Why is the universe made of matter? These 'ghost particle' experiments could help us find out

Scientists have inched a step closer to solving an enduring mystery in physics — why the universe contains any matter at all — thanks to a newly combined analysis from two of the world's leading neutrino experiments.

By pooling nearly 16 years of measurements, the NOvA experiment in the United States and the T2K experiment in Japan have produced the most precise picture yet of how neutrinos and their antimatter twins transform as they travel. The results, published on Oct. 22 in the journal Nature, sharpen the search for subtle differences in how these particles behave — differences that may help explain why matter prevailed over antimatter in the early universe.

If the two are perfectly symmetric, according to the Standard Model of particle physics, the Big Bang should have created equal amounts of matter and antimatter nearly 14 billion years ago. And in fact, because matter and antimatter annihilate on contact, a perfectly balanced universe should have ended in a wash of pure energy. Yet, today's cosmos is overwhelmingly made up of matter, suggesting some subtle mechanism gave matter a slight and still-mysterious advantage early on.

A prime suspect for tipping the scales is the neutrino, a ghostly, almost massless particle that permeates the universe but rarely interacts with anything. This is why scientists often refer to them as "ghost particles." Physicists have long wondered whether neutrinos and antineutrinos behave differently in ways experiments can detect. Even a subtle mismatch, known as CP violation, could illuminate how matter gained its cosmic edge.

"While there is still more to understand, the critical experimental question is clear: can we see this symmetry violation in neutrinos, and if so, how big is it?" Ryan Patterson, a physics professor at the California Institute of Technology and co-lead of the NOvA team, told Space.com.

Neutrinos change 'flavor'

Part of what makes neutrinos so elusive — and so intriguing — is their ability to change identity. They exist in three "flavors," and as they move through space, they oscillate among these types because each flavor is a blend of three mass states. As neutrinos travel, those underlying mass states shift, causing the particles to morph from one flavor to another.

"If you think of the flavors as being like strawberry, chocolate and vanilla, this would be like finding your strawberry ice cream cone turned to chocolate on your way home," a recent Caltech statement explains.

By tracing these flavor changes, scientists can measure the tiny mass differences that govern neutrino oscillations — and by comparing neutrinos with antineutrinos behaviors, they can probe CP violation.

To do this, the NOvA experiment (short for NuMI Off-axis ν_e Appearance) fired a beam of neutrinos from Fermilab near Chicago to a detector 500 miles (800 kilometers) away in Minnesota. Across the Pacific, Japan's T2K (short for Tokai-to-Kamioka) experiment sent its own beam 183 miles (295 kilometers) from the Japan Proton Accelerator Research Complex in Tokai to the massive Super-Kamiokande detector, buried 0.6 miles (about 1 kilometer) beneath a mountain in Kamioka.

Because the experiments operate at different distances and energies, each captures complementary features of neutrino oscillations. Combining their data allows researchers to isolate the subtle parameters that control how neutrinos transform.

A key result of the joint analysis is a sharply refined measurement of one of the most fundamental oscillation parameters, known as the neutrino mass splitting. The collaboration has now constrained this value to just 2 percent, making it one of the most precise measurements ever reported.

"It underlies all the other measurements we make," Patterson said. He added that this progress also opens up avenues to determine the neutrino mass hierarchy, the still-unknown ordering of the three neutrino mass states.

"As of today, we accept the existence of three neutrino families, each associated with distinct masses," Federico Sanchez, an experimental physicist specializing in neutrino physics and a longtime T2K collaborator, told Space.com. "But we still lack a fundamental understanding of why there are precisely three, not two, four or more — and why their mass differences take the specific values we observe."

"The mass hierarchy is not only a cornerstone for many theoretical calculations and predictions but also provides a tangible result that can be directly compared with existing models," he added.

The mass hierarchy affects how neutrinos and antineutrinos oscillate differently — a key part of the search for CP violation. In what is called normal hierarchy, one of the three known neutrino "flavors," muon neutrinos, transform into electron neutrinos more readily than their antimatter counterparts, muon antineutrinos, transform into electron antineutrinos. In the inverted hierarchy, that pattern flips.

The new joint analysis isn't able to say which hierarchy nature prefers. But if future data show the hierarchy is inverted, Patterson says the current dataset already hints that neutrinos may violate CP symmetry. If that data show the normal hierarchy is correct, even more data will be needed to tease apart the competing effects.

"Neutrino physics is a strange field. It is very challenging to isolate effects," Kendall Mahn, a professor at Michigan State University and T2K co-spokesperson, said in the Caltech statement. "Combining analyses allows us to isolate one of these effects, and that's progress."

A new shared 'language' for neutrino science

Beyond the immediate physics results, researchers say one of the collaboration's most significant achievements is the development of an initial common framework — a shared "language" for how neutrino interactions are described across experiments.

Although all experiments are grounded in the same underlying physics, each makes different approximations and methodological choices based on its unique detector design. Among the most critical assumptions are those involving how neutrinos interact with matter, which is essential for accurately reconstructing their energy, and how many neutrinos are produced at a given energy, said Sanchez.

Even small differences in these models can affect the interpretation of oscillation patterns, he noted. By harmonizing these assumptions, the collaboration has created a starting template that future experiments can adopt to ensure their findings are directly comparable.

"Precision in these measurements is critical, as even subtle discrepancies could signal deviations from the model — potentially revealing new physics," Sanchez told Space.com. "The more precise the agreement is the more confident we are that our description is correct."

The timing couldn’t be better. Scientists say such a unified framework will be essential for the next generation of ultra-sensitive experiments — the Deep Underground Neutrino Experiment (DUNE) in Illinois and South Dakota, and the Hyper-Kamiokande in Japan — are under construction and expected to begin operations in 2028. These next-generation detectors will perform measurements far more sensitive than NOvA or T2K, potentially offering definitive evidence of CP violation in the next decade.

And if neutrinos truly do treat matter and antimatter differently, scientists may finally uncover the long-sought reason the universe exists in the form we know today.

A study about these results was published on Oct. 22 in the journal Nature.