Cosmic Shape Shifters

Neutrinos are elusive sub-atomic particles that are often referred to as ‘ghost particles’ and now they also appear to be shape shifting. Neutrinos come in three varieties termed “flavours”: muon neutrinos (νμ), tau neutrinos (ντ) and electron neutrinos (νe). As neutrinos are virtually massless, have no electrical charge and only interact via the weak force (one of the four fundamental forces in nature), they pass easily through matter and have been very difficult to detect and measure.

In 2001 it was discovered that neutrinos can spontaneously switch between the three different flavours — or “oscillate” among the three flavours. This amazing discovery solved the solar neutrino problem (where far few neutrinos were detected from the Sun that predicted by theory, because all the solar neutrino experiments were tuned to a single neutrino flavour, νe, while many of the neutrino had oscillated to another flavour by the time they reached the Earth). It also proved that neutrons had mass, since oscillation between flavours is only possible for particles with mass.

Previous experiments have confirmed interchanges between νμ and ντ neutrinos, and between ντ and νe neutrinos, but until now not between νμ and νe neutrinos.  The latest results from the T2K (Tokai to Kamioka) experiment has found conclusively that muon neutrinos oscillate and transform into electron neutrinos.  The Japanese T2K experiment is an international collaboration between 11 countries, 59 institutions and over 400 physicists.  A muon neutrino beam was sent from the Japan Proton Accelerator Research Complex (J-PARC), situated on the east coast of Japan, to the Super Kamiokande underground detector near the west coast of Japan. The muon neutrino beam travelled a distance of 295 kilometres (185 miles) and was found to have transformed to electron neutrinos at the end of its journey.

T2K beam from J-PARC to Super-Kamiokande. A muon neutrino was sent along the T2K beam and by the end of its journey had transformed into electron neutrinos. (Credit: T2K Collaboration )

The neutrino beam was sent as a pulse every 2.5 seconds, with each pulse containing 8 “bunches” of neutrinos. The prediction was for 4 to 5 electron neutrinos to be present at the final detection point, however the analysis of the T2K data revealed that 28 electron neutrinos appeared in the beam of muon neutrinos.

Neutrino oscillation is considered to be a product of quantum mechanical interference. It is hoped that in the future the data from the T2K experiment will be able to be compared to tests for anti-neutrinos.  The important consequence of these results are that they open the possibility that neutrinos and their anti-particle anti-neutrinos might behave differently.  If true, then this could explain why the Universe ended up with more matter than anti-matter.

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[Sheridan Lacey and Sarah Maddison]

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