Ghost hunting of the universe: unraveling the enigma of the neutrino

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Abstract concept of astrophysics light dimension portal

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The Majorana Demonstrator, a six-year experiment conducted by Indiana University researchers and international collaborators, sought to answer significant questions about the fundamental laws of physics, particularly as they pertain to neutrinos. The study aimed to observe whether neutrinos could be their own antiparticles and the occurrence of neutrinoless double beta decay, which, although not conclusively observed, has provided valuable insights into neutrino decay times, dark matter, quantum mechanics and demonstrated that the research techniques used can be expanded for future work in understanding the composition of universes.

A team of Indiana University researchers, along with international collaborators, is actively engaged in unraveling the fundamental mysteries surrounding the fundamental laws of physics that govern our universe.

For the past six years, a team of Indiana University researchers, along with international collaborators, has been working to unravel the mysteries of the fundamental physical laws that govern our universe. They conducted an experiment known as the Majorana Demonstrator, which greatly improved our understanding of neutrinos, one of the fundamental building blocks of the universe.

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The final report of the experiments was recently published in Neutrinos, tiny particles comparable to electrons but devoid of electric charge, rank as the second most plentiful particles in the universe, trailing only light. Despite this abundance, they prove challenging to study because they do not interact the way other particles do.

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Neutrinos have a profound impact on the universe and physics at every imaginable scale, surprising us down at the particle interaction level and having broad impact up through the cosmic scales, said Walter Pettus, an assistant professor of physics in the IU College of Arts and Sciences. But they are also the most frustrating to study because we know so much about them, yet we have so many gaps.

Nafis Fuad

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Nafis Fuad. Credit: Indiana University

The Majorana Demonstrator, a collaboration of 60 researchers from 24 institutions, was designed to fill many of those gaps at the same time, probing into the most fundamental properties of neutrinos.

One aspect they hoped to observe was whether the neutrino could be its own antiparticle a subatomic particle of the same mass but with the opposite electric charge. Since the neutrino is uncharged, it is the only particle in the universe that could be its own antiparticle. Understanding that would provide insight into why the neutrino has mass in the first place information that would have widespread impacts in understanding how the universe was formed.

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To determine if the neutrino is its own antiparticle, the researchers needed to observe a rare occurrence called neutrinoless double-beta decay. However, this process takes a single atom at least 1026 years significantly longer than the age of the universe. Instead, they chose to observe nearly 1026 atoms over the course of six years.

To observe this incredibly rare decay, the researchers needed the perfect environment. In the Sanford Underground Research Facility in the Black Hills of South Dakota, located a mile underground, they built one of the cleanest and quietest environments on Earth. Extremely sensitive detectors were made of high-purity germanium and were packed in a 50-ton lead shield and surrounded by materials of unprecedented cleanliness. Even the copper used was grown underground in their lab with impurity levels so low they couldnt be measured.

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Pettus and a team of IU students were responsible primarily for analyzing data from the experiment. Graduate student Nafis Fuad, undergraduate senior Isaac Baker, sophomore Abby Kickbush and Jennifer James, a student with the Research Experiences for Undergraduates Program, have been involved in the project. Their focus has been on understanding the stability of the experiment, analyzing details of the recorded waveforms, and characterizing backgrounds.

Walter Pettus

Walter Pettus. Credit: Indiana University

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Its like looking for a tiny needle in a very, very, very big haystack you have to carefully get rid of all the hays (a.k.a. backgrounds) possible, and you dont even know if theres actually a needle in there in the first place or not, Fuad said. Its very exciting to be a part of that search.

While the researchers ultimately did not observe the decay they hoped for, they did discover that the neutrinos scale for decay is longer than the limit they placed on it, which they will test further during the next phase of the experiment. In addition, they recorded other scientific results ranging from dark matter to quantum mechanics that helps provide a better understanding of the universe.

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Through the project, the researchers proved that the techniques they utilized could be used at a much larger scale in a potentially game-changing search that could help explain the existence of matter in the universe.

We didnt see the decay we were looking for, but we have raised the bar on where to look for the physics were going after, Pettus said. True to its name, the Demonstrator advanced critical technologies that we are already leveraging for the next phase of the experiment in Italy. We may not have broken our picture of physics yet, but weve certainly pushed the horizons, and I am very excited about what we have accomplished.

The next phase of the project, called LEGEND-200, has already begun taking data in Italy, with plans to run over the next five years. Researchers aim to observe the decay happening at a magnitude higher sensitivity than the Majorana Demonstrator. Beyond that, thanks to support from the U.S. Department of Energy, the team is already designing the successor experiment, LEGEND-1000.

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Pettus is excited about the future of this work and looks forward to involving more students in the project, both in data analysis and hardware development for LEGEND-1000.

If we discover the neutrino is its own antiparticle, there will still be ground under our feet and stars in the sky, and our understanding of physics doesnt change the reality of the physical laws that always have and continue to govern our universe, Pettus said. But knowing whats down there at the most fundamental level and how the universe works gives us a richer, more beautiful world to live in or possibly just weirder and that pursuit is fundamentally human.

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Reference: Final Result of the Majorana Demonstrators Search for Neutrinoless Double- Decay in 76Ge by I.J. Arnquist et al. (Majorana Collaboration), 10 February 2023, Physical Review Letters.
DOI: 10.1103/PhysRevLett.130.062501

The Majorana Demonstrator was managed by Oak Ridge National Laboratory for the U.S. Department of Energy Office of Nuclear Physics, with support from the National Science Foundation.


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