Recipe for discovery
EAST LANSING, Michigan – About three years ago, Wolfgang “Wolfi” Mittig and Yassid Ayyad set out in search of the universe’s lost mass, better known as dark matter, in the heart of an atom.
Their expedition didn’t lead them to dark matter, but they still discovered something never seen before, something that defied explanation. Well, at least an explanation that everyone can agree on.
“It’s like a detective story,” said Mittig, Hannah Distinguished Professor in Michigan State University’s Department of Physics and Astronomy and 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 find other things that challenge the theory to explain.”
So the team got back to work, conducting more experiments, gathering more evidence to make their findings plausible. Mittig, Ayyad and their colleagues strengthened 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 June 28 in the journal Physical Review Letters. By doing so, they also reveal the interesting physics that is going on in the ultra-small quantum realm of subatomic particles.
Specifically, the team confirmed that when an atom’s nucleus, or nucleus, is filled with neutrons, it can still find a way to a more stable configuration by spewing protons instead.
Shooting in the dark
Dark matter is one of the most well-known 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.
In order for gravity to keep the celestial bodies on track, there must be some invisible mass and plenty of it — six times the amount of ordinary matter we can observe, measure, and characterize. Although scientists believe dark matter is out there, they haven’t found where it is and have found a way to detect it directly.
“Finding dark matter is one of the main goals of physics,” said Ayyad, a nuclear physics researcher at the Galicia Institute of High Energy Physics, or IGFAE, of the University of Santiago de Compostela in Spain.
Speaking in whole numbers, scientists have launched about 100 experiments to try to explain what dark matter really is, Mittig said.
“None of them made it after 20, 30, 40 years of research,” he said.
“But there’s a theory, a very hypothetical idea, that you could observe dark matter with a very special type of nucleus,” said Ayyad, who was previously a detector systems physicist at NSCL.
This theory centers on what is called dark decay. This suggests that certain unstable nuclei, nuclei that naturally fall apart, can throw off dark matter as they disintegrate.
So Ayyad, Mittig and their team devised an experiment that could search for dark decay, knowing the odds were against them. But the stakes aren’t as big as they sound because investigating exotic decay also allows researchers to better understand the rules and structure of the nuclear and quantum worlds.
Researchers have a good chance of discovering something new. The question is what will happen.
Help from hello
When one imagines a nucleus, many may think of a viscous sphere made up of protons and neutrons, Ayyad said. But nuclei can take on strange shapes, including what’s known as a core halo.
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. That keeps 10 of the 11 nuclear particles in a dense central cluster. But one neutron floats away from that nucleus, loosely bound to the other, like the moon revolving around Earth, Ayyad said.
Beryllium-11 is also unstable. After a lifetime of about 13.8 seconds, it is destroyed by what is known as beta decay. One of the neutrons gives off an electron and becomes a proton. This turns the nucleus into a stable form of the element boron with five protons and six neutrons, boron-11.
But according to that highly hypothetical theory, if the decaying neutron was the one in the halo, beryllium-11 could go a completely different route: It could undergo dark decay.
In 2019, researchers launched an experiment at Canada’s national particle accelerator facility, TRIUMF, to look for that hypothetical decay. And they did find an unexpectedly high probability of decay, but it wasn’t dark decay.
It seems that the loosely bound neutrons of beryllium-11 eject electrons like normal beta decay, but beryllium doesn’t follow any known decay path toward boron.
The team hypothesized that the most likely decay could be explained if the state in boron-11 existed as a doorway to other decays, beryllium-10 and protons. For whoever recorded the score, that meant the nucleus had once again become beryllium. Only now it has six neutrons instead of seven.
“This happens only because of the halo nucleus,” Ayyad said. “This is a very exotic type of radioactivity. That’s 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 is met with a healthy dose of both. The state of the “door” in boron-11 does not seem to fit most theoretical models. Without a sound solid theory of what the team saw, different experts interpreted the team’s data differently and offered other potential conclusions.
“We had a lot of long discussions,” Mittig said. “That’s a good thing.”
As useful as the discussion was — and it continues — Mittig and Ayyad knew they had to produce more evidence to support their results and hypotheses. They had to design new experiments.
NSCL Experiment
In the team’s 2019 experiment, TRIUMF produced a beam of beryllium-11 nuclei that the team directed into a detection chamber where the researchers observed different possible decay routes. That includes beta decay to the proton emission process that creates beryllium-10.
For the new experiment, taking place in August 2021, the team’s idea is basically to run reverse reaction time. That is, the researchers would start with a beryllium-10 nucleus and add protons.
Collaborators in Switzerland created a source of beryllium-10, which has a half-life of 1.4 million years, which NSCL can then use to produce radioactive rays with the new reaccelerator technology. The technology vaporizes and injects beryllium into the accelerator and allows the researchers to make highly sensitive measurements.
When beryllium-10 absorbs a proton with the right energy, the nucleus enters the same excited state that the researchers believed they had discovered three years earlier. It will even spew back protons, which can be detected as a sign of the process.
“The results of the two experiments match very well,” said Ayyad.
That’s not the only good news. Unbeknownst to the team, a group of independent scientists at Florida State University had found another way to investigate the 2019 results. Ayyad happened to be attending a virtual conference where the Florida State team presented its initial results, and he was encouraged by what he saw.
“I took a screenshot of the Zoom meeting and sent it straight to Wolfi,” he said. “Then we contacted the Florida State team and looked for ways to support each other.”
The two teams were in touch as they developed their report, and both scientific publications now appear in the same issue of Physical Review Letters. And the new results are already generating buzz in the community.
“The work received a lot of attention. Wolfi will visit Spain in a few weeks to talk about this,” Ayyad said.
The open case of an open quantum system
Part of the excitement is that the team’s work can provide new case studies for what are known as open quantum systems. It’s a scary 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 infinitesimal components of nature: atoms, molecules, and more. This understanding has advanced almost every field of physical science, including energy, chemistry and materials science.
However, most of the frameworks were developed with simplified scenarios in mind. An infinitesimal system of flowers will be isolated in some way from the sea of inputs provided by the world around it. In studying open quantum systems, physicists venture away from ideal scenarios and into the complexities of reality.
Open quantum systems really are ubiquitous, but finding one that’s docile enough to study anything is challenging, especially in the case of the nucleus. Mittig and Ayyad saw potential in their loosely tied core and they knew that the NSCL, and now the 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 was the first published demonstration of stand-alone reaccelerator technology. FRIB, a US Department of Energy Office of Science user facility officially launched on May 2, 2022 is where work can resume in the future.
“Open quantum systems are a common phenomenon, but they are a new idea in nuclear physics,” Ayyad said. “And most of the theorists who do the work are in the FRIB.”
But this detective story is still in its early chapters. To complete the case, the researchers still need more data, more evidence to fully understand what they are seeing. That meant Ayyad and Mittig were still doing their best and investigating.
“We’re going to go ahead and make new experiments,” Mittig said. “The theme through all of this is it’s important to have a good experiment with robust analysis.”
NSCL is a national user facility funded by the National Science Foundation, supporting the mission of the Nuclear Physics program in the Physics Division of the NSF.
Michigan State University (MSU) operates the Facility for Rare Isotope Beams (FRIB) as a user facility for the US Department of Energy’s Office of Science (DOE-SC), supporting the mission of the DOE-SC Office of Nuclear Physics. Hosting what is designed to be the most powerful heavy ion accelerator, FRIB enables scientists to make discoveries about the properties of rare isotopes to better understand nuclear physics, nuclear astrophysics, fundamental interactions and applications to society, including in medicine, homeland security and industry.
US Department of Energy Office of Science
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