Minerals deep inside Earth might contain telltale traces of collisions with dark matter — the elusive stuff that researchers think makes up most of the matter in the Universe. Experiments designed to search for these traces could one day complement or even compete with ongoing efforts to detect dark matter directly.
Researchers using sophisticated detectors sunk deep underground have searched for signs of dark matter for decades. But now, Katherine Freese, a physicist at the University of Michigan in Ann Arbor, and her colleagues suggest that minerals such as halite (sodium chloride) and zabuyelite (lithium carbonate), can act as ready-made detectors1.
Astronomers can detect the gravitational influence of dark matter on the motion of galaxies and galactic clusters, but have never been able to spot it directly. The prevailing explanation for dark matter is that it’s made of material known as weakly interacting massive particles (WIMPs), which interact with normal matter mainly through gravity.
Direct-detection experiments aim to find the faint after-effects of WIMPs colliding with the nuclei of atoms in materials such as germanium, silicon or sodium iodide inside a detector.
Such experiments must be positioned deep underground, to guard against the cosmic rays that bombard Earth’s surface. These rays can also leave faint traces of their collisions with detector materials, which can swamp any potential signals from dark matter. So far, only one experiment — the DAMA/LIBRA experiment at the Gran Sasso National Laboratory in Italy — says it has detected dark matter, but the claim remains unverified.
Freese and her colleagues argue that minerals such as halite and zabuyelite are already deep inside Earth and thus are shielded from cosmic rays. According to the team’s analysis, published last month on the preprint server arXiv, if a WIMP were to smash into the nucleus of an atom of, say, sodium or chlorine, the nucleus would recoil. This would etch a path anywhere from 1 to 1,000 nanometres long in the mineral.
An experiment could extract the minerals — which can be around 500 million years old — from kilometres-deep boreholes that already exist for geological research and oil prospecting. Physicists would need to crack open the extracted minerals and scan the exposed surfaces under an electron or atomic force microscope for the tracks made by recoiling nuclei. They could also use X-ray or ultraviolet 3D scanners to study bigger chunks of minerals faster, but with lower resolution.
Potential collisions with WIMPs will create different signatures in each element of any mineral and thus provide different sources of information. “For example, sodium chloride consists of both sodium and chlorine, so you get multiple signals from just one mineral,” says Freese. “If you do find some positive signals, then you can figure out what kind of WIMP it is based on its scattering off of sodium and its scattering off of chlorine.”
The idea is pretty exciting, says Dan Hooper, a physicist at the Fermi National Accelerator Laboratory in Batavia, Illinois. “Although there are many details yet to be demonstrated before this kind of programme could be realistically implemented, I don’t see any reason why it couldn’t succeed, at least in principle.”
But others, such as physicist Juan Collar of the University of Chicago in Illinois, strike a more cautionary note. In the mid-1990s, physicists considered using the mineral mica as a target for similar searches for dark matter2. But Collar subsequently showed3 that radioactivity from uranium in the minerals would create tracks in the mica that would be impossible to distinguish from those created by WIMPs. He fears a similar fate would befall Freese’s proposal.
“This issue is not limited to mica, but instead affects any mineral containing the ubiquitous natural abundance of uranium and thorium,” he says. “They may be able to find minerals where this problem is reduced, but I think that right now their claims are way too optimistic.”
Freese acknowledges that uranium is a concern, although it's unclear whether the minerals that she and her team propose contain uranium or thorium contaminants. But she adds that it might be possible to identify specific etching patterns that can be created only by a burst of radioactivity, and to ignore them. “The truth is you have to do it and find out,” she says.