The 2015 Nobel Prize in physics has been awarded to Takaaki Kajita of Japan and Arthur B. McDonald of Canada for experiments that demonstrated neutrino oscillations. The discovery of neutrino oscillations eventually led to the conclusion that neutrinos -- one of the most exotic and least understood subatomic particles in the universe -- must have some mass, however small.

“Around the turn of the millennium, Takaaki Kajita presented the discovery that neutrinos from the atmosphere switch between two identities on their way to the Super-Kamiokande detector in Japan,” the Royal Swedish Academy of Sciences said, in a statement released Tuesday. “Meanwhile, the research group in Canada led by Arthur B. McDonald could demonstrate that the neutrinos from the Sun were not disappearing on their way to Earth. Instead they were captured with a different identity when arriving to the Sudbury Neutrino Observatory.”

Neutrinos, which are produced by the decay of radioactive elements, have a unique trait -- they can change, or “oscillate,” between their three known types, or “flavors” -- the electron neutrino, the muon neutrino and the tau neutrino. However, this has been extremely difficult to observe because, despite being one of the most abundant particles in the universe, neutrinos hardly interact with matter. 

The discovery of neutrino oscillation has an important implication -- they hint at the existence of physics beyond the Standard Model, which has so far failed to incorporate one of the four fundamental forces -- gravity.

“The discovery rewarded with this year’s Nobel Prize in Physics have yielded crucial insights into the all but hidden world of neutrinos,” the academy said, in the statement. “The new observations had clearly showed that the Standard Model cannot be the complete theory of the fundamental constituents of the universe.”

Since the Nobel prizes were first handed out in 1901, 199 people have received the prestigious physics award. Only two women -- Marie Curie and Maria Goeppert-Mayer -- have been awarded the Nobel Prize in Physics.