A joint Fermilab/SLAC publication

T2K advances investigation of matter-antimatter imbalance

04/15/20

New results from the T2K experiment in Japan rule out with 99.7% confidence nearly half of the possible range of values that could indicate how neutrinos behave compared to their antimatter counterparts.

Photo of Super-K empty
Kamioka Observatory, ICRR (Institute for Cosmic Ray Research), The University of Tokyo

Physicists are closer than ever to measuring a value that could help answer one of the field’s longstanding questions: Why is the universe filled with matter, and not antimatter? 

Why, after the Big Bang should have produced equal amounts of matter and antimatter, did the two fail to cancel one another out entirely, leaving nothing but energy behind? The answer may lie in physics’ most mysterious particle—the ghostly neutrino—and understanding just how it acts compared to its mirror-twin, the antineutrino.

Scientists involved in the T2K Experiment in Japan have reported new results that eliminate, with 99.7% confidence, nearly half of the possible values of a measurement that could finally tell us whether neutrinos are involved in the imbalance. 

Along with the NOvA experiment in the United States, T2K has been homing in on this value, called the CP violating phase of neutrinos. There’s more work to do to finally determine this value, but these latest results are the strongest constraint yet.

“T2K now has more data with better algorithms and better analysis techniques,” than ever before, says Chang Kee Jung, US principal investigator for the experiment and a professor at Stony Brook University. 

If neutrinos and antineutrinos behave differently, it would be an extremely rare feature in the symmetrical world of physics. It could provide some explanation for why there is more matter than antimatter and offer us a new understanding of the universe. 

Changing flavors 

Neutrinos are the most abundant matter particle in the universe: About 100 trillion pass through your body each second. They come from nuclear reactions, like those in the sun and supernovae. As they travel through the universe, they barely interact with matter. That makes them extremely difficult to study.

Like all other subatomic particles, they have an antimatter particle; theirs is called the antineutrino. These twins behave as mirror opposites to their counterparts, with an opposing electric charge and opposing internal quantum numbers. This is called charge-parity (CP) symmetry. 

But what is special about neutrinos is that when they travel, they oscillate, meaning they change from one “flavor” to another. At the T2K experiment, physicists can send a beam of either muon neutrinos or muon antineutrinos from the Japan Proton Accelerator Research Complex (J-PARC) in Tokai, Japan, nearly 300 kilometers away to the Super-Kamiokande detector. 

As they travel, some oscillate and change their flavor, with muon neutrinos changing to electron neutrinos and muon antineutrinos changing to electron antineutrinos. Jung likens it to ice cream suddenly changing its flavor from chocolate to strawberry. T2K measures just how many of these flavor changes occur and compares the transformations of the neutrino beam to the transformations of the antineutrino beam.

If neutrinos and antineutrinos change flavors differently, that could be evidence of an imbalance called CP violation, which could help them understand why there is so much matter in the universe. It is generally assumed that the Big Bang should have produced equal amounts of matter and antimatter. But if that were the case, all the particles and antiparticles would have met and annihilated each other. 

“The whole universe would just be filled with light,” Jung says. “If we have CP violation, then very early in the Big Bang there could have been a process to allow it to eliminate all the antimatter,” leaving just enough matter behind to form our universe. 

Process of elimination

The amount of CP violation in neutrinos could be described as a specific number, which could be anywhere from -180 to 180. Any number other than 0, -180 or 180 would indicate that there is a difference between the matter and antimatter versions of neutrinos. Neutrino physicists are working to determine this measurement at a confidence level of 5 sigma, at which point physicists would feel comfortable calling it a new discovery.

After years of taking data, T2K has now eliminated the range of values from -2 to 165 with a confidence level of nearly 3 sigma. “If humankind is making a journey to discover CP violation at a 5-sigma level, T2K is now almost halfway there,” Jung says.

In fact, with many of the positive values close to 0 and 180 starting to be ruled out, the measurement could eventually show that CP violation in neutrinos is as big as possible. Considering the only types of particles known to violate CP, quarks, have only a very small imbalance between the behavior of their matter and antimatter particles, this result could be very interesting, says André de Gouvêa, a professor at Northwestern University and a neutrino theorist. “Why would that be? It might say something about how these two values are related to each other. There’s a lot we don’t yet know.”

The NOvA experiment, which is managed by the US Department of Energy’s Fermi National Accelerator Laboratory, is also measuring neutrino oscillations in hopes of narrowing down the potential range of values. NOvA and T2K are slightly different: T2K uses neutrinos with smaller energies, detects them 300 kilometers (about 190 miles) away, and uses a large water tank to detect them. NOvA uses higher energy neutrinos, detects them about 800 kilometers (about 500 miles) away, and uses a detector filled with liquid scintillator. 

It’s important to have multiple experiments looking for the same phenomenon, because “if they get a different answer, that could be a sign that there is some physics that we could have missed,” de Gouvêa says. 

In fact, Jung says, the two experiments are now working together to combine data. “So many people are needed to work on efforts like these,” he says. “Hundreds of people at all different levels, from leaders to people who work directly on the accelerator, beams, detectors and analysis. It takes that kind of collaboration for discovery. Combining our data will give us more confidence as a community in our results.”

Both T2K and NOvA are looking to take data for several more years before two even more powerful long-baseline neutrino experiments come online: the Deep Underground Neutrino Experiment (DUNE) in the United States and Hyper-Kamiokande (Hyper-K) in Japan. 

“In the best of all worlds, T2K and NOVA will have a big hint that CP is violated, and the new experiments will measure the value,” de Gouvêa says. 

These results won’t tell us exactly how the universe manages to have more matter than antimatter, he says, but knowing how neutrinos are involved will be key to understanding particle physics going forward. “It will be very important for making progress,” he says.

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