The Nobel Prize for Physics 2015 is being shared by two scientists who led research into neutrinos, nearly massless particles that are extremely difficult to study because they rarely interact with anything in the physical world.
Takaaki Kajita from Japan and Arthur McDonald from Canada headed separate research teams that made key discoveries about the mysterious particles using huge underground instruments in Japan and Canada.
Neutrinos have no electrical charge, and for many years were assumed to have no mass as well, but Kajita and McDonald were able to demonstrate that neutrinos, which come in three "flavors" or forms — known as tau, electron and muon — can switch from one flavor to another as they travel through space and speed through the Earth.
The ability to swap flavors is evidence that neutrinos do in fact have mass — minuscule, to be sure, but present nonetheless.
Research teams led by McDonald and Kajita were able to capture signs of the elusive particles and catch them in the act of switching between flavors.
McDonald, a professor of particle physics at Queen's University in Kingston, Ontario, said receiving the news of the Nobel Prize was "a very daunting experience."
"Fortunately, I have many colleagues as well, who share this prize with me," he said, referring to Kajita and the members of both research teams. "[It's] a tremendous amount of work that they have done to accomplish this measurement."
Kajita, at the University of Tokyo, characterized the prize win as "kind of unbelievable."
He noted that the work he and McDonald were engaged in was significant because it contradicted commonly-held assumptions about neutrinos.
The Standard Model of physics holds that neutrinos should be massless, but the results obtained by Kajita and McDonald strongly suggest that if they switch between flavors, they must have mass.
Something in the model will likely need revisiting, Kajita said.
"I think the significance is — clearly there is physics that is beyond the Standard Model," he said.
Kajita and his team observed evidence of neutrinos switching their form identities during a 183-mile journey from a proton accelerator lab in Tokai, where they were generated, to a detector buried deep beneath a mountain.
At around the same time, McDonald and his colleagues noted similar behavior in neutrinos coming from the sun — rather than Kajita's artificially-generated ones — at the Sudbury Neutrino Observatory in Canada.
"New discoveries about [neutrinos'] deepest secrets are expected to change our current understanding of the history, structure and future fate of the universe," the Nobel Prize committee said in a statement.
