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

Lead Research Organisation: University of Liverpool
Department Name: Physics

Abstract

It is well understood that the abundance of visible matter in the universe is not sufficient
to explain observed cosmological phenomena. Examples of this include: the rotational
velocity distribution of matter within a galaxy, interactions between galaxy clusters, and
gravitational lensing where there is insufficient luminous matter to do so. It has been postulated
that there is matter within the universe that does not interact via the electromagnetic
force 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 % baryonic
matter, 26 % dark matter, and roughly 69% dark energy [1]. Therefore, over 80% of the
matter in the universe does not interact via the electromagnetic force. Models of physics
beyond the standard model, such as supersymmetry, predict massive particles that only interact
via forces weaker than electromagnetism [2]. Collectively these particles are referred
to as WIMP (weakly interacting massive particle) like. It is therefore predicted that WIMP
particles can interact with atomic nuclei, resulting in a nuclear recoil. This signal should be
directly detectable by a sufficiently sensitive detector [3].
The direct detection of dark matter on earth depends on the local properties of the Milky
Way's dark matter halo. These properties include the dark matter mass density, which is
directly related to the WIMP-nucleon cross section. The field of direct dark matter detection
is wide and highly competitive, but detectors such as LUX and Xenon100 which measure the
WIMP xenon cross-section are of particular relevance to this PhD project. These detectors
consist of a liquid xenon time projection chamber (TPC) used to measure the intensity of
S1 and S2 signals. In a dual-phase xenon TPC, S1 refers to the light produced by prompt
scintillation after an interaction and S2 refers to the electroluminescence produced by drift
electrons during a phase transition. The ratio of these two signals can be used to distinguish
between unwanted background and genuine WIMP interactions.
The University of Liverpool is a collaborator in the LUX-Zeplin (LZ) experiment which
is due to be fully operational by 2021. Currently under construction, it is the largest ever
liquid xenon TPC, designed to operate with a fully active 7 tonnes of liquid xenon [3]. This
large increase in fiducial volume will give LZ the ability to detect interactions with crosssections
2-3 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 120
Hamamatsu R5912 photomultiplier tubes and act as a veto [3]. The first layer consists of
17.5 tonnes of gadolinium-loaded liquid scintillator, the second consists of 228 tonnes of
water. The liquid scintillator is doped with gadolinium because it has a high neutron capture
cross-section. Neutrons interacting in the TPC have a WIMP like signature and it is
therefore essential that they are successfully vetoed. The water surrounding the scintillator
acts as an effective shielding material by thermalising neutrons and absorbing background
radiation from the surrounding rock. It can also act as a cherenkov detector for high energy
background 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 Calibration
System. This system has been specified to produce a known quantity and wavelength
of optical photons in a well documented spectrum. This will allow the group to continuously
calibrate the 120 PMTs surrounding the OD and verify the system's stability. It also
allows for the validation of the optical model of the materials and geometry of the OD by
comparison to simulated events. My hardware responsibility during my PhD project will

Publications

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Studentship Projects

Project Reference Relationship Related To Start End Student Name
ST/S505547/1 01/10/2018 30/09/2022
2113040 Studentship ST/S505547/1 01/10/2018 31/03/2022 Ewan Fraser