THIS is MASSIVE! Less-Masslessness neutrino boffins bag Physics Nobel

'Flavour'-switching particle has mass ... but how much?

Solar filament eruption of August 31, 2012

The Nobel Prize in Physics 2015 has been awarded to Takaaki Kajita of the University of Tokyo, and Arthur B McDonald of Queen's University in Canada, with a citation "for the discovery of neutrino oscillations, which shows that neutrinos have mass".

Neutrinos were classically assumed to be massless by the Standard Model of particle physics, but how this could be remained quite a mystery as up to two-thirds of the number of neutrinos which should theoretically be detectable on Earth appeared to be missing.

However, it was "around the turn of the millennium," according to the Nobel announcement, that Kajita presented the discovery that atmospheric neutrinos had switched between two flavours on their way to the Super-Kamiokande detector in Japan.

Meanwhile, McDonald's research group was demonstrating that solar neutrinos were not "disappearing" on their way to Earth, but were being "captured with a different identity" when they arrived at the Sudbury Neutrino Observatory.

These experiments resulted in the extraordinary conclusion that neutrinos necessarily had to contain mass, however small that mass might be, as it was a prerequisitive for neutrinos to change their "flavour".

For particle physics this was a historic discovery. Its Standard Model of the innermost workings of matter had been incredibly successful, having resisted all experimental challenges for more than twenty years. However, as it requires neutrinos to be massless, the new observations had clearly showed that the Standard Model cannot be the complete theory of the fundamental constituents of the universe.

The hunt is now on to measure the mass of the neutrino, with MIT's Project 8 using a coffee cup-sized particle detector to monitor the movements of individual electrons in a radioactive gas.

In 2012, CERN confirmed that experimental results that seemed to show neutrinos travelling faster than the speed of light were the result of a faulty clock. ®

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