Although it outnumbers regular matter by a ratio of five to one, dark matter is frustratingly elusive. Many experiments have been and are being run to try to hunt down different types of candidate particles, but so far no direct trace has been found of any of them. Now, researchers from Max Planck have proposed a new hypothetical particle that might be behind dark matter – the superheavy gravitino – and outlined just how we might find them.

As far back as the 1930s, astronomers began to notice that galaxies are moving much faster than they should be, based on the mass we could see. Calculations led to the conclusion that there must be far more mass out there that we couldn’t see, and this hypothetical invisible stuff became known as dark matter.

Ever since, astronomers and physicists have been searching for just what dark matter is made of. Many models suggest different types of particles, such as dark photons, axions, weakly-interacting massive particles (WIMPs), “Macro” particles with masses on the scale of a dwarf planet, and even a type of scalar particle older than the Big Bang itself. But despite many elaborate experiments, no traces of any of these exotic particles has turned up yet.

Now, researchers from Max Planck have developed a model which puts forward a brand new candidate – the superheavy gravitino. Gravitinos themselves have long been on the list of potential particles, although generally these are very light. As the name suggests, that’s not the case with superheavy gravitinos.

These hypothetical particles would not only have a much greater mass, but they would interact with normal matter through other forces. Dark matter is generally thought to only interact through gravity, which is what causes the astronomical oddities that clued scientists in on it.

A composite image showing galaxies in optical light, X-ray emissions in pink and invisible mass – or dark matter – in blue NASA/CXC/M. Weiss – Chandra X-Ray Observatory

“The common expectation is that dark matter is made up of an elementary particle, and that it hasn’t been possible to detect this particle yet because it interacts with ordinary matter almost exclusively by the gravitational force,” says Hermann Nicolai, an author of two studies describing the new particles. “In particular, our scheme predicts the existence of superheavy gravitinos, which – unlike the usual candidates and unlike the previously considered light gravitinos – would also interact strongly and electromagnetically with ordinary matter.”

The team calculated that superheavy gravitinos would each have a mass of roughly 0.02 milligrams, or about that of a flea egg. That might not sound very heavy, but for comparison’s sake, it’s about 10 quintillion times heavier than say a proton or a neutron.

With a mass that great, these particles would have to be very diffuse in the universe or they would cause it to collapse. But that works for the theory anyway – having just one particle in 10,000 km3 (2,400 mi3) would be enough to explain the dark matter effects astronomers see.

But what’s particularly interesting is that if superheavy gravitinos are real, we could find traces of them using the Earth itself as a giant detector. After all, we’re bound to have had plenty of them pass through the planet in the last 4.5 billion years. And if they did, they should have left fingerprints behind.

Because superheavy gravitinos would interact with regular matter through the electromagnetic and strong nuclear forces, they could leave ionization tracks in rocks. The problem is, they might be difficult to distinguish from the paths of other particles.

“Ionizing radiation is known to cause lattice defects in crystal structures," says Nicolai. "It may be possible to detect relics of such ionization tracks in crystals that remain stable over millions of years."

The research was published in two articles, appearing in the journals Physical Review Letters and Physical Review D.

Source: Max Planck