The 2015 Nobel prize for physics went to Takaaki Kajita and Arthur B. McDonald for the discovery of neutrino oscillations, showing that neutrinos have mass. This turned out to be the solution to one of the biggest mysteries in particle physics which begins with radioactive decay.
In radioactive decay, atomic nuclei would spit out a particle leaving behind a new nucleus with less energy. It seemed that the nucleus was loosing energy which is not possible from the law of conservation. This law states that energy cannot be gained or lost in any event, the energy you had before is always equal to the energy after. On the 4th December 1940 physicist Wolfgang Pauli postulated that the missing energy went to a third particle in the decay – the neutrino, which would be emitted with the electron. Neutrinos have no charge and were thought to have no mass so solid objects seem as empty space to neutrinos, meaning they can pass through matter without interacting. This made them incredibly difficult to detect.
If a neutrino was emitted when a nucleus decayed it implied that a neutrino colliding with a nucleus could stablalise it and the neutrino would decay, meaning that neutrinos must be emitted in nuclear fusion. It was theorised that the main source of energy from the sun was nuclear fusion so the sun would be emitting a huge number of neutrinos every second and detecting these neutrinos would be strong evidence to that theory.
However Ray Davis’s experiment was only detecting roughly a third of the neutrinos that were expected by the theory of John Bahcall. People were very sceptical of the experiment as trillions of neutrinos went through his tank every second and only ten evidences of the neutrinos were expected to be detected each week. When Davis’s results came through as roughly 1/3 of the expected neutrinos, people were convinced his experiment was wrong and not the theory.
In Japan, physicists were conducting an unrelated experiment to look for a rare kind of nuclear decay and unintentionally found that the number of atmospheric neutrinos their equipment was detecting was also smaller than expected. These atmospheric neutrinos are produced when cosmic rays from space hit the earths atmosphere, spraying out particles including the atmospheric neutrinos. The standard model says there are three flavours of neutrino: electron neutrinos (which are produced by the sun and the only neutrinos that Ray Davis’s experiment was capable of detecting), muon neutrinos and tau neutrinos.
People then began to think that the theory was wrong, but John Bahcall went over his theory and insisted it was mathematically correct, making the solar neutrino problem the biggest mystery in particle physics.
A new theoretic proposal was that neutrinos were able to change back and forth from different flavours, called neutrino oscillations. Suggesting that in the time it takes for the neutrinos to travel from the sun to the earth they can change from electron neutrinos to muon and tau neutrinos (neutrinos the current experiments could not detect). However in order for them to change, time must pass and in order for the neutrinos to have a sense of time they must have a mass – changing the standard model which stated they were massless.
Later in Japan, scientists completed Super Kamiokande, a huge detector that is capable of detecting the electron and tau neutrinos. It could also detect the direction in which the neutrinos were coming from. As the neutrinos were supposedly massless, meaning they have no sense of time or distance, they expected to have an equal number of neutrinos detected from above as below eventhough neutrinos travelling from below were travelling further. Once plotting the results they found that only half of the neutrinos detected above were detected from below.
This was evidence that the neutrinos have a sense of distance which implies they cannot be travelling at the speed of light and do indeed have a mass. The solution to the solar neutrino problem.
A link to a great short film about the the ghost particle recommended by my nuclear physics lecturer can be found below.