Extending the physics reach of the LUX and LZ dark matter experiments by lowering the scintillation threshold

Dark matter experiments searching for weakly interacting massive particles (WIMPs), which are hypothesised to constitute the dark matter content of the Universe, probe other important physical processes producing very low energy (∼keV) nuclear recoils. These include the scattering of (non-WIMP) light dark matter particles and the coherent nuclear scattering of B-8 and He3-proton fusion (hep) solar neutrinos. Noble liquid dual-phase detectors, and particularly the Large Underground Xenon experiment (LUX), have placed stringent upper limits on the spin-independent WIMP-nucleon scattering cross-section at masses above ∼6 GeV. LUX was located at the Sanford Underground Research Facility (SURF), one mile underground, where it completed two successful runs between 2013 and 2016, utilising 300 kg of liquid xenon (LXe). The site will now be used by the forthcoming LUX- ZEPLIN (LZ) experiment, which is planned to start running in late 2020 and will utilise 10 tonnes of LXe. In this work we present a novel analysis technique for LXe time projection chambers (TPCs) that allows for a lower threshold by relying on events with a prompt scintillation signal consisting of single detected photons. The energy threshold of the LUX dark matter experiment was primarily determined by the smallest scintillation response detectable, which previously required a 2-fold coincidence signal in its photomultiplier arrays, enforced in data analysis. The technique presented here exploits the double photoelectron emission effect observed in some photomultiplier models at vacuum ultraviolet wavelengths. We demonstrate this analysis using an electron recoil calibration dataset and place new constraints on the spin-independent scattering cross-section of WIMPs down to 2.5 GeV WIMP mass using the 2013 LUX dataset. This new technique is promising to enhance light WIMP and astrophysical neutrino searches in next-generation liquid xenon experiments. To this end, we investigate the possible improvement in sensitivity brought about by applying this technique in LZ.Open Acces