (mass near 1TeV) decay process, increasing the sensitivity to the production process of heavy hyper symmetric particles and further lowering the multi jets background of the standard model, such as alpha T, razor, mT2, mT2W and mCT. While the LHC Run- I phase did not find new particles (the new physical signals exceeded the standard model in the background of 5 Sigma or above), some new physical signs were still found (referring to the new physical signals beyond the standard model background at about 3 sigma), such as the signs that the ATLAS experimental group was found near the mass of the Z boson. Using the sub minimum supersymmetric model, the first two generation of Squark pairs explained the Z-peak beyond the signs observed by the ATLAS experimental group. It requires that the branch ratio of the cascade decay of Squark, Q to qX2-0, qZX1-0, which is close to 100%. close to 100%, is mainly singlino and X20 is BINO. To meet the other experimental results of the LHC Run- I phase. Now LHC Run- II is running, its centroid can reach 13TeV, so the cross section of supersymmetric particles will be greatly raised. So, even if the integral brightness is not very high at the initial stage of operation, there will be a more significant phenomenon than LHC Run- I, we expect to be in LHC Run-. The stage II stage has an exciting result.
【学位授予单位】：河南师范大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：O572.2

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