Ghostly neutrinos caught shape-shifting in the night


Ghostly particles are more active in the dark. For the first time, a neutrino detector has shown that the particles change form as they pass through Earth. And since neutrinos from the sun inevitably pass through Earth from the sunlit side, it's night-time when the detector observes the effect.


Neutrinos are nearly massless and notoriously shifty, scarcely interacting with most other matter. They come in three flavours – electron, muon and tau – and can flip between them without warning, an effect called neutrino oscillation.


The sun's core should produce electron neutrinos in a range of energies, but detectors see fewer high-energy ones than predicted.


One theory is that the missing neutrinos morph into other flavours as they move through dense plasma on their way out of the sun. Interacting with the less dense matter in Earth then makes some of them change back. If so, a detector should see more high-energy neutrinos at night, when the particles pass through the planet's bulk before reaching the detectors.


Neutrinos matter


The Super-Kamiokande detector in Japan has now shown that that is indeed what happens. "In the solar neutrino community, people have been waiting for this," says team member Andrew Renshaw at the University of California, Irvine. "This is the first direct evidence that there is a matter effect to the neutrino oscillations."


Measuring how big of an effect matter has on neutrino flipping will be important for calibrating other experiments that use neutrinos to investigate why there is more matter than antimatter in the universe and look for new physics.


"It's an expected effect, and if it hadn't been there, we'd have been very distressed," says Asher Kaboth at Imperial College London, who works on the T2K neutrino oscillation experiment also in Japan. "But it appears that it is what we expect, which is good for helping us get to the next measurements we want to do."


Journal reference: http://ift.tt/1lx6bW1


If you would like to reuse any content from New Scientist, either in print or online, please contact the syndication department first for permission. New Scientist does not own rights to photos, but there are a variety of licensing options available for use of articles and graphics we own the copyright to.