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Fundamental Physics at the Low Background Frontier

Final Report Summary - DARKFRONTIER (Fundamental Physics at the Low Background Frontier)

The nature of dark matter is one of the fundamental questions about the universe today, at the forefront of experimental and theoretical physics research. From a variety of astrophysical measurements we now know that dark matter makes up approximately 25% of the energy density of the universe. However, the properties of dark matter particles are unknown. Many types of experiments search for dark matter: direct detection experiments seek to observe the interactions of ambient dark matter with atomic nuclei in terrestrial detectors; indirect detection experiments search for dark matter pair annihilation products in the cosmos; collider experiments seek to produce dark matter and identify it by its decay products. These approaches all suffer from backgrounds which can mimic a dark matter interaction signature.
Neutron backgrounds are particularly dangerous in direct detection dark matter experiments because on a single-event basis, neutron elastic scattering is indistinguishable from a dark-matter induced signal. There are two relevant sources of this background: neutrons from (α,n) reactions, and from cosmic ray spallation. The neutron interaction probability is at least twenty orders of magnitude larger than the dark matter interaction probability, and the neutron flux has large uncertainty. This work addressed neutron backgrounds in two new ways: (i) neutron tagging and (ii) active veto development. These new methods for addressing neutron backgrounds were employed in a highly competitive dark matter search with a novel Liquid Argon (LAr) detector, in the DEAP/CLEAN single-phase detector development programme. The DEAP-3600 detector has 3.6 tonne LAr target mass, and dark matter cross section sensitivity reach an order of magnitude beyond current experimental results for heavy dark matter at the TeV mass scale. This regime is highly complementary with LHC dark matter searches.
There is great interest in LAr dark matter searches because of the potential for detectors at the kilotonne scale, with design similar to highly successful astroparticle physics experiments like SNO and Borexino. Success depends critically on demonstrating the required background suppression. The single-phase LAr detector development work of this proposal demonstrated for the first time that pulse shape discrimination in LAr can distinguish between nuclear vs. electronic recoil interactions at low energies, that active veto rejection of neutron backgrounds is effective in large-scale LAr detectors, and resulted in the first dark matter search result from the DEAP-3600 detector, which is the leading result in liquid argon at high masses. Demonstration of this background rejection potential paves the way for the next generation of larger-scale LAr experiments- at the 100 tonne scale, such a detector would be a new kind of observatory for fundamental physics at the low background frontier, testing predicted properties of dark matter, neutrinos, supernovae, and stellar evolution.