WIMPs and underground experiments

The key features of a WIMP are that they are relatively heavy (with a mass anywhere from the mass of a proton to the mass of a heavy nucleus) and interact only feebly with ordinary matter. To give a sense of the scale of the weakness of the expected interaction, almost all of the WIMPs that the Earth encounters as it passes through the dark matter halo will pass straight through the Earth without interacting with any ordinary matter!

The rarity of the interaction presents a challenge for experiments that hope to search for WIMPs, because the other types of particles that can also interact in WIMP detectors can overwhelm the signal. These other particles form two broad categories: those associated with the radioactive decay of naturally occurring isotopes, and high energy cosmic ray particles.

The difficulty of trying to detect the WIMP signal above the cacophony of interactions by other particles is a bit like to trying to hear a dripping tap in a rainstorm, so most of the efforts made by WIMP experiments are aimed at silencing the storm by reducing these backgrounds.

Naturally-ocurring radioactivity

Our world is filled with trace amounts of radioactive isotopes and when these decay, they emit gamma rays and other particles which can be mistaken for a WIMP. The effects of this endemic radioactivity can be reduced by shielding the WIMP detector to block emissions coming from outside the detector, and by ensuring that the detector material itself contains as little radioactive contamination as possible. This latter problem is important enough that characterising radioimpurities in detector materials is a key focus of the Centre’s Metrology program.

Cosmic rays

Cosmic rays are high energy particles that originate from space and travel downwards towards the Earth. Because of the energies and types of particles involved, shielding a WIMP detector from cosmic rays in a way analogous to radioactive decays is not feasible. However, a deep underground location such as a mine can shield almost all of these cosmic rays, reducing them to a manageable level. This is the basic physics motivation for building the Stawell Underground Physics Laboratory (SUPL), which will be located 1025 m underground in the Stawell Gold Mine in regional Victoria. SUPL’s overburden of rock will reduce the cosmic ray background by about a million-fold, allowing it to house the large scale WIMP direct detection experimental program in Australia.