Low Energy Nuclear Recoil Response in Xenon Gas for Low Mass Dark Matter WIMP Search
Sofka, Clement James
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Over 80 years of astrophysical observations suggest that the observable luminous matter makes up ? 5% of the total energy density in the Universe. The remaining ~ 95% comes from matter and energy that has not been observed directly. Discovering these "dark" sources of matter/energy is the single most important concern in the modern quest for understanding Nature. We live in an epoch that is almost certainly characterized by a at, expanding Universe. Coupling this with the wealth of astrophysical surveys, we are able to probe the vastness of space, and develop theories of space-time evolution, going back in time several billions of years. The evidence suggests that the Universe began in a Big Bang, underwent a brief moment of Inflation, then cooled and began forming the structures (atoms, molecules, stars, galaxies, etc.) we observe plainly today. An integral part of this consistent story of the Universe's birth and cosmic evolution is the existence of cold dark matter in the form of Weakly Interacting Massive Particles (WIMPs) and dark energy. Initial cosmological considerations suggested that WIMPs were some type of Standard Model (SM) particle, but even the best-case estimates lead to matter energy densities that come up well short without a significant modification of the underlying theory of gravity. The best proposed WIMP candidate has surfaced from efforts motivated by particle physics. A new type of WIMP arises out of Supersymmetry (SUSY). The Lightest Supersymmetric Particle (LSP), a neutralino, seems to fit perfectly into both particle physics and cosmology. First estimates from a Minimal Supersymmetric Standard Model (MSSM) placed the WIMP in the mass range of O(10) - O(10^(3)) GeV/c^(2). However, there is mounting evidence in recent years that suggests the existence of a low mass WIMP as a suitable dark matter candidate. Some of the most sensitive detectors to low mass WIMPs employ noble liquids as a target medium. Groups using noble liquid detectors are currently limited to the detection of relatively higher mass WIMPs because of detector threshold limits, background effects, or a lack of fundamental understanding of very low energy nuclear recoils (< 3 keVnr). This work is aimed at studying these very low nuclear recoil energies in xenon to improve noble element detector sensitivities and develop a fundamental understanding of nuclear stopping power theories originally studied by Lindhard et al. in the 1960's. We present the nuclear recoil results from measurements using a nearly mono-energetic beam of neutrons aimed at high-pressure gaseous xenon (HPXe) in a time projection chamber (TPC). This work demonstrates the viability of future low mass dark matter WIMP and other rare event searches (e.g. Neutrinoless Double Beta Decay, 0 ) using high pressure noble gases.