Hunting the Ghosts of the Universe: Unraveling the Neutrino Enigma

The Majorana Demonstrator, a six-year research initiative led by scholars from Indiana University along with international colleagues, aimed to address profound queries about fundamental physics laws, with a focus on neutrinos. The project’s goal was to determine if neutrinos could be their own antiparticles and if neutrinoless double-beta decay could be observed. Even though the latter was not definitively seen, it delivered crucial insights about neutrino decay durations, dark matter, quantum mechanics, and confirmed that the applied research methodologies can be expanded for future endeavors to comprehend the makeup of the universe.

The group of investigators from Indiana University, working hand-in-hand with international allies, has been passionately involved in solving the essential mysteries associated with the foundational laws of physics that control our universe.

Over the past six years, these scholars have been diligently working on an experiment known as the Majorana Demonstrator, which has greatly enhanced our comprehension of neutrinos, the essential building blocks of the universe.

The concluding findings of the experiment were recently disclosed in the Physical Review Letters.

Neutrinos, minute particles that are akin to electrons but lack an electric charge, are the second most abundant particles in the universe, just behind light. Despite their prevalence, they are difficult to study as their interaction is unlike other particles.

“Neutrinos have a deep impact on the universe and physics at all conceivable scales, astonishing us at the particle interaction level and having a wide influence up to cosmic scales,” stated Walter Pettus, an assistant professor of physics at the IU College of Arts and Sciences. “However, they are the most challenging to study as our knowledge about them is extensive yet incomplete.”

The Majorana Demonstrator, a joint venture of 60 researchers from 24 institutions, was designed to bridge many of these knowledge gaps, delving into the most intrinsic properties of neutrinos.

One of their objectives was to confirm whether a neutrino could be its own antiparticle – a subatomic particle identical in mass but with reverse electric charge. As neutrinos are uncharged, they stand as the only particles in the universe that could potentially be their own antiparticles. Understanding this could offer insight into why neutrinos possess mass in the first place – data that could potentially unravel the mystery of the universe’s formation.

To ascertain if a neutrino is its own antiparticle, the researchers attempted to observe a rare event called neutrinoless double-beta decay. However, this process requires a single atom at least 10^26 years – significantly longer than the age of the universe. So, they opted to monitor nearly 10^26 atoms over a six-year span.

For observing this extremely rare decay, an ideal environment was essential. The Sanford Underground Research Facility in the Black Hills of South Dakota provided such an environment. Located a mile underground, this facility boasts one of the cleanest and quietest environments on Earth. Highly sensitive detectors made of pure germanium were enclosed in a 50-ton lead shield and surrounded by unprecedentedly clean materials. Even the used copper was cultivated underground in their lab with immeasurably low impurity levels.

A team of IU students, led by Pettus, were primarily responsible for interpreting data from the experiment. Graduate student Nafis Fuad, senior undergraduate Isaac Baker, sophomore Abby Kickbush, and Jennifer James, a Research Experiences for Undergraduates Program student, contributed to the project. They focused on understanding the stability of the experiment, analyzing the recorded waveforms, and characterizing backgrounds.

“It’s akin to searching for a microscopic needle in an overwhelmingly large haystack – you have to meticulously eliminate all possible distractions (a.k.a. backgrounds) and you don’t even know if the needle is there in the first place,” Fuad explained. “Being a part of this hunt is thrilling.”

Although the researchers were unable to observe the decay they anticipated, they did ascertain that the neutrino’s decay timeframe is longer than the limit they proposed, which they will scrutinize further in the next phase of the experiment. Additionally, they noted other scientific outcomes – ranging from dark matter to quantum mechanics – which aids in improving our understanding of the universe.

The experiment proved that the techniques employed can be scaled up, paving the way for future research that could fundamentally alter our understanding of the universe’s existence.

“We may not have observed the decay we were hunting for, but we have broadened the horizon of where to seek the physics we’re chasing after,” Pettus said. “True to its name, the Demonstrator pioneered critical technologies that we’re already applying in the next phase of the experiment in Italy. We may not have shattered our understanding of physics yet, but we’ve certainly pushed boundaries, and I am immensely thrilled about what we have achieved.”

The subsequent phase of the project, dubbed LEGEND-200, has already started collecting data in Italy, with plans to run for the next five years. Researchers aim to observe decay at a magnitude higher sensitivity than the Majorana Demonstrator. Furthermore, with support from the U.S. Department of Energy, the team is already designing the successor experiment, LEGEND-1000.

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 doesn’t change the reality of the physical laws that always have and continue to govern our universe,” Pettus said. “But knowing what’s 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.”

Reference: “Final Result of the Majorana Demonstrator’s Search for Neutrinoless Double-β 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 overseen by Oak Ridge National Laboratory for the U.S. Department of Energy Office of Nuclear Physics, with support from the National Science Foundation.

Frequently Asked Questions (FAQs) about Neutrino Research

More about Neutrino Research

• Indiana University Physics Department
• The Majorana Demonstrator Project
• Sanford Underground Research Facility
• About Neutrinos
• LEGEND Experiment
• U.S. Department of Energy Office of Nuclear Physics