The discovery of dark matter is of fundamental importance to cosmology, astrophysics, and elementary particle physics. A broad range of observations indicate that 80 to 90% of the matter in the universe is in some new form of matter. The resulting Standard Cosmology holds that a quarter of the energy density of the universe is a new fundamental form of matter not yet encountered in the laboratory, and most of the remainder is dark energy. The solution to this mystery may lie in the existence of some new generic class of Weakly Interacting Massive Particles, or WIMPs, whose existence is also motivated by physics beyond the standard model of particle interactions. WIMP detection experiments require operation deep underground to prevent cosmic ray interactions from causing false positive signals. The LUX and ZEPLIN collaborations, building on their pioneering efforts in liquid xenon (LXe) dark matter detectors and their broad expertise in low background techniques and rare event searches, propose here a plan for the design of a 20-ton LXe detector (LZ20). The LZ20 detector will use established two-phase LXe technology with readout of primary and secondary scintillation signals for particle identification together with 3d position reconstruction to exploit the self-shielding of the LXe, removal of surface artifacts and calibration of position dependent response functions. This detector, installed at the 4850 level, with a fiducial mass of 13.5 tons and an initial operating period of 1000 days, will suppress internal and external backgrounds from electromagnetic and neutron interactions to below a single event and achieve a sensitivity to WIMP-interactions down to cross section of 10 to the power negative 48 cm2. At this exposure, the sensitivity will begin to encounter irreducible backgrounds from solar and atmospheric neutrino interactions this experiment therefore represents the definitive WIMP search that can be performed with liquid Xe.
Regarding Broader Impacts Dark Matter science is broadly appealing to the public, building on the natural curiosity of the public as well as the connection and familiarity with gravity. The collaboration proposes to develop a range of activities, from tours and displays to partnerships with local schools and individuals, to foster an interest in science and science careers. Dark matter experiments demand the development of new technologies that are ultra-sensitive to radiation, and new methods for achieving ultra-low radioactive backgrounds. There is an overlap of these technologies and methods with those used in a wide range of nuclear security contexts. Through direct technology development or through the training of personnel that transfer to industry or defense labs later in their careers, the collaboration expects significant impact from LZ20 particularly because of the involvement of LLNL. This proposal includes a description of postdoctoral mentoring plans.
