The Veto System of the LZ Dark Matter Experiment

Abstract

A large body of astrophysical and cosmological evidence suggests the majority of the mass density of the universe is composed of non-luminous, weakly/non interacting kind of matter, dubbed dark matter. Weakly Interacting Massive Particles (WIMPs) are a leading candidate for the make-up of dark matter, motivated by both theoretical and cosmological arguments. A dual-phase Xenon Time Projection Chamber (TPC) is employed by the LUX-ZEPLIN experiment to detect WIMP-nucleon interactions. This ultra-clean experiment benefits greatly in sensitivity from the inclusion of a gadolinium-doped liquid scintillator (GdLS) Outer Detector (OD) to veto troublesome neutron backgrounds thermal neutron captures on gadolinium nuclei.

To achieve this purpose, an array of PMTs observes scintillation light from the interaction of gamma-rays produced by these captures in the GdLS. This work presents the successful calibration of these PMTs, and a series of tests exploring the anomalous increase in PMT dark rate when exposed to water. An independent OD trigger is defined in order to boost statistics of high energy OD events facilitating future studies intending to probe the ambient neutron flux and muon events. This trigger was found to be maximally efficient for pulses of size equivalent to neutron capture signals.

The dominant source of false vetos in the OD comes from gamma-rays produced by $^{238}$U, $^{232}$Th, and $^{40}$K contaminants in the walls of the Davis cavern in which LZ is housed. The spectrum of these gammas is measured, finding that a layer of shotcrete effectively screens the underlying gamma spectrum. Thus this shotcrete layer is responsible for the measured radioactive contamination levels of 220$pm$60~Bq/kg of $^{40}$K, 29$pm$15 Bq/kg of $^{238}$U, and 13$pm$3 Bq/kg of $^{232}$Th. This measurement informs the background model and understanding of the false veto rate.

Additionally, an attempt is made to identify and measure fast neutrons via proton recoils in the OD by exploiting timing differences between proton and gamma interactions. Although discrimination was not achieved due to smearing effects from reflections, it was found that proton recoil pulses are encapsulated by the veto threshold in LZ and thus removed from the WIMP search dataset of LZ. Finally, the world-leading results of the first WIMP search run of LZ are reported.