Dark matter has never been directly detected; could a gold mine in Australia be part of the answer?
Deep underground, in a gold mine in western Victoria, physicists from across Australia and the world are preparing for the construction of the Stawell Underground Physics Laboratory (SUPL). To be built one kilometre underground, this is the first laboratory of its kind in the southern hemisphere. Being located so far under the surface makes this laboratory uniquely valuable for particular experiments in particle physics, geophysics and radiation biology. But the main focus of SUPL will be hunting for elusive dark matter particles: the exotic matter that makes up about a quarter of the total energy content of the universe, but which cannot be seen.
Professor Elisabetta Barberio from the University of Melbourne is leading the SUPL project. She said: "The reason why we started this enterprise of building a lab in the southern hemisphere, is to catch dark matter. There are different kinds of experiments looking at different properties of dark matter, and it's not clear what we are going to see. We don’t really know what it is.”
The idea that dark matter is comprised of exotic new particles, fits elegantly with what we already know about dark matter from cosmology. This dark matter is crucial "extra mass" that holds together galaxies and clusters of galaxies, playing an important role in the evolution of the universe. Dark matter does not emit or absorb any form of radiation, so no direct evidence of the presence of such particles has ever been found. This makes determining the nature of dark matter one of the greatest open questions in modern physics.
The goal of an underground laboratory, is to keep background radiation from cosmic rays — and radiation from radioactive isotopes in the surrounding structure — excluded from any experiment. These background radiation sources can be many orders of magnitude more intense than the rare phenomena being studied, so they can easily drown out the signal. To capture these extremely rare events, we need extraordinary circumstances — such as the depths of a mine — to eliminate radiation.
In astrophysics, there are a number of inferred indications that dark matter exists, such as galaxy rotation curves and gravitational lensing. Gravitational lensing occurs when a large mass of matter in space bends light, distorting our vision of a galaxy present behind the mass, just like a lens does. This happens with any matter, including dark matter. Where lensing is observed, but there is no regular matter that can account for this lensing effect, this implies the existence of dark matter. The ability to directly detect it would be a great advance for modern physics and cosmology.
"If we see dark matter, in direct detection, we'll start understanding where it comes from, and what this particle is," Professor Barberio said. "We can start studying the characteristics. We can start making models and try to put together all the pieces to understand what it is."
Dark matter is thought to be composed of elementary particles we generally term weakly interacting massive particles (WIMPs). These are very different from the tiny particles like quarks and electrons that make up the matter we can see. WIMPs are abundant throughout the universe, but do not interact with other particles, via electromagnetism or the strong nuclear force, as familiar matter does. This means WIMPs simply pass through familiar matter with a very low chance of ever interacting with it. We usually see matter through its interaction with light, but without these forces WIMPs are practically invisible. The only way that WIMPs can be "seen" is via the weak nuclear interaction, or via their mass and interaction with gravity on large scales — which we observe amongst the stars.
The search for WIMPs and an understanding of their properties has led to a rich and interesting collaboration between different disciplines of physics. Astrophysicists and cosmologists use their observations and models to understand how much dark matter is present in the universe. Theoretical particle physicists work with models, such as supersymmetry, to predict the existence of new, weakly-interacting particles which are promising dark matter candidates. Experimental physicists use particle accelerators, such as the Large Hadron Collider, to determine if such particles can be artificially generated in collider experiments.
The investigation into WIMPs as candidates for dark matter promotes a rich collaboration across the international physics community, with several underground laboratories attempting direct detection. SUPL will conduct a range of experiments similar to those in Snolab, Canada, the Sanford Underground Research Facility in the United States (where neutrinos from the sun were first detected in the 1960s), the Gran Sasso National Laboratory (LNGS) in Italy and the Boulby Underground Laboratory in the UK — upon which the SUPL design is based. SUPL will be constructed in the Stawell gold mine about 240km west of Melbourne. As the existing mine is 1.6km deep, already equipped with electricity and fibre optics, and contains many large caverns, it is an ideal site for building dark matter experiments.
The University of Melbourne, the ARC Centre of Excellence for Particle Physics at the Terascale (CoEPP), and the Australian Nuclear Science and Technology Organisation are leading the development of SUPL, along with the University of Adelaide, the Australian National University and Swinburne University.
