Scientists at Indiana University have achieved a breakthrough in understanding the universe thanks to a collaboration between two major international experiments studying neutrinos, which are ubiquitous, tiny particles that stream through everything in space but barely interact with anything around them. The results , published in the journal Nature, bring researchers closer to answering one of the biggest questions in science: why is the universe filled with matter, such as stars, planets, and life, instead of nothing?
The discovery stems from a first-of-its-kind joint analysis between the NOvA experiment in the United States and T2K in Japan, two of the world's most advanced long-distance neutrino experiments. Together, the projects are helping scientists peer into the invisible world of neutrinos and their antiparticles, shedding light on why the universe did not annihilate itself in the first instant after the Big Bang.
In both experiments, neutrinos are fired from particle accelerators and detected after traveling long distances underground. The challenge is immense: out of trillions upon trillions of particles, only a handful leave detectable traces. Sophisticated detectors and software then reconstruct these rare events, providing clues about how neutrinos transform as they move.
The study reflects IU's decades of leadership in particle physics. IU researchers have played key roles in building detector components, analyzing data, and training early-career scientists. Mark Messier , Distinguished Professor and Chair of the Physics department within the College of Arts and Sciences at IU Bloomington, has occupied leadership roles with the project since 2006. Also involved in the project from IU are physicists Jon Urheim and James Musser (Emeritus), Astronomy Professor Stuart Mufson (Emertius), and Jonathan Karty in the Chemistry department in the College at IU.
Tiny Particles, Enormous Questions
Neutrinos are among the most abundant particles in the universe. They have no electric charge and nearly no mass, making them extraordinarily difficult to detect. But that same elusiveness makes them scientifically priceless.
Understanding neutrinos could help explain one of the greatest puzzles in cosmology: why the universe is made of matter. Theoretically, the Big Bang should have produced equal parts matter and antimatter, which would have annihilated each other completely; when a particle meets its mirror opposite, both disappear in a burst of energy. But when the Big Bang occurred something tipped the balance, creating a greater abundance of matter, which led to the formation of stars, galaxies, and life today.
Physicists suspect that neutrinos may hold the answer. Neutrinos come in three types, or "flavors," electron, muon, and tau, essentially three versions of the same tiny particle. Neutrinos possess the unusual ability to oscillate and transform from one "flavor" to another as they travel through space, and the way these oscillations occur, and whether they differ between neutrinos and their antimatter counterparts, could reveal why matter won out over antimatter in the early universe.
Uniquely, the new Nature study combines data from two of the world's premier neutrino observatories. NOvA (the NuMI Off-axis νe Appearance experiment) sends a beam of neutrinos through the Earth 810 kilometers from its source at the Fermi National Accelerator Laboratory near Chicago to a 14,000-ton detector in Ash River, Minnesota. Japan's T2K shoots a beam of neutrinos 295 kilometers from the J-PARC accelerator in Tokai to the giant Super-Kamiokande detector under Mount Ikenoyama.
Why? Analyzing data jointly from both experiments significantly improves scientists' ability to pin down how neutrinos behave, a task that has challenged researchers for decades. This is important because, according to a press release from Nature, "Combining the analyses takes advantage of the complementary sensitivities of the two experiments and demonstrates the value of collaboration." With NOvA using a longer baseline through Earth, and T2K using a shorter but more intense beam, researchers were able to cross-check their findings with unprecedented precision.
By merging their datasets, the research teams achieved a more accurate measurement of the parameters that govern neutrino oscillation, especially those related to detected asymmetry between neutrinos and antineutrinos. The joint study's results focus on something called CP symmetry (charge-parity symmetry), reflecting the idea that matter and antimatter should behave like perfect mirror images; the rules of physics should stay exactly the same for both.
But that's not what scientists observe, because the universe is made almost entirely of matter, with hardly any antimatter left behind from the Big Bang. The study's results suggest an imbalance in how neutrinos and antineutrinos oscillate, suggesting they violate CP symmetry. Meaning, neutrinos may act differently from their antimatter twins, and that hint could be the first step toward explaining why our universe contains matter at all.
"We've made progress on this really big, seemingly intractable question: why is there something instead of nothing?" said Professor Messier. "And, we've set the stage for future research programs that aim to use neutrinos to tackle other questions."
The work underscores how large-scale scientific projects pay dividends well beyond physics. The technologies developed to detect neutrinos, from high-speed electronics to advanced data processing, find applications downstream in industry. The joint study is funded by a grant from the U.S. Department of Energy.
"There has been transformative technological innovation across all sectors of society that's come out of high-energy physics," noted Messier. "Further, next-generation scientists immerse themselves in data science, in machine learning, artificial intelligence, and in electronics, and then go into industries with the deep skills they've gained while trying to answer these really difficult questions."
The NOvA and T2K teams include hundreds of scientists from more than a dozen countries, representing a global partnership spanning the U.S., Europe, and Japan. The combined analysis highlights the positive outcomes when scientists share resources and expertise.
In this light, IU Ph.D. students currently involved in the joint study include Reed Bowles, Alex Chang, Hanyi Chen, Erin Ewart, Hannah LeMoine, and Maria Manrique-Plata. Moreover, Messier and colleagues have supervised numerous IU graduate and undergraduate students on NOvA since the experiment started in 2014.
This collaboration offers a glimpse of how future major experiments in particle physics may operate. For Indiana University and its partners, the discovery paves the way for research that expands on the joint study's findings.
"As a physicist I find it fascinating that a huge question, like why there's matter in the universe instead of antimatter, can be broken down into smaller, step-by-step questions," said Messier. "Instead of being dumbstruck by the enormity of it, we can actually make progress toward an answer about why we're here in the universe."
