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About three years ago, Wolfgang “Wolfi” Mittig and Yassid Ayyad set out to find the universe’s missing mass, better known as dark matter, at the heart of an atom.
Wolfgang Mittig, Hannah Professor Emeritus at MSU and faculty member of the Facility for Rare Isotope Beams
Their expedition did not lead them to dark matter, but they did find something that had never been seen before, something that defied explanation. Well, at least one explanation that everyone could agree on.
“It’s been something like a detective story,” said MittigHannah Distinguished Professor of Michigan State University Department of Physics and Astronomy and a faculty member of the Facility for Rare Isotope Beams, or FRIB.
“We started looking for dark matter and we didn’t find it,” he said. “Instead, we found other things that were hard to explain by theory.”
So the team got back to work, doing more experiments, gathering more evidence to make sense of their discovery. Mittig, Ayyad and their colleagues bolstered their case at the National Superconducting Cyclotron Laboratory, or NSCL, at Michigan State University.
Working at NSCL, the team found a new path to their unexpected destination, which they detailed on June 28 in the newspaper. Physical examination letters. In doing so, they also revealed some interesting physics brewing in the ultra-small quantum realm of subatomic particles.
In particular, the team confirmed that when an atom’s nucleus, or nucleus, is overloaded with neutrons, it can always find a way to get to a more stable configuration by spitting out a proton instead.
A shot in the dark
This Hubble Space Telescope image is centered on what’s called a Low Surface Brightness, or LSB, galaxy (blue), surrounded by more familiar galaxies (yellow). Astrophysics believes that more than 95% of the matter found in LSBs is dark matter. Credit: ESA/Hubble & NASA, D. Calzetti
Dark matter is one of the most famous things in the universe that we know the least about. For decades, scientists have known that the cosmos contains more mass than we can see based on the trajectories of stars and galaxies.
For gravity to keep celestial objects tethered to their paths, there had to be mass invisible and in large quantities – six times the amount of regular matter that we can observe, measure and characterize. Although scientists are convinced that dark matter exists, they have yet to figure out where and how to detect it directly.
“Finding dark matter is one of the main goals of physics,” said Ayyad, a nuclear physics researcher at the Galician Institute for High Energy Physics, or IGFAE, at the University of Santiago de Compostela in Spain.
Speaking in round numbers, scientists have launched about 100 experiments to try to shed light on what exactly dark matter is, Mittig said.
“None of them succeeded after 20, 30, 40 years of research,” he said.
Yassid Ayyad holding a component used in the research team’s experiment in 2019, when he was a sensing systems physicist at the National Superconducting Cyclotron Laboratory.
“But there was a theory, a very hypothetical idea, that you could observe dark matter with a very particular type of nucleus,” said Ayyad, who was previously a detection systems physicist at NSCL.
This theory centered on what she calls dark decadence. He postulated that some unstable nuclei, nuclei that naturally fall apart, could shed dark matter by collapsing.
So Ayyad, Mittig and their team designed an experiment that could seek dark decay, knowing the odds were stacked against them. But the bet wasn’t as big as it looks, because probing alien decays also allows researchers to better understand the rules and structures of the nuclear and quantum worlds.
The researchers had a good chance of discovering something new. The question was what it would be.
Help from a halo
When people imagine a nucleus, many may think of a lumpy ball made up of protons and neutrons, Ayyad said. But the nuclei can take on strange shapes, including so-called halo nuclei.
Beryllium-11 is an example of a halo nucleus. It is a form, or isotope, the element beryllium which has four protons and seven neutrons in its nucleus. It holds 10 of these 11 nuclear particles in a tight central cluster. But a neutron floats away from that core, loosely bound to the rest of the core, much like the moon ringing around Earth, Ayyad said.
In the team’s experiment published in 2019, beryllium-11 decays by beta decay into an excited state of boron-11, which decays into beryllium-10 and a proton. In the new experiment, the team accesses the state of boron-11 by adding a proton to beryllium-10, i.e. running the reaction reversed in time.
Beryllium-11 is also unstable. After a lifetime of about 13.8 seconds, it collapses by what is called beta decay. One of its neutrons ejects an electron and becomes a proton. This transforms the nucleus into a stable form of the five-proton, six-neutron boron element boron-11.
But according to this very hypothetical theory, if the neutron that decays is the one in the halo, beryllium-11 could follow an entirely different path: it could undergo dark decay.
In 2019, researchers launched an experiment at Canada’s national particle accelerator facility, TRIUMF, to search for this highly hypothetical decay. And they found decay with surprisingly high probability, but it wasn’t dark decay.
It seemed that the weakly bound beryllium-11 neutron ejected an electron like normal beta decay, but the beryllium did not follow the known decay path to boron.
The team hypothesized that the high probability of decay could be explained if a boron-11 state existed as a gateway to another decay, to beryllium-10 and a proton. For anyone counting, that meant the nucleus had reverted to beryllium. Only now he had six neutrons instead of seven.
“It just happens because of the halo core,” Ayyad said. “It’s a very exotic type of radioactivity. This was in fact the first direct evidence of proton radioactivity from a neutron-rich nucleus.