Centre for Dark Matter Particle Physics

The Adelaide node is involved in the construction of the SABRE South direct-detection experiment. This experiment will hunt for a candidate type of particle known as WIMPs (Weakly-Interacting Massive Particles), and will be located around 1,000 metres below ground in the Stawell Underground Physics Laboratory (SUPL) in western Victoria.

We are contributing effort in a number of areas, including:

• modelling and experimental characterisation of the background radiation in SUPL;
• simulation of the overall behaviour of the experiment, using GEANT4
• the optical calibration system for the liquid veto;
• crystal purity measurements;
• the fluid-handling system (high-purity nitrogen, liquid scintillator);
• and to broader areas such as organisation and equipment specifications.

We also undertake investigations into the theoretical side of dark matter physics, including the following:

• using the SABRE experiment for the detection of axions (another class of candidate DM particles, of much lower mass than WIMPs);
• the inelastic boosted dark matter (IBDM) model, which will be tested by the SABRE experiment;
• a two-component dark matter model, taking into account the gravitational focusing effect, which will attempt to put constraints on the dark matter parameter space by analysing the experimental data of direct detections;
• dark photon searches from the global QCD analysis (with JAM Collaboration at Jefferson Lab, USA).

Published in the Journal of High-Energy Physics

The existence of dark matter has been firmly established from its gravitational interactions, yet its precise nature continues to elude us despite the best efforts of physicists around the world. The key to understanding this mystery could lie with the dark photon, a theoretical massive particle that may serve as a portal between the dark sector of particles and regular matter.

In our recent work - a collaboration between CSSM, the Adelaide node of the ARC Centre of Excellence for Dark Matter Particle Physics and Jefferson Laboratory - we study the potential effects a dark photon could have on the interpretation of existing experimental results from the deep inelastic scattering process.

Specifically, we make use of the state-of-the-art Jefferson Angular Momentum collaboration (JAM) global analysis framework for parton distribution functions, modifying the underlying theory to allow for the effect of a dark photon. We show that the dark photon hypothesis is preferred over the Standard Model hypothesis at a significance of 6.5 sigma, which constitutes strong evidence, albeit indirect, for a particle discovery.

The ARC Centre of Excellence for Dark Matter Particle Physics (CDMPP) aims to discover what this mysterious stuff, dark matter, is made of. It is five times as plentiful as the baryonic matter we have explored so successfully, and of which we are composed. We do not know what kind of particle it is, but we (along with a very large number of people around the world) want to understand this. One key approach is the SABRE experiment that is being built in a new laboratory in a gold mine (1km underground) in Stawell, which will hopefully shed light on this question in a few years. 

The present work, published in Physical Review Letters¹, explores the possibility that dark matter is in the form of  a dark massive photon. In particular, we explore the discovery potential of the new tool enabled by the upgrade at Thomas Jefferson National Accelerator Facility (JLab) in the United States. 

This new tool is parity-violating electron scattering. Parity violation is, simply put, the difference between what happens in the laboratory and what happens when you view the experiment in a mirror. The differences are very small, typically less than a part per million, but incredibly precise measurements enable us to observe this and use it as a signal of the existence of this new particle.

We show that the effects of this new particle may reconcile a surprising discrepancy between the neutron density in a lead nucleus inferred from the PREX experiment at JLab, and that predicted by nuclear structure theory. We also show that there may be corrections as large as 10% in the valence quark distributions extracted from data taken at HERA in Germany. Finally, it is shown (see figure below) that a future experiment comparing electron and positron deep inelastic scattering on the deuteron could provide a vital test of the existence of such a particle.

¹ A. W. Thomas, X.-G. Wang and A. G. Williams, Physical Review Letters 129 (2022) 011807