New observations of neutrinos: one of the universe’s smallest sub-atomic particles

Parliamentary Yearbook reports on observations of one of the universe’s smallest and least understood particles - neutrinos - by the international T2K project. Neutrinos exist in three types. 

The T2K project has recently conclusively established - through observation for the first time - that muon neutrinos transform into electron neutrinos as they travel. It is hoped that the findings will help solve one of the great mysteries of the universe.

The T2K (Tokai-to-Kamioka) project is a multinational experimental project led by Japan and part-funded by the UK’s Science and Technology Facilities Council (STFC). It was designed to detect and study some of the least understood particles in the universe, including tiny particles called neutrinos.

Neutrinos are invisible particles which are one of the basic building blocks of nature.  They have no electrical charge and very tiny masses, making them extremely hard to detect. Their mass is thought to be tiny even by the standards of subatomic particles (particles smaller than an atom). This allows them to travel at close to the speed of light. They only interact via weak force which means they can pass straight through large amounts of dense matter - like the Earth - unimpeded.  For example, every second hundreds of billions of neutrinos can travel through your thumbnail.

Neutrinos occur in three types or “flavours”: electron neutrinos, muon neutrinos and tau neutrinos.  T2K’s recent experiment has provided conclusive proof that muon neutrinos transform into electron neutrinos through a process known as neutrino oscillation.

The T2K project is spread across several sites in Japan. The J-PARC accelerator in the village of Tokai, on the east coast of Japan, produces a beam of neutrinos - primarily muon neutrinos - which is transmitted via a local detector in Toakai to the Super-Kamiokande detector, located 295km away the Kamioka Zinc Mine near the west coast of Japan.  The neutrino beam is produced when high-energy protons strike a graphite target.  This results in short-lived secondary particles (such as kaons and pions). As these particles decay, neutrinos are produced.  To increase the volume of neutrinos reaching the Super-Kamiokande detector, three huge magnetic horns are used to focus the direction of secondary pions.

In the experiment, an analysis of data observed at the Super-Kamiokande detector associated with a neutrino beam transmitted from the J-PARC accelerator the presence of more electron neutrinos than would be expected. The observation is the first of its kind to explicitly see a unique flavour of neutrinos appear at the detection point (in Super-Kamiokande) from a beam initially consisting of a different type of neutrino (in Tokai).

It was previously known that neutrinos transform from one kind but this particular transformation had never been conclusively observed and is regarded as a major milestone.

Commenting on the results, Professor Alfons Weber, a British collaborator on the T2K from the STFC and Oxford, said in a statement:  “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.”

The findings may play an important role in helping to answer one of the most important questions about the universe: ‘Why is there so much more ‘matter’ than ‘antimatter’ in the universe?’  Every particle has a corresponding anti-particle (with a charge opposite to that of its counterpart). This is known as anti-matter.  Physicists have long puzzled over why, if anti-matter and matter are equal but opposite, should there be so much more matter in the universe than antimatter.

It is anticipated that T2K’s recent findings will be compared to future data with anti-neutrinos to test whether muon and electron neutrinos oscillate in a different manner to their antimatter counterparts.  It is believed that this asymmetry could help explain why anti-matter is so comparatively rare.

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