Swimming against the tide: Scientists unveil the unique diet of garden eels

Halos and Dark Matter: A Recipe for Discovery: No, scientists still don’t know what dark matter is. But MSU scientists helped uncover new physics while searching for it.

About three years ago, Wolfgang “Wolfi” Mittig and Yassid Ayyad set out to find the missing mass of the universe, better known as dark matter, at the heart of an atom.

Their expedition didn’t take them to dark matter, but they still found something that had never been seen before, something that defied explanation. Well, at least an explanation everyone could agree on.

“It was kind of a detective story,” said Mittig, Hannah Distinguished Professor in Michigan State University’s Department of Physics and Astronomy and a faculty member at the Facility for Rare Isotope Beams (FRIB).

“We started looking for dark matter and didn’t find it,” he said. “Instead, we found other things that were difficult for the theory to explain.”

So the team got back to work, running more experiments and gathering more evidence to make their discovery meaningful. Mitten, Ayyad and their colleagues supported their case at the National Superconductor Cyclotron Laboratory (NSCL) at Michigan State University.

Working at NSCL, the team found a new path to their unexpected goal, which they detailed in the Journal on June 28 Physical Verification Letters. In doing so, they also revealed interesting physics underway at the ultra-small quantum scale of subatomic particles.

In particular, the team confirmed that when an atom’s nucleus or nucleus is crowded with neutrons, it can still find a way to a more stable configuration by spewing out a proton instead.

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 tell from the trajectories of stars and galaxies.

For gravity to hold celestial objects in their orbits, there had to be invisible mass, and lots of it — six times the amount of normal matter that we can observe, measure, and characterize. Although scientists are convinced that dark matter is out there, 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 (IGFAE) at the University of Santiago de Compostela in Spain.

To speak in round numbers, scientists have launched about 100 experiments to try to figure out exactly what dark matter is, Mittig said.

“None of them have had success 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 specific type of nucleus,” said Ayyad, who was previously a detector systems physicist at NSCL.

This theory focused on what she calls a dark decay. It postulated that certain unstable nuclei, nuclei that naturally fall apart, could shed dark matter as they decay.

So Ayyad, Mitten and their team designed an experiment that could look for a dark decay, knowing the odds were stacked against them. But the gamble wasn’t as big as it sounds, because studying exotic decays also allows researchers to better understand the rules and structures of the atomic and quantum worlds.

The researchers had a good chance of discovering something new. The question was what would happen.

Help from a halo

When people picture a nucleus, many might think of it as a lumpy ball of protons and neutrons, Ayyad said. But cores can take strange shapes, including so-called halo cores.

Beryllium-11 is an example of halo nuclei. It is a form or isotope of the element beryllium, which has four protons and seven neutrons in its nucleus. It holds 10 of those 11 core particles in a tight central cluster. But a neutron floats far away from that nucleus, loosely bound to the rest of the nucleus, much like the moon orbits the Earth, Ayyad said.

Beryllium-11 is also unstable. After a lifetime of about 13.8 seconds, it decays through what is known as beta decay. One of its neutrons ejects an electron and becomes a proton. This turns the atomic nucleus into a stable form of the element boron with five protons and six neutrons, boron-11.

But according to this very hypothetical theory, if the neutron that’s decaying is the one in the halo, beryllium-11 could go a very different route: it could undergo dark decay.

In 2019, researchers launched an experiment at Canada’s national particle accelerator facility, TRIUMF, to look for this very hypothetical decay. And they found a decay with an unexpectedly high probability, but it wasn’t a dark decay.

It appeared that the beryllium-11’s loosely bound neutron was ejecting an electron as in normal beta decay, but the beryllium did not follow the known decay path to boron.

The team hypothesized that the high probability of the decay could be explained if one state in boron-11 existed as a gateway to another decay, beryllium-10 and a proton. To anyone keeping count, that meant the core had gone back to beryllium. Only now it had six instead of seven neutrons.

“It’s only happening 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 was greeted with a healthy dose of both. This “initial state” in boron-11 appeared to be incompatible 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 conversations,” said Mittig. “It was a good thing.”

As useful as the discussions were – and continue to be – Mitte and Ayyad knew they needed to gather more evidence to support their findings and hypotheses. You would have to design new experiments.

The NSCL experiments

In the team’s 2019 experiment, TRIUMF generated a beam of beryllium-11 nuclei, which the team aimed into a detection chamber, where the researchers observed various possible decay pathways. This included beta decay to the proton emission process that produced beryllium-10.

For the new experiments, which took place in August 2021, the team’s idea was to essentially perform the time-reversed reaction. That means the researchers would start with beryllium-10 nuclei and add a proton.

Collaborators in Switzerland created a beryllium-10 source with a half-life of 1.4 million years, which NSCL could then use to generate radioactive beams using new Rebe accelerator technology. The technology vaporized and injected the beryllium into an accelerator, allowing the researchers to make a highly sensitive measurement.

When beryllium-10 absorbed a proton with the right energy, the nucleus entered the same excited state that the researchers thought they discovered three years earlier. It would even spit out the proton again, which can be detected as a signature of the process.

“The results of the two experiments are very compatible,” said Ayyad.

That wasn’t the only good news. Unbeknownst to the team, an independent group of scientists from Florida State University had found another way to study the 2019 result. Ayyad happened to be attending a virtual conference where the Florida State team was presenting their preliminary findings, 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 reached out to the Florida State team and found a way to support each other.”

The two teams were in contact as they prepared their reports, and both scientific papers now appear in the same issue of Physical Review Letters. And the new results are already causing a stir in the community.

“The work gets a lot of attention. Wolfi will visit 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 you can think of the concept as the old saying goes: “Nothing exists in a vacuum”.

Quantum physics has provided a framework for understanding nature’s incredibly tiny components: atoms, molecules, and more. This understanding has advanced virtually every area of ​​science, including energy, chemistry, and materials science.

However, much of this framework was designed with simplified scenarios in mind. The super small system of interest would be somewhat isolated from the sea of ​​inputs provided by the world around it. When 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 manageable enough to learn is a challenge, especially at the core. Mitte and Ayyad saw potential in their loosely bound cores and they knew 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 standalone Rebe accelerator technology. FRIB, a U.S. Department of Energy Office of Science user facility that officially opened on May 2, 2022, is where work can continue 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 doing the work are at FRIB.”

But this detective story is still in its infancy. To close the case, researchers need more data, more evidence, to fully understand what they are seeing. That means Ayyad and Mitten are still doing what they do best and investigating.

“We’re going ahead and doing new experiments,” said Mittig. “The theme in all of this is that it’s important to have good experiments with strong analytics.”

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