A distinguished group of researchers based at Indiana University, in collaboration with international scientific partners, is dedicated to demystifying the profound questions tied to the foundational principles of physics that govern our cosmos. Over the course of the last half-decade, this research group from Indiana University, in partnership with a network of international scholars, has been striving to decode the enigmas intrinsic to the essential laws of physics that command our cosmos. One notable project they undertook is the Majorana Demonstrator, an experiment that has considerably amplified our comprehension of neutrinos, among the universe’s core components.

Neutrinos are minute particles that bear resemblance to electrons but lack electric charge. They are second only to light in terms of abundance in the universe. Their plentiful presence notwithstanding, these particles pose substantial challenges in terms of research, as they do not interact as other particles do. “Neutrinos’ influence pervades all aspects of the universe and impacts physics across all conceivable scales. They surprise us with their interactions at the quantum level and make significant contributions to cosmic phenomena,” says Walter Pettus, an associate professor of physics at the College of Arts and Sciences, Indiana University. “However, they’re equally perplexing to study because our knowledge about them is riddled with holes.”

The Majorana Demonstrator project, which involved 60 scientists from 24 institutions, was conceived to bridge these knowledge gaps by investigating the most fundamental properties of neutrinos. One particular element the researchers wanted to explore was the possibility of neutrinos being their own antiparticles – subatomic particles with the same mass but opposite electric charge. Given the neutrino’s lack of charge, it could potentially be its own antiparticle. Discerning this possibility could elucidate why neutrinos possess mass, which could have broad implications in understanding the origins of the universe.

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In their quest to ascertain if the neutrino could be its own antiparticle, the researchers aimed to witness a rare phenomenon known as neutrinoless double-beta decay. However, a single atom undergoing this process takes a staggering 10^26 years – significantly more than the universe’s age. Therefore, the researchers monitored nearly 10^26 atoms over a span of six years. For the observation of such an extraordinarily rare decay, an immaculate environment was necessary. The researchers found this ideal setting in the Sanford Underground Research Facility in the Black Hills of South Dakota, situated a mile below the surface. They built one of the cleanest and quietest laboratories on Earth, with ultra-pure germanium high-precision detectors enclosed in a 50-ton lead shield, and materials of unequaled purity all around them.

Even the copper used was cultivated underground in their lab, with a level of impurity so minute that it couldn’t be measured. Pettus and a group of IU students primarily oversaw the data analysis from the experiment. Graduate student Nafis Fuad, undergraduate seniors Isaac Baker, sophomore Jennifer James, and Abby Kickbush, a participant in the Research Experiences for Undergraduates Program, contributed to the project. They focused on understanding the experiment’s stability, analyzing the recorded waveform specifics, and characterizing backgrounds.

“Trying to find a single needle in an enormous haystack is a fitting analogy for our research. You must carefully remove all possible ‘hay’ (i.e., backgrounds), without even knowing if there’s a needle to find,” explained Fuad. “Participating in this search is exhilarating.” Despite not observing the anticipated decay, the researchers uncovered that the neutrino’s decay scale is beyond the limit they had established, a finding they intend to probe further in the next experimental phase. They also recorded other scientific outcomes, from dark matter to quantum mechanics, contributing to a more comprehensive understanding of the universe.

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Throughout the project, they demonstrated that the methodologies employed can be scaled up for a potentially revolutionary search that might explain the existence of matter in the universe. “We may not have observed the decay we were seeking, but we’ve raised the bar on our pursuit of the physics we’re targeting,” Pettus expressed. “In keeping with its name, the Demonstrator project advanced crucial technologies that we’re already applying in the next phase of our experiment in Italy. We may not have shattered our understanding of physics yet, but we’ve undoubtedly broadened our horizons. I’m thrilled with our achievements.”

The subsequent phase of the project, known as LEGEND-200, has already started gathering data in Italy and plans to operate for the next five years. The researchers hope to witness the decay at a sensitivity magnitude higher than that of the Majorana Demonstrator. Beyond this, with the support of the U.S. Department of Energy, the team has started designing the follow-up experiment, LEGEND-1000. Pettus anticipates the progression of this work and plans to involve more students in the project, both in terms of data analysis and hardware development for LEGEND-1000.

“Discovering that the neutrino is its own antiparticle won’t change the physical reality of our universe – the fundamental laws of physics remain intact. But such knowledge deepens our understanding of the most fundamental levels of the universe and how it operates. It renders our world richer, possibly weirder, and makes our journey a fundamentally human pursuit,” Pettus added.

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