Dark matter searches with LZ detector and their interpretation using Effective Field theory

众所周知，宇宙中可见物质的丰富程度不足以解释观测到的宇宙现象。这方面的例子包括：星系内物质的旋转速度分布，星系团之间的相互作用，以及没有足够发光物质的引力透镜效应。据推测，宇宙中存在不通过电磁力相互作用的物质，因此是“透明的”。这被统称为暗物质。目前的天体物理模型计算出宇宙中的能量为5%重子物质，26%暗物质，大约69%暗能量[1]。因此，宇宙中超过80%的物质不会通过电磁力相互作用。超出标准模型的物理模型，如超对称模型，预测大质量粒子只通过弱于电磁力的力相互作用。这些粒子统称为WIMP（弱相互作用大质量粒子）。因此，预测wimp粒子可以与原子核相互作用，导致核后坐力。这种信号应该能被足够灵敏的探测器[3]直接探测到。对地球上暗物质的直接探测依赖于银河系暗物质晕的局部特性。这些特性包括暗物质的质量密度，它与wimp -核子的横截面直接相关。直接探测暗物质的领域很广，竞争也很激烈，但是像LUX和Xenon100这样测量wimp氙横截面的探测器与这个博士项目特别相关。这些探测器由一个液态氙时间投影室（TPC）组成，用于测量s1和S2信号的强度。在双相氙气TPC中，S1是指相互作用后由提示闪烁产生的光，S2是指相变过程中漂移电子产生的电致发光。这两个信号的比值可以用来区分不需要的背景和真正的WIMP相互作用。利物浦大学是LUX-Zeplin （LZ）实验的合作者，该实验将于2021年全面投入使用。目前正在建设中，它是有史以来最大的液态氙TPC，设计为7吨液态氙[3]。基准体积的大幅增加将使LZ能够检测比LUX低2-3个数量级的横截面的相互作用。利物浦的LZ小组都是外部探测器物理小组的成员。LZ OD将围绕TPC形成两层，这两层由120个hamamatsu R5912光电倍增管同时观测，并充当否决[3]。第一层由17.5吨装载钆的液体闪烁体组成，第二层由228吨水组成。由于液体闪烁体具有较高的中子俘获截面，因此掺加了钆。在TPC中相互作用的中子具有类似WIMP的特征，因此它们被成功地否决是至关重要的。闪烁体周围的水通过热化中子和吸收周围岩石的背景辐射，起到了有效的屏蔽材料的作用。它还可以充当切伦科夫探测器，探测高能背景，如宇宙射线介子及其相互作用产生的子粒子。利物浦集团的主要职责是生产光学校准系统。该系统已被指定产生已知数量和波长的光子在一个良好的记录光谱。这将使团队能够持续校准外径周围的120个pmt，并验证系统的稳定性。它还允许通过与模拟事件的比较来验证材料的光学模型和外径的几何形状。我在博士项目中的硬件责任将

It is well understood that the abundance of visible matter in the universe is not sufficientto explain observed cosmological phenomena. Examples of this include: the rotationalvelocity distribution of matter within a galaxy, interactions between galaxy clusters, andgravitational lensing where there is insufficient luminous matter to do so. It has been postulatedthat there is matter within the universe that does not interact via the electromagneticforce and as a result is 'transparent'. This is collectively referred to as dark matter.Current astrophysical models calculate the energy in the universe to be 5 % baryonicmatter, 26 % dark matter, and roughly 69% dark energy [1]. Therefore, over 80% of thematter in the universe does not interact via the electromagnetic force. Models of physicsbeyond the standard model, such as supersymmetry, predict massive particles that only interactvia forces weaker than electromagnetism [2]. Collectively these particles are referredto as WIMP (weakly interacting massive particle) like. It is therefore predicted that WIMPparticles can interact with atomic nuclei, resulting in a nuclear recoil. This signal should bedirectly detectable by a sufficiently sensitive detector [3].The direct detection of dark matter on earth depends on the local properties of the MilkyWay's dark matter halo. These properties include the dark matter mass density, which isdirectly related to the WIMP-nucleon cross section. The field of direct dark matter detectionis wide and highly competitive, but detectors such as LUX and Xenon100 which measure theWIMP xenon cross-section are of particular relevance to this PhD project. These detectorsconsist of a liquid xenon time projection chamber (TPC) used to measure the intensity ofS1 and S2 signals. In a dual-phase xenon TPC, S1 refers to the light produced by promptscintillation after an interaction and S2 refers to the electroluminescence produced by driftelectrons during a phase transition. The ratio of these two signals can be used to distinguishbetween unwanted background and genuine WIMP interactions.The University of Liverpool is a collaborator in the LUX-Zeplin (LZ) experiment whichis due to be fully operational by 2021. Currently under construction, it is the largest everliquid xenon TPC, designed to operate with a fully active 7 tonnes of liquid xenon [3]. Thislarge increase in fiducial volume will give LZ the ability to detect interactions with crosssections2-3 orders of magnitude lower than LUX.The LZ group at Liverpool are all members of the Outer Detector (OD) physics group.The LZ OD will surround the TPC in 2 layers, these are simultaneously observed by 120Hamamatsu R5912 photomultiplier tubes and act as a veto [3]. The first layer consists of17.5 tonnes of gadolinium-loaded liquid scintillator, the second consists of 228 tonnes ofwater. The liquid scintillator is doped with gadolinium because it has a high neutron capturecross-section. Neutrons interacting in the TPC have a WIMP like signature and it istherefore essential that they are successfully vetoed. The water surrounding the scintillatoracts as an effective shielding material by thermalising neutrons and absorbing backgroundradiation from the surrounding rock. It can also act as a cherenkov detector for high energybackground such as cosmic ray muons and the daughter particles produced by their interactions.The primary responsibility of the Liverpool group is the production of an Optical CalibrationSystem. This system has been specified to produce a known quantity and wavelengthof optical photons in a well documented spectrum. This will allow the group to continuouslycalibrate the 120 PMTs surrounding the OD and verify the system's stability. It alsoallows for the validation of the optical model of the materials and geometry of the OD bycomparison to simulated events. My hardware responsibility during my PhD project will