Scientists Who Tracked Down Sneaky Neutrinos Win Nobel Physics Prize

They travel through water, rock and even solid metal, and they’re passing through your body right now. Neutrinos are everywhere — second only to photons in terms of how common they are across the universe.

And they were, for a while, masters of disguise, switching their “flavors” and fooling physicists. Because they don’t carry an electric charge, particle physicists assumed they had no mass.

The Nobel Prize in Physics was awarded Tuesday to two scientists who figured out how to identify them.

Image: Arthur B. MacDonald | Takaaki Kajita
Arthur B. MacDonald, professor Emeritus at Queen's University in Canada and Japan's Takaaki Kajita won the 2015 Nobel Prize for Physics for their discovery that neutrinos, labeled nature's most elusive particles, have mass, the award-giving body said on Tuesday. Reuters | AFP - Getty Images

Takaaki Kajita of the University of Tokyo and Arthur McDonald of Queen’s University in Canada will share the award for their experiments that show the invisible yet ubiquitous particles have mass.

“New discoveries of the neutrino’s closely guarded secrets are expected to change our understanding of the history, structure and future fate of the universe,” the Nobel Committee wrote.

“Some were created already in the Big Bang, others are constantly being created in various processes in space and on Earth — from exploding supernovas, the death of massive stars, to reactions in nuclear power plants and naturally occurring radioactive decays,” they added.

“Even inside our bodies an average of 5,000 neutrinos per second is released when an isotope of potassium decays.”

Kajita worked with the Super-Kamiokande, a huge tank of water more than half-a-mile below the Earth’s surface. Most neutrinos pass right through it, too — but every once in a while, one collides with an atomic nucleus or an electron in the water.

Kajita figured out and announced in 1998 that neutrinos can oscillate from one “flavor” to another, something they need mass to do.

McDonald used this slightly different underground water tank at the Sudbury Neutrino Observatory and noticed it captured only three neutrinos a day — one-third of what it should have. He figured out the other two-thirds had switched identifties.

“A decisive piece of the puzzle fell in place when Sudbury Neutrino Observatory, SNO, performed their measurements of neutrinos arriving from the Sun,” the Committee said.

"Yes, there certainly was a Eureka moment in this experiment when we were able to see that neutrinos appeared to change from one type to the other in traveling from the Sun to the Earth," McDonald told a news conference.

Now physicists know there are three flavors, or varieties, of neutrino. They are roughly described as electron neutrino, muon neutrino and tau neutrino.

The Nobel Prize in medicine was awarded Monday to a Chinese researcher who discovered the malaria drug artemisinin and Canadian and Japanese researchers who helped develop the drug ivermectin. The prize in chemistry will be awarded Wednesday.