As the first site in the southern hemisphere to host an experiment of this type, SUPL will compliment experiments at existing northern hemisphere facilities such as the LNGS in Italy. Having detectors in both hemispheres allows for more confidence in these searches for dark matter particles. As the solar system moves around the galaxy, the Earth orbits the sun. This means that the Earth’s speed, relative to a stationary “halo” of dark matter throughout the galaxy, should modulate up and down over the course of a year.
This means the particle detection rate should also oscillate over the year. “Most of the experiments have seen nothing, but there is one experiment that has, for many years, seen a very strong signal,” Professor Barberio said, referring to the DAMA/LIBRA experiment at LNGS. This experiment has found some tentative evidence that this modulation is occurring.
"When earth goes with the same direction as the sun around the galaxy, the velocity of earth around the galaxy increases. In the opposite direction, it decreases," Professor Barberio explained. "Suppose dark matter is like the wind that you feel when you're on a bicycle. You can imagine if you put a detector on Earth, depending on the position of Earth during the year, you can feel more or less."
A complementary detector on the other side of the world should detect this same annual modulation, providing an elegant check that this modulation is not due to some background effect. If a seasonal variable such as temperature or atmospheric changes which affect background radiation levels is responsible, then we will see a different result in the southern hemisphere. Similar to the DAMA/LIBRA detector, an identical pair of dark matter detectors in Stawell and Gran Sasso (Italy) will allow for this detection.
International scientists from Princeton University and Italy’s National Institute of Nuclear Physics are also involved, with the eventual goal of having an identical pair of dark matter detectors, similar to the DAMA/LIBRA detector, located at both Stawell and Gran Sasso.
Although direct detection of dark matter has been the primary driver of scientific interest in the establishment of SUPL, it is not the only area of research that will benefit from access to a deep underground laboratory. Having a laboratory with low background radiation lends itself to the study of other particle physics phenomena, such as neutrino oscillation and nuclear double-beta decay.
SUPL could also help with biological research. For example, the response of cells to low doses of ionising radiation can be studied by comparing the health of cells in the underground environment to cells which are kept above ground. There is some evidence that low doses of radiation are beneficial to living organisms, preventing disease and stimulating repair mechanisms in the cell. If this is true, the cells grown in the underground environment may be less healthy than the control cells exposed to normal background radiation. According to Professor Barberio: “Life probably evolved thanks to cosmic radiation.”
The study of microorganisms that live in the unusual environment deep in the Earth is another interesting area that underground laboratories can help us investigate. “Australia is a very old continent, so the primordial crust is still present. Primordial organisms that live underground can give us insight into how life came about — or if there is life on other planets — because we always assume you need water to have life," said Professor Barberio. As these environments can contain very little water: “this has big implications, because life could have emerged without water”.
The initial stage of the SUPL project — involving the construction of the laboratory itself and its supporting infrastructure — is estimated to cost $3.5m and take up to five years. Victorian Premier Daniel Andrews toured the site earlier this year, pledging $1.75m in state funding for the project, which was later matched by the federal government. With the necessary funding in place — and detailed design, site characterisation and underground measurements underway — construction work should start early next year. The laboratory is expected to boost the local economy by bringing an estimated $550m in cumulative economic output to the region over 10 years.
However, building a high-tech physics lab in a deep underground mine is not without challenges. Excluding all traces of environmental radioactivity from the experiment means building a full clean-room environment at the bottom of a dirty, dust-filled mine. All materials, such as the sodium iodide crystals that make up the detectors, must be meticulously chosen and purified in order to exclude the traces of radioactivity found everywhere on the surface. "Everything must be clean from radioactivity," Professor Barberio said. When people enter the lab they will change into cleanroom clothing and carefully clean all materials and equipment, ensuring that all dust and contamination from outside is excluded. This need to be fastidious about cleanliness has planners “really worried about water,” as absolutely everything needs to be clean of radioactivity.
Despite these challenges, the building of an underground radiation-free laboratory will put Australia at the cutting edge of particle physics without the need for a multi-billion-dollar particle collider.. With the nature of dark matter such an important, but poorly understood part of modern physics, SUPL will be at the forefront of research. As SUPL will be the first southern hemisphere site to attempt direct detection, this is not only an exciting project for the physics community in Australia, but for the rest of the world as well. With any luck, we might just shine some light on this dark puzzle.