Billions of mysterious particles called neutrinos bombard your body every day. But catching even one neutrino is a huge effort. Nearly all neutrinos pass through people — and even our planet Earth — without a trace.
"There are 65 million neutrinos going through your thumbnail every second," said Robert Svoboda, a University of California, Davis, physics professor who has studied neutrinos for more than 25 years. "Only one will stop in your body during your lifetime."
Svoboda collaborated on one of the two neutrino experiments honored this year by the 2015 Nobel Prize in Physics. The Nobel prizes went to Takaaki Kajita, leader of the Super-Kamiokande Collaboration in Japan, and Arthur McDonald, who leads the Sudbury Neutrino Observatory Collaboration in Canada.
Researchers want to understand neutrinos because the particles carry information about the inner workings of stars. The intense nuclear reactions inside stars, supernovas and nuclear reactors all produce neutrinos. Svoboda also thinks neutrinos may have played a role in creating more matter than antimatter just after the big bang. The big bang should have produced equal parts matter and antimatter, but our universe has mostly matter and seemingly very little antimatter, he said.
A weighty discovery
Svoboda joined the Super-Kamiokande experiment, or Super-K, in the early 1990s. Built in an old mine in Japan, the Super-K experiment centers on a 50,000-ton tank of purified water (about 13 million gallons). The tank's sheer size means researchers can catch a few neutrinos randomly colliding with molecules in the water. Sensors monitor the faint light flashes created by these neutrino collisions. Super-K snags about 20 to 30 neutrinos on a typical day, Svoboda said.
Physicists at Super-K already knew neutrinos come in three types, also called flavors: electron, muon and tau. The names refer to the particles produced when a neutrino collides with matter. (UC Davis physicist Phil Yager, professor emeritus, helped capture the first direct evidence of the tau neutrino in the late 1990s.)
"Flavor is a name given to something we don't quite understand," Svoboda said. "We don't know why there are three flavors. We're probably missing something very important, and don't know it yet."
The Super-K group discovered that neutrinos oscillated, switching between different types. The Sudbury Neutrino Observatory confirmed that neutrinos switch flavors on their journey from the Sun. The oscillation observations confirmed that neutrinos have a very tiny mass. Before the experiment, most physicists thought neutrinos were massless particles. These results meant the Standard Model of the universe and its particles was wrong.
New neutrino experiments may help solve some of these riddles. Svoboda is a leading scientist on the Deep Underground Neutrino Experiment (DUNE), a mega-neutrino detector at Fermilab in Batavia, Illinois. The DUNE experiment could lead to a better estimate of the mass of the Universe, and an improved measure of neutrino mass.
Whatever the results from DUNE, Svoboda expects more surprises from scientists studying neutrinos. "This is the fourth Nobel prize for neutrino research, and it won't be the last one," Svoboda said.
— Becky Oskin, content strategist in the UC Davis College of Letters and Science