Indiana University researchers have contributed to significant advances in our understanding of the universe through the partnership of two major international neutrino experiments. Neutrinos are very small, almost massless particles that constantly pass through space, planets, and even our bodies, but they almost never interact with anything. Research results published in journal nature Scientists can get closer to answering a profound question: why does the universe contain matter such as stars, planets, and life rather than just the sky?
This breakthrough result resulted from an unprecedented joint analysis of data from the US NOvA experiment and Japan’s T2K experiment. These two long-range neutrino projects are among the most sophisticated of their kind. The combined results will allow researchers to better study neutrinos and their antimatter, and provide insight into why the universe did not explode on its own immediately after the Big Bang.
In both experiments, scientists use particle accelerators to generate beams of neutrinos that are sent across vast distances underground to giant detectors. They are very difficult to detect. Of the countless particles produced, only a fraction leave a measurable signal. We then use advanced detectors and powerful software to reconstruct these rare interactions and study how the neutrinos change as they travel.
Indiana University has played a major role in this research for decades. IU scientists have helped build detector systems, interpret data, and mentor young researchers. Mark Messier, Distinguished Professor and Chair of the Department of Physics in Bloomington College of Arts and Sciences, has played a leadership role on the project since 2006. Other IU researchers involved include physicist John Urheimand James Masser, professor emeritus, astronomy professor Stuart Mouffson, professor emeritus, and Jonathan Carty of IU’s Department of Chemistry.
The mystery of neutrinos and matter-antimatter
Neutrinos are one of the most common particles in the universe. They have no charge and little mass, making them very difficult to detect. But those same properties make it an invaluable tool for investigating the deepest laws of physics.
One of the biggest mysteries in cosmology is why the universe is dominated by matter. The Big Bang should have produced equal amounts of matter and antimatter. When matter and antimatter meet, they annihilate each other with explosive energy. If the early universe had contained exactly equal amounts of both, they would all have disappeared. Instead, slight imbalances favored matter, allowing the formation of galaxies, stars, planets, and life.
Scientists believe neutrinos may help explain that imbalance. Neutrinos come in three types, or “flavors,” known as electrons, muons, and tau. As it travels through space, it can switch from one flavor to another in a process called vibration. If neutrinos and antineutrinos oscillate differently, that difference could indicate why matter eventually became dominant.
NOvA and T2K partner up
The new study in Nature stands out because it combines data from two major neutrino observatories. NOvA (NuMI Off-Axis νe Appearance Experiment) sends a beam of neutrinos from Fermi National Accelerator Laboratory near Chicago to a 14,000-ton detector in Ash River, Minnesota, 810 kilometers away. Meanwhile, Japan’s T2K project is firing a beam 295 kilometers away from the J-PARC accelerator in the Tokai region to the giant Super-Kamiokande detector beneath Mt. Ikenoyama.
By analyzing the results together, the researchers improved their ability to measure how neutrinos behave. According to a press release from nature“Combining the analysis takes advantage of the complementary sensitivities of the two experiments and demonstrates the value of collaboration.” NOvA’s longer distance through Earth and T2K’s shorter but more powerful beam provide complementary strengths, allowing scientists to compare and refine measurements with very high precision.
By pooling the datasets, the team was able to better determine the parameters that control neutrino oscillations, particularly those related to the difference between neutrinos and antineutrinos. The results focus on CP symmetry (charge parity symmetry), the principle that matter and antimatter obey the same physical laws and behave as mirror images of each other.
However, the observable universe is overwhelmingly made up of matter, and there is very little antimatter left over from the Big Bang. The overall findings suggest that there may be differences in the way neutrinos and antineutrinos oscillate, indicating a possible violation of CP symmetry. Simply put, neutrinos may not behave exactly like antimatter. The subtle differences may provide important clues to why the material has survived.
“We have made progress on this very large and seemingly intractable question of why there is something rather than nothing,” Professor Messier said. “And the stage is set for future research programs aimed at using neutrinos to tackle other problems.”
Technology, training and global collaboration
Large-scale particle physics experiments often yield benefits beyond basic science. The technologies developed to detect neutrinos, such as high-speed electronics and advanced data analysis systems, are often commercialized in industry. This collaborative research effort is supported by funding from the U.S. Department of Energy.
“Revolutionary innovations stemming from high-energy physics are occurring in all areas of society,” Messier said. “Additionally, the next generation of scientists will enter the industry immersed in data science, machine learning, artificial intelligence, and electronics, leveraging the deep skills they have gained trying to answer these very difficult questions.”
The NOvA and T2K collaboration involves hundreds of scientists from more than a dozen countries in the United States, Europe, and Japan. The analysis they shared proves the scientific power of international cooperation.
IU Ph.D. students currently contributing to collaborative research include Reed Bowles, Alex Zhang, Hanyi Chen, Erin Ewart, Hannah LeMoyne, and María Manrique Plata. Since NOvA began in 2014, Messier and colleagues have mentored many IU graduate and undergraduate students working on the experiment.
This partnership provides a preview of how future large particle physics projects will operate. For Indiana University and its collaborators, this result opens the door to more precise studies that build on this work.
“As a physicist, I find it interesting that big questions like why there is matter and not antimatter in the universe can be broken down step by step into smaller questions,” Messier said. “Instead of being dumbfounded by its enormity, we can actually make progress toward the answer to why we exist in this universe.”
