Signatures of the Invisible: New Considerations for Xenon Dark Matter Searches


The nature of dark matter (DM) remains one of the largest open questions in modern physics despite the many searches for particle dark matter around the world. The search for dark matter particles with masses above 5 GeV/$c^2$ is led by the dual-phase xenon time-projection chamber (LXe-TPC) detector technology which observes recoils of xenon nuclei induced by a collision from a DM particle. One of the premier experiments using this technology for DM searches is the LUX-ZEPLIN (LZ) collaboration, with an active target of 7 tonnes of liquid xenon (LXe). Dark matter searches in LXe­TPCs require sensitivity to extremely rare, small energy depositions, which is only attainable through elimination and understanding of the backgrounds present. In this work, I present an overview of the LZ detector and experiment, and discuss the projected results. As these detectors become increasingly sensitive to small energy depositions, previously ignored and subdominant backgrounds become important to understand. I discuss the effect of a class of electron recoil (ER) backgrounds that has been systematically ignored in the standard ER calibration, which treats all ERs as valence shell recoils. Auger cascades arising from neutrino or Compton scatters off inner-shell atomic electrons produce detector responses that cause them to look more like nuclear recoil signal events rather than background electron recoils. This effect increases the significance of electron recoils stemming from neutrino-electron scattering. I present the results of the XELDA experiment which calibrated this effect for L-shell scatters. The LZ experiment will probe the majority of the parameter space above the ``neutrino fog” for DM masses above 5 GeV/$c^2$, thus increasing sensitivity at this mass range may prove to be impractical for continuing a dark matter search, with current techniques. I present a method, by way of doping an LXe-TPC with light noble elements, such as hydrogen, to extend the mass reach of LXe-TPC DM searches to the largely unexplored region of DM masses below 5 GeV/$c^2$, for both spin-dependent and spin-independent DM-nucleon interactions. This method, if implemented in an LZ-scale detector, can enable sensitivity to DM with masses down to hundreds of MeV/$c^2$.

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