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

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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 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 - 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 3.5-year R&D project is 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 Boulby (UK), pioneered the LXe technology for dark matter searches. The design is inherently scalable and, with sizes large enough to benefit maximally from self-shielding, LXe 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, and an ultimate G3 experiment will build on that and extend the search sensitivity right down to where new neutrino backgrounds will become dominant for masses of dark matter candidates of >5 GeV - and also to 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 design of the G3 experiment.