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Neutrinos Hint at Why There Is More Matter Than Antimatter in the Universe

One of the great mysteries for physicists is why there is more matter than antimatter in the universe. This question lies at the core of our understanding of the laws of nature and the evolution of the cosmos.

One of the ways this mystery is being explored is by using neutrinos – subatomic, electrically neutral particles – to help solve this puzzling question. It is thought that finding asymmetry in the physical properties of neutrinos and antineutrinos may help us understand the origin of the current prevalence of matter over antimatter in the universe. This asymmetry is referred to as a charge conjugation-parity, or CP, violation.

To address this issue, a particle physics experiment, known as Tokai to Kamoika, or T2K,  run by an international collaboration of 500 members including Michigan State University scientists, has published a paper in this week’s issue of Nature that suggests that differences between neutrinos and antineutrinos might help to shed light on this.

Group photo in a group meeting.

T2K Collaboration members pose for a shot during a meeting in 2019. MSU’s Kendall Mahn is fifth from the left in the front row in the blue blouse. Credit: T2K experiment, http://t2k-experiment.org

“Hundreds of people worked for nearly a decade to produce this result,” said Kendall Mahn, an experimental high-energy physicist in the MSU Department of Physics and Astronomy who served as analysis coordinator for T2K and oversaw the development of the analysis for this paper. “It is a testament to how patient we must be for science. It’s thrilling to learn more about how the universe works.”

The T2K Collaboration used the Super-Kamiokande detector to observe neutrinos and antineutrinos generated at the Japanese Proton Accelerator Research Complex, or J-Parc.

An electron neutrino candidate as seen in Super-Kamiokande. The diffuse, electron-like Cherenkov ring signals that neutrino oscillation has occurred as almost all the neutrinos produced at J-PARC are known to be muon neutrinos. Credit: T2K experiment, http://t2k-experiment.org

As they travel through the Earth, these particles oscillate between different physical properties (electron, muon and tau) known as flavors. The T2K collaboration found a mismatch in the way neutrinos and antineutrinos oscillate by recording the numbers that reached Super- K with a flavor different from the one they had been created with.

After analyzing nine years’ worth of data, the T2K experiment reached a level of statistical significance high enough to provide an indication that CP violation occurs in these fundamental particles. More precise measurements are needed to confirm these findings. However, these measurements do strengthen previous observations and pave the way toward a future discovery.

“It’s a very exciting result, which has only been possible thanks to the hard work of a great many people,” said Luke Pickering, a research associate in Mahn’s lab who is a convener of the T2K working group that develops the neutrino interaction model, which is used to interpret the physical significance of the Super-K data. “The ‘hint’ that nature is giving us is getting pretty compelling, I’m looking forward to seeing what more data can tell us!”

A new generation of experiments under construction might provide an answer to the problem of the ‘missing’ antimatter in the next 10 years – the Hyper-Kamiokande (located near Super-Kamiokande), expected to start in 2027; DUNE in the United States, due to start in 2025; and JUNO in China, which intends to go live in 2022.

If confirmed by future studies, these findings would represent a groundbreaking advance toward understanding the fundamental reason for the predominance of matter over antimatter in our universe.

In addition to Mahn and Pickering’s participation, two of Mahn’s former students, Andrew Cudd and Jacob Morisson, graduated with T2K-based theses in the past year.

The United States T2K collaborating team is composed of 13 institutions [Boston University, University of California, Irvine, University of Colorado, Colorado State University, Duke University, University of Houston, Louisiana State University, Michigan State University, University of Pennsylvania, University of Pittsburgh, University of Rochester, Stanford Linear Accelerator Center, and Stony Brook University] is funded by the U.S. Department of Energy, Office of Science.

Val Osowski and Kim Ward via MSU Today

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