The LUX-ZEPLIN (LZ) Dark Matter Search

Direct dark matter experiments search for the extremely rare and very faint collisions expected to occur between the cold dark matter particles that are believed to permeate our galaxy and the nuclei of atoms in the active medium of a radiation detector. Weakly Interactive Massive Particles, or WIMPs, are hypothetical elementary particles weighing as little as a few protons or as much as several heavy nuclei; besides gravity, they would interact by the weak nuclear force, thereby providing a mechanics for their possible detection. The motivation to search for WIMP dark matter is strong, with theories that attempt to explain what lies beyond the Standard Model of Particle Physics providing very good candidates.

The LUX-ZEPLIN (LZ) experiment will deploy a 7-tonne liquid xenon WIMP detector at the 4850-foot level of the Sanford Underground Research Facility (SURF) after a 3-year construction phase. In this most quiet of environments, shielded from cosmic rays and local backgrounds, LZ will then operate for at least 3 years to hopefully find a handful of tell-tale signs of WIMP interactions. LZ will be so sensitive that it will be limited only by backgrounds from astrophysical neutrinos, which provide a fundamental limit to these searches. However, before neutrinos set in, this experiment has the potential to finally shed light on the nature of dark matter.

This project will address the fundamental nature of the dark matter in the universe, of great importance to particle physics, astrophysics and cosmology. Our science output will directly impact these fields by constraining models of physics beyond the Standard Model and structure formation and evolution in the universe.

The technologies we are developing find wider and immediate benefit to other areas of physics within the STFC remit, notably long baseline neutrino oscillation experiments and neutrino-less double beta decay (0vBB) searches. Construction and operation of a multi-tonne noble gas time projection chamber impacts the UK and international neutrino physics communities seeking to develop detectors based on similar principles and technology. Challenges such as high voltage delivery, light collection, sensor readout, purification and electron transport, and long-term stability are all paralleled for proposed experiments such as LBNF, which could also be located at SURF, and with significant UK involvement. Similarly, LZ will develop and employ leading ultra-low background techniques to construct an instrument capable of detecting the rarest particle interactions. Experiments searching for rare 0vBB decay have similar requirements on stringent radio-purity. The UK 0vBB community and their international partners will benefit from our material screening instrument development and radiation detection techniques.

Our outputs developing low-background material screening technology will find further application in a diverse range of applications outside of physics. Mass-spectrometry instrumentation capable of detecting trace contaminants at parts-per-trillion levels in small samples provides unprecedented sensitivity for food safety, pharmaceutical, environmental, forensic and clinical research. Commercial service providers are limited in their capability due to regular exposure to high concentrations of impurities in their samples. We will validate our system against food safety and pharmaceutical standards from mid-2017. Demonstration of successful screening for toxicity in spiked samples, beginning with titanium dioxide nanoparticles, has the potential for improving safety standards and impacting UK health.

We will also investigate feasibility of measuring impact of uranium and thorium in silicon electronics chips from 2017. The alpha decays from U and Th progeny cause 'Single Event Effects' resulting in faults in integrated circuits; a problem that is increasingly significant as chips become smaller, and an area of active research. Coupled to our underground ultra-low background gamma spectroscopy capability, we can uniquely provide measures of the full U and Th chains, at sub-mBq/kg sensitivity, accounting for the complete alpha particle emission rates. Pre-screening Si and other construction materials with both mass spectrometry and gamma spectroscopy will allow manufacturers to increase reliability of their components and foster greater economic competitiveness of UK industry.

Our research provides training for students and staff in a wide range of specialist skills that are readily transferrable to commercial, financial and industrial sectors. Radiation detection technology and techniques are applicable to medical imaging and defence, particularly screening for special nuclear material. Data processing, analysis, statistical methods, programming, and Monte Carlo simulations are regularly employed in the banking and investment sectors. Construction of vacuum systems, gas recirculation and purification plants, photosensors and many other systems in LZ provide valuable engineering skills, as does project management training.

Finally, given the readily accessible nature of the topic of dark matter and deep underground operation, our research has always captured the public's imagination, providing significant outreach and engagement opportunity for dissemination of knowledge.