Difference Between Neutrinos And Antineutrinos Could Mean A Whole New Physics

The neutrino observatory getting filled with water as two physicists sit in a dingy. Kamioka Observatory, ICRR (Institute for Cosmic Ray Research), The University of Tokyo

Why the world is made of matter and not antimatter is a difficult question to answer, but now a new study might offer a tantalizing new glimpse of physics beyond our current theories.

The Tokai to Kamioka (T2K) neutrino experiment is studying how neutrinos spontaneously change from one type to another. In its first data release, researchers showed that although they were expecting the same number of events from neutrinos and antineutrinos, they actually observed 32 neutrino oscillations compared with only four antineutrinos. The results are published in Physical Review Letters.

The detection is still preliminary, but it's definitely exciting. The discovery that the rate of neutrino and antineutrino oscillation is different tells us that there’s a new asymmetry in the physical laws. Matter and antimatter are not equal and opposite in our universe.

There have been other phenomena breaking this law, called charge-parity (CP) symmetry, but none so far can explain why there is so much more matter than antimatter.

"While the data sets are still too small to make a conclusive statement, we have seen a weak preference for large CP violation and we are excited to continue to collect data and make a more sensitive search for CP violation,” said T2K collaborator and Kavli IPMU Project Assistant Professor Mark Hartz in a statement.

The current theory of fundamental physics is called the Standard Model and it has worked extremely well in explaining and predicting fundamental particles and forces. Yet we know it is limited for several reasons – a simple one is that it doesn’t include gravity.

Physicists are looking for holes in the theory, like the CP symmetry violation, and they hope that when we find where things break down, we can work out how to make a better theory. 

T2K has finished collecting another data set and more results will be released later this year, but given how difficult it is to observe neutrinos, researchers think that it will take another decade before we can have a more solid confirmation of this effect.

"If we are lucky and the CP violation effect is large, we may expect 3 sigma evidence, or about 99.7 percent confidence level, for CP violation by 2026," Hartz added.

Neutrinos come in three varieties, or “flavors”: electron, muon, and tau. Every neutrino has an antimatter counterpart and it’s said they “oscillate”, meaning they can change from one flavor into another. The T2K experiment shoots beams of muon neutrinos and muon antineutrinos through the Earth's crust at a detector located 295 kilometers (183 miles) away. On the way, some of the muon neutrinos and antineutrinos turn into the electron variety, and it is these events that researchers are studying.

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