Karsten Heeger talks neutrinos

Wright Lab Director Karsten Heeger arrived at Yale in July.
Wright Lab Director Karsten Heeger arrived at Yale in July. Photo by Jennifer Lu.

Physics professor Karsten Heeger, who came to Yale from the University of Wisconsin in July, is the new director of the A. W. Wright Nuclear Structure Laboratory. His research focuses on neutrinos: tiny and mysterious subatomic particles that were once thought to be massless but are now believed to be the key to further understanding our universe. Heeger spoke with the News on Monday to discuss neutrinos, their significance and one of his ongoing research projects in China.

Q: What are neutrinos and why do they matter?

A: Neutrinos are particles that come out of nuclear reactions. They are elementary particles that move closely to the speed of light. All this time we thought that neutrinos had no mass at all. Neutrinos have been produced since the beginning of the big bang, and they’re still in our universe. We thought they were just like light, but then we found that they do have a tiny mass. This has profound implications [for figuring out] how the universe evolved and the role neutrinos play in our universe. It may turn out that the neutrinos are important for the structure of the universe, for galaxy formation, or they might explain why we live in a universe of matter instead of a universe of anti-matter. At the beginning of time, there was a big bang — a gigantic explosion of sorts. Then, the universe was filled with particles and anti-particles. One of the big questions in cosmology is, “Where did all the anti-matter go? Why do we live in a world filled matter and not anti-matter?” One interesting thing about neutrinos is that they may be their own antiparticle. Usually particles and antiparticles are very distinct. If you bring them together, they annihilate and produce energy. But neutrinos may be in fact their own antiparticle. If we discover that, it would be a very new way of looking at matter and the universe as a whole.

Q: What results have been discovered recently in the field of neutrino physics?

A: We’ve studied neutrinos produced in the sun, for example, to figure out what happens inside the sun. There are three different types of neutrinos (called “flavors,” like ice cream) and the sun only emits one type of neutrino: the electron neutrino. About 12 years ago, we found that when we measured the neutrinos that come from the sun with our detector on earth, all three flavors are actually involved. That was a big surprise. This meant that the three flavors of neutrinos — electron, muon and tau — are, in fact, not distinct. They can transform into each other. This phenomenon is known as “neutrino oscillation,” which demonstrates that a quantum mechanical effect is taking place and also shows that neutrinos must have mass, since it is required for this effect to happen. So by looking at the sun and trying to understand how the sun burns and what is happening inside the sun, we learned something very profound about the nature of neutrinos and what happens at a quantum mechanical level. This effect of neutrino oscillation has been studied in many experiments with accelerators, and it has been studied in experiments with neutrino nuclear reactors. I’m involved in another experiment looking at neutrinos coming out of a commercial nuclear reactor. Every one of these experiments confirmed this effect: the three flavors of neutrinos are not fixed in time or space.

Q: Could you summarize some results of your previous work in China?

A: What we discovered with this experiment is that neutrinos can actually oscillate over a distance of a kilometer or two. Previous experiments found that neutrinos can change between the sun and the earth or over a distance of couple hundred kilometers. We found that there is a component of oscillation that is actually faster. Over the distance of just a mile, a neutrino can change flavor. This turnout is an important ingredient to understanding how neutrinos and antineutrinos might behave. The other important fact is that when you look at neutrinos from the sun, you are actually measuring neutrinos. Out of a nuclear power plant, you get antineutrinos. That’s the way the nuclear reaction works. We have made a precise measurement of antineutrino oscillations.

Q: The Daya Bay experiment conducted in China has over 200 scientists from six different countries or territories working on it. What is it like working with such a large and diverse group?

A: The main issue is that the way we do science is culturally driven. You’d think that physics and the other sciences should be pretty universal, and it is. But the way scientists interact has a cultural component. Whenever you have a big project like this, there is an aspect of sociology and cultural understanding that comes in to making such an experiment work. So besides just getting the science right, to have success in the end, you need to configurate the group. That is, I think the biggest challenge in an international project.

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