Rocks May Hold Dark Matter Fossils
The rocks beneath our feet may contain vestiges—in the form of nanometer-wide tracks—of past dark matter interactions. Previous searches for these so-called dark matter fossils have come up empty, but a team of researchers from Sweden and Poland believes that recent advances in material analysis techniques warrant a new campaign to “dig” them up. The team predicts that certain minerals from deep drill cores could show signs of dark matter with a sensitivity surpassing that of current search methods.
Fossil track identification was originally developed to study nuclear fission in old rocks. When a high-energy fission product barrels through the crystal structure, it leaves an amorphous track that is a few nanometers wide and a few micrometers long. Dark matter particles could produce similar—but shorter—tracks through collisions with nuclei, but a search published in 1995 using atomic force microscopy found no such signatures.
For this updated study, the researchers propose using present-day techniques, such as helium-ion beam microscopy and small-angle x-ray scattering, to directly map the entire volume of a gram’s worth of rock with nanometer resolution. Such a feat was previously impossible. They suggest obtaining samples from low-radioactivity minerals in 10-km-deep boreholes, which should contain fewer tracks from other particles like cosmic rays.
The team estimates that a cubic centimeter of ancient rock could hold hundreds to thousands of dark-matter-induced tracks, which, they say, would be identifiable by their unique distribution of track lengths. Their calculations also suggest that the sensitivity of a gram-sized “paleo-detector”—whose rocky material has been “searching” for dark matter for roughly a billion years—could be greater than current ton-sized detectors that have only been running for a few years.
This research is published in Physical Review D.
Michael Schirber is a Corresponding Editor for Physics based in Lyon, France.