About three years ago, Wolfgang "Wolfi" Mittig and Yassid Ayyad began looking for dark matter --also known as missing mass in the universe. Although their explorations did not discover dark matter, scientists discovered something that had never been seen before and could not be explained. "It's like a detective story," said Mittig, a Hanner specialist in the Department of Physics and Astronomy at Michigan State University and a faculty member of the Rare Isotope Beam Facilities (FRIB for short). "We started looking for dark matter, but we didn't find it. Instead, we found something else that was challenging to explain the theory." To make their findings meaningful, the team continued to work, conducting further tests and accumulating more data. Mittig, Ayyad and their colleagues reinforced their argument at the National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University .
researchers discovered a new route to their unexpected destination while working at NSCL, which they disclosed in the journal Physical Review Express . In addition, they reveal the intriguing physics’ work in the ultra-small quantum field of subatomic particles.
Scientists specifically show that even if a atom center or nucleus is filled with neutron , it can find a route to a more stable configuration by spitting out a proton.
Dark matter is one of the most well-known but least known things in the universe. Scientists have known for decades that the universe contains more mass than we can perceive according to the motion of stars and galaxies. Although researchers are convinced of the existence of dark matter, they have not found its location and have not designed how to detect it directly. "Find dark matter is one of the main goals of physics," said Ayyad, a nuclear physics researcher at the Galician Institute of High Energy Physics (IGFAE) at the University of Santiago de Compostela in Spain. Scientists have launched about 100 experiments to try to shed light on what dark matter is. "After 20, 30, 40 years of research, no one succeeded," he said. "
"But there is a theory, a very hypothetical idea that you can use a very special nucleus to observe dark matter," said Ayyad, who was a detector system physicist in NSCL. The core of the theory of
is its so-called dark decay. It assumes that some unstable nuclei, i.e., naturally disintegrating nuclei, can discard dark matter when it disintegrates.
So Ayyad, Mittig and their team designed an experiment that could look for dark decay, and they knew that this possibility was bad for them. But the gambling isn't as big as it sounds, as detecting heterogeneous decay also gives researchers a better understanding of the rules and structures of the nuclear and quantum worlds.
researchers have a great chance to discover something new. The question is what that would be. When people imagine a nucleus, many people may think of a blocky sphere composed of protons and neutrons. But atomic nuclei can take strange shapes, including so-called halo nuclei.
beryllium-11 is an example of halo nucleus. It is a form of beryllium element, or isotope , and its nucleus contains four protons and seven neutrons. It keeps 10 of these 11 nuclear particles in a tight central cluster. But there is a neutron floating away from the core, loosely bonding with the rest of the nucleus, a bit like the moon orbiting the Earth.
beryllium-11 is also unstable. After a lifespan of about 13.8 seconds, it disintegrates by so-called β decay . One of its neutrons shoots out an electron and becomes a proton. This transforms the nucleus into a stable form of boron element with five protons and six neutrons, i.e. boron-11.
But according to that very hypothetical theory, if the decayed neutron is the one in the halo, then beryllium-11 may take a completely different route. It may experience a dark decay.
In 2019, researchers launched an experiment at the National Particle Accelerator Facility in Canada to find that very hypothetical decay .And they did find a decay with unexpected high probability, but it was not a dark decay. It looks like the loosely bound neutron of beryllium-11 ejecting an electron like a normal beta decay, but beryllium does not follow the known decay path to become boron.
The research team hypothesized that if one state in boron-11 exists as the inlet for another decay, i.e., decays to beryllium-10 and a proton, then the high probability of this decay can be explained. For anyone who scores, this means that the nucleus becomes beryllium again. It's just now it has six neutrons instead of seven. "This happens only because of the halo nucleus. It's a very peculiar type of radioactivity. It's actually the first direct evidence of proton radioactivity from neutron-rich nuclei."
However, science welcomes review and doubt, and the team's 2019 report was affected by both health. The kind of "doorway" state in Boron-11 seems to be incompatible with most theoretical models. Since there is no solid theory to explain what the team sees, different experts have different interpretations of the team's data and have come up with other potential conclusions.
"We have a lot of long discussions," Mittig said. "This is a good thing."
Although these discussions are beneficial -- Mittig and Ayyad know they must generate more evidence to support their results and hypotheses. They have to design new experiments.
NSCL experiments
In the team's 2019 experiment, TRIUMF produced a bunch of beryllium-11 nuclei, which the team introduced into a test chamber where the researchers observed different possible decay routes. This includes the beta decay to the proton emission process that produces beryllium-10.
For the new experiment conducted in August 2021, the team's idea is to respond to essentially running time reversal. That is, the researchers will add a proton starting from the beryllium-10 nucleus.
Swiss collaborators have created a source of beryllium-10 with a half-life of 1.4 million years, and then NSCL can use new reaccelerator technology to produce radioactive beams. The technology evaporates beryllium and injects it into the accelerator, allowing researchers to make highly sensitive measurements.
When beryllium-10 absorbs a proton of proper energy, the nucleus enters the same excited state that researchers think they discovered three years ago. It even spits out protons, which can be detected as a marker of the process. "The results of these two experiments are very compatible," Ayyad said.
This is not the only good news. What the team didn't know is that an independent team of scientists at Florida State University has designed another way to detect 2019 results. Ayyad happened to attend a virtual meeting where the Florida team showed its initial results and he was encouraged by what he saw.
He said, "I got a screenshot of the Zoom meeting and sent it to Wolfi immediately. We then contacted the team in Florida and developed a mutually supportive approach."
The two teams kept in touch while writing the report, and both scientific publications are now in the same issue of Physical Review Letters. And the new results have already caused a sensation in society.
"This work has attracted a lot of attention. Wolfi will visit Spain in a few weeks to talk about the issue," Ayyad said.
A public case about open quantum systems
is partly excited because the team's work can provide a new case study for so-called open quantum systems. Quantum Physics provides a framework to understand the incredible tiny components of nature: atoms, molecules and more, more. This understanding drives nearly every field of physical science, including energy, chemistry and materials science.
However, most of the content of this framework is developed with simplification in mind. The ultra-small system in question will somehow be isolated from the ocean of input provided by the world around it. When studying open quantum systems, physicists are moving away from idealized scenarios and entering the complexity of reality.
Open quantum systems are simply everywhere, but finding a quantum system that is practical enough to learn something is challenging, especially when it comes to atomic nuclei. Mittig and Ayyad saw the potential of loosely bound nuclei, and they knew that NSCL, and now FRIB could help develop it.
NSCL is a user facility for National Science Foundation , serving the scientific community for decades, and it chairs the work of Mittig and Ayyad, the first public display of independent reaccelerator technology. FRIB is a user facility in the U.S. Department of Energy’s Office of Science and officially launched on May 2, 2022, and is a place where you can continue to work in the future.
"Open quantum systems are a common phenomenon, but they are a new idea in nuclear physics," Ayyad said. "And most theorists doing this work are in FRIB."
But this "detective story" is still in the early chapter. To complete this "case", researchers still need more data and more evidence to fully understand what they are seeing. This means Ayyad and Mittig are still doing what they do best, conducting investigations.
"We are continuing to do new experiments," Mittig said. "The theme throughout this is that it is important to have good experiments and strong analysis."
