About three years ago, Wolfgang “Wolfi” Mittig and Yassid Ayyad went looking for the universe’s missing mass, better known as dark matter, at the heart of an atom.
Their expedition didn’t lead them to dark matter, but they still found something that had never been seen before, something that defied explanation. Well, at least one explanation everyone could agree on.
“It’s been something of a detective story,” said Mittig, the Hannah Distinguished Professor in the Department of Physics and Astronomy at Michigan State University and a faculty member at 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 have been difficult for theory to explain.”
So the team went back to work, doing more experiments and 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 on NSCL, the team found a new path to their unexpected destiny, which they detailed in the diary. Physical Review Letters. In doing so, they also revealed some interesting physics at work in the ultrasmall quantum realm of subatomic particles.
In particular, the team confirmed that when an atom’s nucleus is overloaded with neutrons, it can still find its way to a more stable configuration by spitting out a proton instead.
A shot in the dark
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 paths of stars and galaxies.
For gravity to keep celestial objects tethered to their paths, there had to be invisible mass, and plenty of it: six times the amount of regular matter that we can observe, measure, and characterize. Although scientists are convinced that dark matter is out there, they have yet to find where and figure out 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 around 100 experiments to try to clarify what exactly dark matter is, Mittig said.
“None of them have been successful after 20, 30, 40 years of research,” he said.
“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 sensing systems physicist at NSCL.
This theory focused on what it calls a dark decay. He postulated that certain unstable nuclei, nuclei that naturally fall apart, might shed dark matter as they fall apart.
So Ayyad, Mittig and their team designed an experiment that could search for dark decay, knowing the odds were stacked against them. But the gamble wasn’t as big as it sounds because probing exotic 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 would that be.
help of a halo
When people imagine a nucleus, many might think of a lumpy ball made up of protons and neutrons, Ayyad said. But the nuclei can take on strange shapes, including what are known as halo nuclei.
Beryllium-11 is an example of a halo nucleus. Is it a way, or isotope, from the element beryllium that has four protons and seven neutrons in its nucleus. It holds 10 of those 11 nuclear particles in a tight central group. But a neutron floats away from that core, loosely attached to the rest of the core, like the moon resonating around Earth, Ayyad said.
Beryllium-11 is also unstable. After a lifetime of about 13.8 seconds, it falls apart by what is known as beta decay. One of its neutrons ejects an electron and becomes a proton. This transforms the nucleus into a stable form of the element boron with five protons and six neutrons, boron-11.
But according to that same hypothetical theory, if the decaying neutron is the one in the halo, beryllium-11 could take an entirely different route: it could undergo dark decay.
In 2019, researchers launched an experiment at Canada’s National Particle Accelerator Facility, TRIUMF, in search of that hypothetical decay. And they found a decay with an unexpectedly high probability, but it wasn’t a dark decay.
It appeared that the loosely bound neutron from beryllium-11 was ejecting an electron like normal beta decay, but the beryllium was not following the known decay path to boron.
The team hypothesized that the high probability of decay could be explained if a state existed in boron-11 as a gateway to another decay, to beryllium-10 and a proton. To anyone keeping count, that meant the core had been converted back to beryllium. Only now it had six neutrons instead of seven.
“This happens just because of the core of the halo,” Ayyad said. “It is a very exotic type of radioactivity. It was actually the first direct evidence for the radioactivity of protons from a neutron-rich nucleus.”
But science welcomes scrutiny and skepticism, and the team’s 2019 report received a healthy dose of both. That “gateway” state in boron-11 did not seem compatible with most theoretical models. Without a solid theory to make sense of what the team saw, different experts interpreted the team’s data differently and offered other possible conclusions.
“We had a lot of long discussions,” Mittig said. “It was a good thing.”
As beneficial as the discussions were, and continue to be, Mittig and Ayyad knew they would have to generate more evidence to support their results and hypotheses. They would have to design new experiments.
NSCL experiments
In the team’s 2019 experiment, TRIUMF generated a beam of beryllium-11 nuclei that the team directed into a detection chamber where the researchers observed different possible decay pathways. That included the process of beta decay to proton emission that created beryllium-10.
For the new experiments, which were carried out in August 2021, the team’s idea was to essentially run the reaction in reverse time. That is, the researchers would start with beryllium-10 nuclei and add a proton.
Collaborators in Switzerland created a source of beryllium-10, which has a half-life of 1.4 million years, that NSCL could use to produce radioactive beams with new reacceleration technology. The technology evaporated and injected the beryllium into an accelerator and allowed the researchers to make a very sensitive measurement.
When beryllium-10 absorbed a proton of the correct energy, the nucleus entered the same excited state that the researchers believed they had discovered three years earlier. It would even spit out the proton again, which can be detected as the signature of the process.
“The results of the two experiments are very compatible,” said Ayyad.
That was not the only good news. Unbeknownst to the team, an independent group of Florida State University scientists had come up with another way to test the 2019 result. Ayyad attended a virtual conference where the Florida State team presented their preliminary results, and what saw encouraged him.
“I took a screenshot of the Zoom meeting and immediately sent it to Wolfi,” he said. “Then we reached out to the Florida State team and found a way to support each other.”
The two teams were in contact while developing their reports, and both scientific publications now appear on the same issue of Physical Review Letters. And the new results are already generating excitement in the community.
“The work is getting a lot of attention. Wolfi will visit Spain in a few weeks to talk about this,” Ayyad said.
An open case on open quantum systems
Part of the excitement is that the team’s work could provide a new case study for what are known as open quantum systems. It’s an intimidating name, but the concept can be thought of as the old adage, “nothing exists in a vacuum.”
Quantum physics has provided a framework for understanding the incredibly tiny components of nature: atoms, molecules, and much, much more. This understanding has advanced virtually every area of physical science, including energy, chemistry, and materials science.
Much of that framework, however, was developed considering simplified scenarios. The supersmall system of interest would be somewhat isolated from the ocean of information provided by the world around it. By studying open quantum systems, physicists are venturing away from idealized scenarios and into the complexity of reality.
Open quantum systems are literally everywhere, but finding one that is tractable enough to learn anything is a challenge, especially in matters of the nucleus. Mittig and Ayyad saw potential in their loosely bound nuclei and knew that NSCL, and now FRIB, could help develop it.
NSCL, a National Science Foundation user facility that has served the scientific community for decades, hosted Mittig and Ayyad’s work, which is the first published demonstration of independent accelerator technology. FRIB, a US Department of Energy Bureau of Science user facility that officially launched on May 22022 is where the work can continue into the future.
“Open quantum systems are a general phenomenon, but they are a new idea in nuclear physics,” Ayyad said. “And most of the theorists who are doing the work are at FRIB.”
But this detective story is still in its first chapters. To complete the case, the investigators still need more data, more evidence to make full sense of what they are seeing. That means Ayyad and Mittig are still doing what they do best and investigating.
“We go ahead and do new experiments,” Mittig said. “The theme of all this is that it’s important to have good experiments with robust analysis.”