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

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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