For half a decade, a team of researchers from Indiana University and their international collaborators have been on a quest to answer pivotal questions about the most basic physical laws governing our universe. The Majorana Demonstrator experiment has been instrumental in advancing the understanding of one of the universe’s most elementary constituents: neutrinos.

Neutrinos, subatomic particles akin to electrons but lacking electric charge, rank second only to light as the most plentiful particles in the universe. Yet, they are among the most elusive and difficult to measure because they do not interact in the same way as other particles. “Neutrinos exert a profound influence on the universe and on physics at all conceivable scales, both at the level of individual particle interactions and on the grand scale of the cosmos,” remarked Walter Pettus, an assistant professor of physics at IU College of Arts and Sciences. “However, studying them can be frustrating, as our knowledge is extensive, but riddled with gaps.”

A collaborative effort of 60 researchers from 24 institutions, the Majorana Demonstrator was designed to address many of these gaps by probing the most fundamental properties of neutrinos. Whether the neutrino may be its own antiparticle—a subatomic particle with the same mass but opposing electric charge—was an intriguing subject that the researchers intended to address. Neutrinos are ideally positioned to potentially act as their own antiparticles because they are the sole uncharged particle in the cosmos. Unraveling this mystery could shed light on why neutrinos possess mass, a revelation with significant implications for understanding the formation of the universe.

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The researchers want to examine a remarkably uncommon occurrence known as neutrinoless double-beta decay in order to determine if the neutrino is its own antiparticle. However, this process takes at least 10^26 years for a single atom, which is significantly longer than the universe’s age. Instead, they opted to observe nearly 10^26 atoms over a six-year period. The researchers needed an impeccably pristine environment to observe this extraordinarily rare decay. So, in the Sanford Underground Research Facility, hidden deep within South Dakota’s Black Hills, scientists built one of the purest and silent settings on Earth. Extremely sensitive detectors made of high purity germanium were encircled by materials of unsurpassed cleanliness and enclosed in a 50 ton lead barrier.

Even the copper utilized was produced underground in their laboratory, boasting such low impurity levels that they were immeasurable. Pettus and a group of IU students were mainly responsible for analyzing data from the experiment. They concentrated on determining the stability of the experiment, deciphering the specifics of the recorded waveforms, and describing backgrounds. The researchers ultimately did not see the degradation they were expected to see. Instead, they discovered that the neutrino’s scale for decay is longer than the limit they had imposed. They plan to further test this in the next phase of the experiment. Additionally, they obtained other scientific results related to dark matter and quantum mechanics, which help to enrich our understanding of the universe.

The researchers demonstrated that their techniques could be employed on a much larger scale in a potentially revolutionary search that could elucidate the existence of matter in the universe. The next phase of the project, LEGEND-200, is already underway in Italy, with plans to run for the next five years. The researchers aim to observe decay at a sensitivity level that is an order of magnitude higher than that of the Majorana Demonstrator. Thanks to support from the U.S. Department of Energy, they are also designing the successor experiment, LEGEND-1000. Pettus is enthusiastic about the project’s future and plans to involve more students in both data analysis and hardware development for LEGEND-1000.

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Pettus stated, “Should we find out that the neutrino is indeed its own antiparticle, our surroundings will stay the same, with terra firma below and celestial bodies above, since our enhanced comprehension of physics doesn’t change the longstanding physical laws governing our universe.” He added, “Nonetheless, exploring the deepest elements of the cosmos and decoding its mechanics allows us to experience a more complex, enthralling world to live in—or maybe simply a more peculiar one. This quest is an inherent part of being human.” The Majorana Demonstrator was overseen by Oak Ridge National Laboratory for the U.S. Department of Energy Office of Nuclear Physics and received support from the National Science Foundation. This groundbreaking research marks a significant step toward deepening our understanding of the universe and the fundamental laws that shape it.

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