The 2015 Nobel Prize in Physics has been awarded to Takaaki Kajita, from the University of Tokyo, Japan, and to Arthur McDonald from Queen’s University, Canada. The Royal Academy of Sweden wanted to recognize the impact these two scientists had in improving our understanding of neutrino physics.
Neutrinos are fundamental particles akin to the electron but carrying no electric charge. Neutrinos only interact with gravity and with the "weak nuclear force," responsible for nuclear decays that power nuclear fission power stations. In addition, they are the only identified candidate for dark matter, although there are others that are speculated.
They are produced in a number of ways, including the nuclear reaction in the Sun, radioactive decays, supernova explosions and when cosmic rays hit the atmosphere. There are three families of neutrinos: electronic, muonic and tau. Neutrinos do not interact much with normal matter and they are so light that every second, 65 billion neutrinos are passing through every square centimeter (about the area of a fingernail) of your body.
This flux of neutrinos from the Sun is at the center of the Nobel Committee motivation. A significant discrepancy was observed between the predicted electron neutrinos from the Sun's interior and those observed by the scientists. This deficit of solar electron neutrinos was explained by Italian physicist Bruno Pontecorvo, who argued that if neutrinos had mass they could mutate from one type to the other, known as oscillation. Physicists struggled with the idea for decades both from a theoretical and experimental point of view, but it was resolved in 2001 by the team lead by Kajita using the Super Kamiokande experiment in Japan, and the team lead by McDonald at Sudbury Neutrino Observatory in Canada.
Super Kamiokande looked at neutrinos formed in the atmosphere, noticing how they oscillated between two types; the Canadian experiment detected all three families of neutrinos, and the sum of all the neutrinos detected was found to be exactly the expected flux from the Sun. The missing neutrino puzzle which had confused physicist since the 1960s could therefore be solved just by assuming neutrino change between the three types.
Neutrino oscillation is of great theoretical and experimental significance. The existence of this oscillation implies that neutrinos have mass, although much smaller than what we are currently capable of measuring.