Hunting for 'Cracks' in the Standard Model

In the world of research, often the unprecedented, unusual, and unexpected makes the discovery – and sometimes it is the absence of a surprise that moves science forward.

For example, nearly a mile underground, in a remote South Dakota town that once drew prospectors during the Black Hills gold rush, a highly sensitive array of detectors has been gathering terabytes of data on how electrons decay. Analysis by above-ground scientists showed that the way we understand the universe still holds true, culminating in a study published in Nature Physics in April.

With the help of computing power from Lawrence Berkeley National Laboratory’s National Energy Research Scientific Computing Center (NERSC), researchers from the Majorana Collaboration – which includes more than 50 researchers from more than 20 institutions in four countries – used detectors at the Sanford Underground Research Facility (SURF) in Lead, South Dakota, to search for violations of quantum mechanics.

And though finding such violations is “extremely unlikely,” it is nonetheless useful to look for “cracks in the Standard Model of particle physics when we have the most sensitive experiment for such studies,” said Clint Wiseman, a research scientist at the University of Washington and a corresponding author on the new paper titled “Search for charge non-conservation and Pauli exclusion principle violation with the Majorana Demonstrator.”

“Each of these violations of quantum mechanics has kind of its own signature,” Wiseman said. “So basically, there would be a little peak in the data that we really don't expect to see, but if we did, it would indicate there's something funny happening with the Pauli exclusion principle or the lifetime of the electron.”

Setting more stringent limits

The Pauli exclusion principle, a bedrock of quantum mechanics, essentially holds that two electrons cannot simultaneously occupy the same quantum state. The other potential violation the scientists searched for has to do with the rule that charges cannot be created or destroyed, known as charge conservation.

A theoretical violation of charge conservation implies that electrons may have a finite lifetime. That’s why the conservation of electric charge can be tested by searching for the decay of electrons to chargeless particles with lighter mass, such as neutrinos and photons. And while one possible way an electron could theoretically decay is already tightly constrained by a different experiment, the researchers set out to improve limits on a different decay channel.

Utilizing high-purity germanium radiation detectors in the underground facility, which is specially designed to keep background radioactivity as low as possible, the scientists looked for unexpected spikes in the data, triggered by ionization in the germanium.

The group used two datasets: one where the detectors sat in the underground environment – one of the most “radiation-quiet” places in the solar system – to listen for unusual signals and one where the researchers introduced a radiation source into the experiment.

The scientists’ null result – meaning the absence of unusual signals – helped further strengthen confidence in the Standard Model and improve our understanding of how the universe works.

“You use your null result to constrain the space of possible theories that people have proposed,” Wiseman said. “So you set tighter and tighter limits when you don’t find anything and that sort of reins the theory in and constrains it to match the real world.”

Tapping into NERSC’s resources, the scientists stored, cleaned, and combed through mountains of data. Their experiment’s result for the mean lifetime of the electron decaying to three neutrinos or dark matter is the best in more than two decades.

“That’s a remarkable improvement,” said Inwook Kim, a postdoc at Lawrence Livermore National Laboratory and another corresponding author on the paper.

NERSC as a ‘workhorse’ to store, process data

One of the biggest challenges for the scientists was cleaning the data, making sure they filtered out enough noise to properly read their measurements while at the same time not accidentally removing any unusual signals they had their eyes peeled for. The infrastructure at NERSC – including the Cori supercomputer, which has since been retired – proved to be the study’s “workhorse,” Wiseman said.

“When it comes time to do processing, to filter out all the noise, they have the computing resources to carefully analyze datasets many terabytes in size,” he said, referring to NERSC.

The researchers’ work was also proof positive that the data collected at the Majorana Demonstrator had scientific value beyond its original intent. The Majorana Demonstrator was initially conceived and designed to search for one of the rarest forms of radioactive decay, known as neutrinoless double-beta decay.

“Our work has proven that these kinds of detectors can be used for multiple purposes,” Kim said.

Alan Poon, who helped put together the detectors as part of the Majorana Collaboration and is a senior physicist and head of the Neutrinos Program in Berkeley Lab’s Nuclear Science Division, agreed.

“The most exciting part for me is that we can use the Majorana Demonstrator, an instrument designed for something else, for this type of analysis,” he said. “It shows me and the funding agency that the experiment is not a one-trick pony.”

The original experiment, searching for neutrinoless double-beta decay, ran from 2015 through 2021 and was managed by Oak Ridge National Laboratory (ORNL) for the U.S. Department of Energy Office of Nuclear Physics with support from the National Science Foundation. Scientists from the Majorana Collaboration published their final results in February 2023. This newer experiment was also supported by the DOE Office of Nuclear Physics.

Some of the detectors used at the Majorana Demonstrator will now be deployed at the latest iteration of the experiment in Italy, known as LEGEND-200, which is already using NERSC as its platform for computation, simulation, and analysis. The next phase – LEGEND-1000 – has NERSC as its baseline site for analysis and simulation.

Violations could ‘change the understanding of our universe’

Wiseman and Kim’s work is the latest attempt to probe and test the Standard Model, physicists' current best theory of how elementary particles interact. Questioning some of those core principles not only opens the door for potentially spectacular discoveries, it also helps undergird modern physics.

“That's been the main thrust of studies in physics for the last 20 years,” Wiseman said. “We're looking for violations, the things that are not predicted by the Standard Model, because it's so comprehensive.”

Even a small violation of the Pauli exclusion principle, for instance, would have enormous ramifications.

“It would change our understanding of the universe entirely,” Kim said.