Physicists soon will be closer than ever to answering fundamental questions about the origins of the universe by learning more about its tiniest particles.
University of Cincinnati Professor Alexandre Sousa in a new paper outlined the next 10 years of global research into the behavior of neutrinos, particles so tiny that they pass through virtually everything by the trillions every second at nearly the speed of light.
Neutrinos are the most abundant particles with mass in the universe, so scientists want to know more about them.
They are created by nuclear fusion reactions in the sun, radioactive decay in nuclear reactors or the Earth’s crust or in particle accelerator labs. As they travel, they can transition between one of three types or “flavors” of neutrinos and back.
But unexpected experimental results made physicists suspect there might be another neutrino flavor, called a sterile neutrino because it appears immune to three of the four known “forces.”
“Theoretically, it interacts with gravity, but it has no interaction with the others, weak nuclear force, strong nuclear force or electromagnetic force,” Sousa said.
In a new white paper published in the Journal of Physics G: Nuclear and Particle Physics, Sousa and his co-authors discuss experimental anomalies in neutrino exploration that have baffled researchers. The paper was a product of the Particle Physics Community Planning Exercise, referred to as “Snowmass 2021/2022.”
Representatives in high energy physics gather every 10 years to collaborate on the future of particle physics in the United States and its international partners.
Sousa was a corresponding author of the paper that discusses some of the most promising projects coming in the next decade.
UC Professor Jure Zupan, UC Associate Professor Adam Aurisano, UC visiting scholar Tarak Thakore, UC postdoctoral fellow Michael Wallbank and UC physics students Herilala Razafinime and Miriama Rajaoalisoa also contributed to the paper.
“Progress in neutrino physics is expected on several fronts,” Zupan said.
Besides the search for sterile neutrinos, Zupan said physicists are looking at several experimental anomalies—disagreements between data and theory—that they will be able to test in the near future with the upcoming experiments.
Learning more about neutrinos could upend centuries of our understanding about physics. Several neutrino projects have been recognized with the world’s top scientific award, the Nobel Prize, most recently with the discovery of neutrino oscillations receiving the 2015 Nobel Prize in Physics. Countries such as the United States are investing billions of dollars into these projects because of the immense scientific interest in pursuing these questions.
One question is why the universe has more matter than antimatter if the Big Bang created both in equal measure. Neutrino research could provide the answer, Sousa said.
“It might not make a difference in your daily life, but we’re trying to understand why we’re here,” Sousa said. “Neutrinos seem to hold the key to answering these very deep questions.”
Sousa is part of one of the most ambitious neutrino projects called DUNE or the Deep Underground Neutrino Experiment conducted by the Fermi National Accelerator Laboratory. Crews have excavated the former Homestake gold mine 5,000 feet underground to install neutrino detectors. It takes about 10 minutes just for the elevator to reach the detector caverns, Sousa said.
Researchers put detectors deep underground to shield them from cosmic rays and background radiation. This makes it easier to isolate the particles generated in experiments.
The experiment is set to begin in 2029 with two of its detector modules measuring neutrinos from the atmosphere. But starting in 2031, researchers at Fermilab will shoot a high-energy beam of neutrinos 800 miles through the Earth to the waiting detector in South Dakota and a much closer one in Illinois. The project is a collaboration of more than 1,400 international engineers, physicists and other scientists.
“With these two detector modules and the most powerful neutrino beam ever we can do a lot of science,” Sousa said. “DUNE coming online will be extremely exciting. It will be the best neutrino experiment ever.”
The paper was an ambitious undertaking, featuring more than 170 contributors from 118 universities or institutes and 14 editors, including Sousa.
“It was a very good example of collaboration with a diverse group of scientists. It’s not always easy, but it’s a pleasure when it comes together,” he said.
Meanwhile, Sousa and UC’s Aurisano are also involved in another Fermilab neutrino experiment called NOvA that examines how and why neutrinos change flavor and back. In June, his research group reported on their latest findings, providing the most precise measurements of neutrino mass to date.
Another major project called Hyper-Kamiokande, or Hyper-K, is a neutrino observatory and experiment under construction in Japan. Operations there could begin as early as 2027 as it, too, looks for evidence of sterile neutrinos, among other research questions.
“That should hold very interesting results, especially when you put them together with DUNE. So the two experiments combined will advance our knowledge immensely,” Sousa said. “We should have some answers during the 2030s.”
UC’s Zupan said these multibillion-dollar projects hold promise for answering core questions about matter and antimatter and the origins of the universe.
“So far we know of only one such parameter in particle physics that has a nonzero value, and has to do with the properties of quarks,” Zupan said. Whether or not something similar also is present for the neutrinos is an interesting open question.”
Sousa said scientists around the world are working on many other neutrino experiments that could provide answers or generate new questions.
Reference: M A Acero et al, White paper on light sterile neutrino searches and related phenomenology, Journal of Physics G: Nuclear and Particle Physics (2024). DOI: 10.1088/1361-6471/ad307f
