XENON FUTURES: R&D for a Global Rare Event Observatory - Phase 1

Astrophysical and cosmological evidence has established the Standard Model of Cosmology which suggests that less than 5% of the universe is made of matter that we believe we understand, with the remaining >95% shared between dark matter and dark energy. Dark matter is suggested to contribute ~27% to the total energy density of the universe.

Existing dark matter detectors, including those under construction in 2019 such as LUX-ZEPLIN (LZ), are primarily geared towards detecting the collisions of WIMPs (Weakly Interacting Massive Particles) with ordinary nuclei - contained in a liquid xenon target in the specific case of LZ. With LZ entering its data-taking and exploitation phase during 2020, it is timely to start research and development for future liquid xenon detectors now.

This 18-month project delivers the first phase of a wider R&D programme for developing and preparing UK contributions for a future liquid xenon Generation 3 (G3) Global Rare Event Observatory. The ZEPLIN programme of experiments, operated at the Boulby mine (UK), pioneered the liquid xenon technology for dark matter searches. The design is inherently scalable and, with sizes large enough to benefit maximally from self-shielding, these detectors have been the world-leading technology in searches for dark matter scattering ever since.

The proponents have delivered successfully the UK contribution to the Generation 2 (G2) experiment LZ which is about to switch on, and an ultimate G3 experiment will build on that effort and extend the search sensitivity right down to where new neutrino backgrounds will become dominant for masses of dark matter candidates of >5 GeV. Through this R&D, a G3 experiment will also address lower masses, which are less well-constrained but equally well-motivated candidates.

The main aims of this R&D phase are to enhance the scientific reach of a G3 experiment through demonstrating sensitivity via the Migdal effect, exploring xenon doping with light elements, developing scalable VUV-SiPM readout systems as potential replacements for PMTs, reducing backgrounds (especially radon), and producing significant contributions to the conceptual and preliminary designs of the G3 experiment.

This project will address technical challenges related to the future pre-construction and construction of a large liquid xenon detector for dark matter particles. The fundamental nature of the dark matter in the universe is of great importance to particle physics, astrophysics and cosmology.

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 neutrinoless double beta decay (0vBB) searches. 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.

We recognise the importance of translational activities and the opportunities for societal and industrial engagement through our low-background expertise and facilities. We target specific impact activities that align with technical development required for constructing a next generation LXe observatory and which also deliver rapid and high impact or openings for further development, particularly in sectors identified in the UK's Industrial Strategy. We will deploy a new state-of-the-art ICP-MS instrument to measure heavy metal toxicity in food and water in the UK, in partnership with Agilent and Analytix Ltd. We specifically target As and TiO2 and aim to establish sample preparation and rapid analysis protocols using novel matrix separation techniques with the UK Food Standards Agency. We shall develop a new 'SAGe' well-type detector with Mirion with both low-background and energy resolution of BEGe instruments; unavailable at present anywhere in the world but key for diverse sectors from defence to electronics. With XIA LLC we will develop their world leading UltraLo-1800 device to gain a factor 5x, sufficient for penetrating the electronics packaging sector for soft error rate and single site interaction investigations, as per international JEDEC standards. Finally, with Public Health England we aim to test radon barriers for lowering the intrinsic radon and background in devices deployed to monitor radon within homes and the workplace.

Our research provides training for students and staff in a wide range of specialist skills that are readily transferable 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 cryogenic vacuum systems, photosensors and cold readout electronics with mixed low-noise analogue and digital front-ends 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.