Background isolation techniques for future liquid xenon dark matter observatories

LUX-ZEPLIN is a generation-2 Dark Matter experiment being commissioned in 2020 at the Sanford Underground Research facility in Lead, South Dakota (USA). It will use 10 tonnes of liquid xenon and 500 photomultiplier tubes (PMT) to detect extremely rare events of dark matter interactions with regular matter. For the unambiguous detection of elusive dark matter particles rigorous data analysis procedures are vital, supported by a detector calibration programme and associated simulations. The experiment will use a suite of internally dispersed (83mKr, 131Xe, 220Rn and CH4 labelled tritium) and externally deployed sealed neutron (AmLi, 205, 206BiBe, 252Cf, 88YeBe) and gamma (57Co, 22Na, 133Ba, 228Th) radiation sources to provide high statistics calibration of the response to background events and WIMP signal. Rare-event experiments require cutting-edge data analysis, supported by extremely accurate simulations tuned existing and future calibration data, and a model of the detector response. All data analysis steps and supporting simulations and calibrations have to be extremely robust. A very important step is to extract and identify single photons from the pulse trains that the digitizers record from the electronics chain comprising PMT, cabling, amplifier board and further cabling. Algorithms to do this exist and have been tested at a level suitable for existing experiments, however, further improvements can be made by incorporating new techniques and expanding capabilities of the portfolio of simulation tools.

The goal of this project is to develop specific analysis tools, supported by detailed simulations to provide an optimal understanding of the detector response to calibration sources. The main project objective is to enhance the science reach of the detector by lowering the detection threshold through improvements to the photon detection algorithms, event reconstruction and triggering capability. In addition to using data from the LZ detector, this will also involve generating simulations of particular events in a xenon time projection chamber (re-using calibration events, for example), events from data recorded in previous experiments and using all possible information that is available in order to provide and validate suitably enhanced algorithms. The novelty of the project is ensured by focussing on detector elements that so far have not been covered by simulation and analysis, either not at all or only partially. There is a further novel aspects to the project that is related to future photon sensing devices, an area that is ideally matched to the experimental capabilities of the Oxford laboratory. This will involve developing cryogenic electronics and cabling solutions for a future dark matter detector. A crucial novel element of this research will be the integration of the electronic readout chain for such sensors, building on previous work at Oxford that developed a detector electronics response model.

The project is mostly computer based, using GEANT-based simulation, the ROOT data analysis package and the LZAp analysis framework. It is carried out within the LZ Collaboration of which Oxford is a member. The small element of hardware development will be carried out at Oxford, mainly, in collaboration with Imperial College London, Bristol and Liverpool.