Dark Matter Searches with LUX-ZEPLIN

The LZ direct dark matter detector utilizes scintillation photons and electroluminescence photons created by electrons released by ionization to detect particle interactions. For the first science result in 2022, LZ relied upon a fast chain of simulations with parametrised response functions fit to calibration data to build the signal and background models used in setting initial limits. However, such simulations cannot handle the tail of distributions needed to be understood for the full planned exposure of 1000 days. A full chain simulation that tracks the emitted light and charge will be necessary to maximize LZ's sensitivity.

LZ has such a simulation, completed prior to detector construction for the third mock data challenge, and used such that mock waveform events could be created to go through the entire reconstruction and analysis chain. Oxford played a leading role in this simulation development. Nicholas Fieldhouse's initial work will focus on the full simulation chain making waveforms that look precisely like waveforms recorded in the LZ detector. Improved simulations from electric field modelling code will be required to build ionization signals from first principles that appear as jagged as LZs do, in contrast to the previous LUX detector. Once an improved full chain simulation is in hand, and verified against extended calibration data sets, it can then be used for improving event processing and testing analyses.

Nicholas' projects can then explore the use of the full simulation chain in two ways. The first will be improving our ionization signal reconstruction of integer numbers of extracted electrons by using the pulse substructure. This improved reconstruction can help lower the energy threshold, reduce the uncertainty on the measured energy, and hopefully increase discrimination against electronic recoil backgrounds. The second task would be developing new cuts against accidental coincidence events where the ionization signal is linked to the emission of electrons from the electrodes or from electrons trapped on the liquid surface. A large simulation dataset will play a key role in developing these cuts against rare events that have otherwise proved difficult to eliminate in our first science run.

Finally, Nicholas will have hardware experience working on R&D studies of electron emission from electrodes as designs for the next generation liquid xenon TPC dark matter experiment. These hardware studies will benefit from the analysis and simulation work done for LZ, and can hopefully provide useful feedback to the LZ analysis too.