2015 Nobel Prize in Physics awarded to neutrino oscillation experimenters

The Royal Swedish Academy of Sciences on October 6 awarded the 2015 Nobel Prize in Physics to Takaaki Kajita and Arthur B. McDonald in recognition of key contributions each made to experiments demonstrating neutrino oscillations.

Discovered experimentally in 1956, neutrinos are the most numerous subatomic particles in the entire cosmos after photons. Reactions between cosmic radiation and the Earth’s atmosphere create many neutrinos. Others are produced in nuclear reactions inside the sun.

There are three types of neutrinos according to the Standard Model of particle physics: electron, muon and tau neutrinos. Kajita in 1998 discovered that atmospheric muon neutrinos, those produced when cosmic rays interact with the atmosphere, switched between two identities while traveling toward the Super-Kamiokande detector in Japan. In 2002, McDonald demonstrated that neutrinos from the sun were not disappearing on the way to Earth. Instead the neutrinos were oscillating from sun based electron-type neutrinos into muon or tau neutrinos before arriving at the Sudbury Neutrino Observatory in Canada.

Physicists wrestled for decades prior to Kajita’s and McDonald’s experiments to explain why up to two-thirds of theoretically calculated neutrinos seemed to be missing from measurements performed on Earth. Kajita and McDonald discovered these neutrinos weren’t missing but rather had oscillated and changed identities. This realization led to the far-reaching conclusion that neutrinos, for a long time considered massless, must have some mass, however small.

This understanding in turn “revealed the first apparent crack in the Standard Model” of particle physics, according to documents (I, II) released by the Royal Swedish Academy of Sciences in coordination with the prize announcement. “It has become obvious that the Standard Model cannot be the complete theory of how the fundamental constituents of the universe function.”

The Royal Swedish Academy believes that several questions regarding neutrinos must be answered before new theories beyond the Standard Model can be developed. “What are the neutrinos’ masses?” they ask. “Why are they so lightweight? Are there more types than the three currently known? Are neutrinos their own antiparticles? Why are they so different from other elementary particles?”

In an attempt to answer these and other questions about neutrinos, more than 800 scientists from around the world are coming together around a proposed flagship, the Deep Underground Neutrino Experiment (DUNE). Based at Fermilab, DUNE will be the first international mega-science project hosted by the US Department of Energy dedicated to studying neutrino oscillation.

“Neutrinos may provide the key to answering some of the most fundamental questions about the nature of our universe,” says Fermilab. “Neutrinos may play a key role in solving the mystery of how the universe came to consist of matter rather than antimatter. They could also unveil new, exotic physical processes that have so far been beyond our reach.”

The DUNE project will use Fermilab’s particle accelerators to create neutrinos and then send them 800 miles through the Earth to the Sanford Underground Research Facility, a new neutrino detector in South Dakota. The neutrinos will travel at close to the speed of light, covering the distance between the facilities in a fraction of a second. Once the facility is operational, researchers will measure neutrino oscillations looking for reasons why the universe is matter-dominated. They will also use the DUNE detector to further understand the formation of black holes by searching for neutrinos originating from exploding stars.