# Synopsis: Rocks May Hold Dark Matter Fossils

If dark matter interactions occurred inside ancient rocks, they could have left detectable traces in the rocks’ crystal structure.

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

Michael Schirber is a Corresponding Editor for Physics based in Lyon, France.

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