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Where has all the anti-matter gone?

Posted on 20/04/2017
Kamioka Observatory, ICRR (Institute for Cosmic Ray Research), The University of Tokyo

Kamioka Observatory, ICRR (Institute for Cosmic Ray Research), The University of Tokyo

One of the ongoing mysteries of the universe is why, although matter and anti-matter were created in equal proportion in the Big Bang, the universe today is dominated by matter. Anti-matter is made of anti-particles: for every particle there exists a corresponding antiparticle, exactly matching the particle but with opposite charge. When matter and anti-matter come into contact, they disappear emitting lots of energy. So why, if there were equal quantities of them in the beginning of time, our universe today is made of matter?

The answer to this dilemma may be coming from studying the behaviour of tiny, extremely weakly interacting particles: neutrinos. Neutrinos and their anti-matter particles come in three flavours, called electron, muon, and tau. Several experiments have shown that neutrinos can oscillate, meaning they can spontaneously change their flavour – a quite surprising trait that challenges our current knowledge! By studying the difference in this flavour changing behaviour between neutrinos and anti-neutrinos, the cause of the universe’s matter and anti-matter difference may be illuminated.

Dr Asher Kaboth, Lecturer in the Centre for Particle Physics of Royal Holloway, leads an analysis on the T2K experiment. The T2K experiment investigates how neutrinos change from one flavour to another as they travel. A beam of muon neutrinos is generated at the East coast of Japan and directed across the country to the Super-Kamiokande detector in the mountains of western Japan. Neutrinos are so tiny and weakly interacting it is extremely difficult to detect them – as a result, Super Kamiokande is located …under the mountain. Hoping that tons of mass will make neutrinos interact, the detector is located in a mine 3,300 ft) underground. In addition, it consists of a cylindrical stainless steel tank that holds 50,000 tons of ultra-pure water. There are more than 11.000 photomultipliers in it, working like eyes trying to see neutrino traces. The beam of the T2K experiment is measured once before it leaves the J-PARC site, where it is generated, and again at Super-K: the change in the measured intensity and composition of the beam is used to provide information on the properties of neutrinos.

The analysis led by Dr. Kaboth is the first from the experiment that includes data from both neutrino and anti-neutrino operation, and was recently reported in Physics Review Letters, where it is selected as an Editor’s Pick.

The analysis is the world’s most precise measurement of the difference in oscillation behaviour between neutrinos and anti-neutrinos, and hints that there may be a difference between them. T2K continues to take data to investigate this intriguing possibility, and will release new results in summer 2017.


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