Low Energy Nuclear Recoil Band Tuning in the LUX-ZEPLIN Experiment

Centuries of astrophysical observations have indicated the presence of an in-
visible, weakly interacting matter known as dark matter, with the most popular candidate being the Weakly Interacting Massive Particle (WIMP).

This thesis focuses on calibrations aspect of the underground liquid xenon
experiment, LUX-ZEPLIN. LZ uses a dual-phase time projection chamber (TPC) to observe a WIMP-nucleon scattering signal, with its second science result released in
Aug. 2024. Accurate discrimination between electronic recoil (ER) and nuclear recoil (NR) signals at low energies is vital for improving dark matter detector sensitivities.

The main focus of this thesis is on LZ’s lowest energy neutron source, Yttrium
Beryllium (YBe), a photoneutron source that was first deployed after the first science result. Obtaining a clean selection of the YBe relied on removing an unexpected population of above-anode gas events obscuring the NR region. These events occur above the anode that drift down and create light in the high-field region. This selection work resulted in obtaining a sample of 231 YBe events, fitting the 1σ and
2σ contours from YBe simulations.

Using this selection data, a 6-parameter fit was conducted for the NR model
in LZ called Noble Element Scintillation Technique (NEST), using Markov Chain
Monte Carlo. This is a novel high dimensional fit for tuning the low energy NR region using the lowest energy calibration source in LZ. The fitting process impacted the lower mass WIMPs significantly, quantified through assessing the impact on the number of generated WIMPs from the default NEST models and the fitted models.
Using three different simulated mass WIMPs (i.e. 6 GeV, 10 GeV and 100 GeV)
and comparing both simulated models showed a 12% reduction in the number
of generated 6 GeV WIMP events, a 5% reduction for 10 GeV WIMP events but
minimal impact on 100 GeV events.

These results obtained higher uncertainties than those from the default model, leading to questions surrounding whether there were sufficient statistics to constrain
the model for a 6-dimensional fit. A parallel tuning analysis was conducted using a larger simulated YBe dataset, to investigate the uncertainties, showing a similar reduction in generated events and a smaller uncertainty to the previous tuning, yet still higher than the default NEST.

Both tuning analyses impact the intended low energy and mass WIMP region most, with indication of potential overestimation from the currently used NEST model parameters. Yet the higher uncertainties may suggest that the data or model
might not have sufficiently constrained the parameters requiring the need for further statistics and research.