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.
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 Mittig, Hannah Professor Emeritus 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 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
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 the celestial objects attached to their paths, there had to be an invisible mass 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.
“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, of the element beryllium that 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.
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 decay probability 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 only happens because of the halo core,” Ayyad said. “It’s a very exotic type of radioactivity. It was actually the first direct evidence of proton radioactivity from a neutron-rich nucleus.”
But science welcomes scrutiny and skepticism, and the team’s 2019 report got a healthy dose of both. This “gate” state in boron-11 did not seem compatible with most theoretical models. Without a solid theory that makes sense of what the team saw, different experts interpreted the team’s data differently and offered other potential conclusions.
“We had a lot of long talks,” Mittig said. “It was a good thing.”
As beneficial as the discussions were – and continue to be – Mittig and Ayyad knew they would need to generate more evidence to support their findings and hypotheses. They should design new experiences.
In the team’s 2019 experiment, TRIUMF generated a beam of beryllium-11 nuclei that the team directed into a detection chamber where researchers observed different possible decay pathways. This included the proton emission beta decay process that created beryllium-10.
For the new experiments, which took place in August 2021, the team’s idea was basically to run the reaction reversed in time. In other words, researchers would start with beryllium-10 nuclei and add a proton.
Collaborators in Switzerland have created a source of beryllium-10, which has a half-life of 1.4 million years, which NSCL could then use to produce radioactive beams with new reaccelerator 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 right energy, the nucleus entered the same excited state the researchers thought they discovered three years earlier. It would even spit out the proton, which can be detected as the signature of the process.
“The results of the two experiments are very compatible,” Ayyad said.
That wasn’t the only good news. Unbeknownst to the team, an independent group of Florida State University scientists had come up with an alternate way to probe the 2019 result. Ayyad attended a virtual conference where the Florida State team presented his preliminary results, and he was encouraged by what he saw.
“I took a screenshot of the Zoom meeting and immediately sent it to Wolfi,” he said. “Then we contacted the Florida State team and found a way to support each other.”
The two teams were in contact during the development of their reports, and the two scientific publications now appear in the same issue of Physical Review Letters. And the new results are already creating buzz in the community.
“The work is getting a lot of attention. Wolfi will be going to Spain in a few weeks to talk about it,” 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 so-called open quantum systems. It’s an intimidating name, but the concept can be seen 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 more. This understanding has advanced virtually every field of physical science, including energy, chemistry, and materials science.
Much of this framework, however, was developed with simplified scenarios in mind. The super small system of interest would be isolated in some way from the input ocean provided by the world around it. By studying open quantum systems, physicists venture away from idealized scenarios and into the complexity of reality.
Open quantum systems are literally everywhere, but finding one that’s manageable enough to learn something from is a challenge, especially when it comes to the core. Mittig and Ayyad saw the potential in their loosely bound cores and they 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 autonomous reaccelerator technology. FRIB, a U.S. Department of Energy Office of Science user facility that officially launched on May 2, 2022, is where work can continue going forward.
“Open quantum systems are a general phenomenon, but it’s a new idea in nuclear physics,” Ayyad said. “And most of the theorists doing the work are at FRIB.”
But this detective novel is still only in its first chapters. To complete the case, researchers still need more data, more evidence to make sense of what they are seeing. This means Ayyad and Mittig are still doing what they do best and investigating.
“We’re moving forward and having new experiences,” Mittig said. “The theme through all of this is that it’s important to have good experiences with solid analytics.”