August 20, 2013
It has been hypothesised for some time now that hard to detect and nearly massless particles known as neutrinos can oscillate between three different types and now this has been finally confirmed by an international group of scientists conducting experiments in Japan. They have detected muon neutrinos disappearing and electron neutrinos appearing.
Photomultiplier tubes used for electron and muon detection in Super-K.
Neutrinos are elusive particles that have a tiny mass, come in three types, or flavours, and originate from nuclear reactions, such as those that happen in the Sun or nuclear reactors on Earth, or radioactive decays of elements like Uranium-238, Thorium-232 or even Potassium-40 in human body. They are everywhere and they travel at speeds close to the speed of light, passing through matter like bullets through the fog. This makes them really hard to detect. Billions of them passed through you since you started reading this sentence, yet they are harmless.
There are three types of neutrinos – tau neutrino, muon neutrino and electron neutrino. Back in 2003, it was confirmed by Sudbury Neutrino Observatory, Ontario, Canada, that neutrino of electron variety can oscillate into a neutrino of muon variety (confirming Raymond Davis' and John Bahcall's decades long experiments and calculations as accurate). Until now however, scientists only knew that the number of electron neutrinos at the source (Sun in the Davis's and Bahcall's experiment) and the sum of muon neutrinos and electron neutrinos at the detector were identical, with muon neutrino to electron neutrino ratio of two to one, as predicted by Bahcall's calculations. Now, the scientists taking part at the T2K experiment (Tokai to Kamioka, Japan) have observed one variety disappearing and another arriving.
"Up until now the oscillations have always been measured by watching the types disappear and then deducing that they had turned into another type. But in this instance, we observe muon neutrinos disappearing and we observe electron neutrinos arriving - and that's a first," said Prof Alfons Weber, British collaborator on T2K from the UK's Science and Technology Facilities Council (STFC) and Oxford University.
T2K experiment is taking place in Japan, with some 500 scientists from all over the world cooperating on two locations – Japan Proton Accelerator Research Centre (J-Parc) on the eastern cost and Super-Kamiokande facility (Super Kamioka Neutrino Detection Experiment), 300 km away, on the west cost of the country. The former creates a beam of muon neutrinos, firing them underground towards the latter. The Super-K contains a tank filled with 50,000 tonnes of ultra-pure water, surrounded by optical detectors. When a neutrino interacts with a water, tiny flashes of light are emitted and detected by light-sensitive photomultiplier tubes. These reactions are relatively rare, since neutrinos pass through most of the matter, but the sheer number of neutrinos emitted from the source makes it possible to gather enough data over time to come to conclusions.
This observation allows for the possibility that neutrinos and anti-neutrinos might behave differently. If that is true, it could explain why we live in a universe with significantly more matter than antimatter. Neutrino oscillations are governed by a matrix of three angles that have non-zero values. Previous research has shown that two of the matrix angles have non-zero values and now T2K's observation confirms that the third one, called theta-one-three, also has non-zero value.
This allows for the oscillations of neutrinos and anti-neutrinos to be different, thus displaying an asymmetrical behaviour called charge parity (CP) violation. CP-violation is known to occur in quarks, building particles of protons and neutrons, but it is not significant enough to tip the scales in favour of matter over antimatter in early days after the Big Bang. If it also occurs in neutrinos, especially massive neutrinos believed to have existed in the early Universe, it could be an explanation the science has sought for a long time. To confirm this, it is likely that much more powerful experiments will be needed. "We have the idea for a Hyper-Kamiokande which will require an upgrade of the accelerator complex," said Prof Weber. "And in America there's something called the LBNE, which again would have bigger detectors, more sensitive detectors and more intense beams, as well as a longer baseline to allow the neutrinos to travel further."
Source: BBC news