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Muon Neutrino To Electron Neutrino Oscillation Conclusively Shown
New submitter Chris Greenley writes "The T2K long baseline neutrino experiment in Japan has just announced conclusive evidence for electron to muon neutrino oscillation at the 7.5 sigma level. (The level needed for discovery is 5 sigma.) This experiment generates a focused beam of electron neutrinos using an accelerator in the J-PARC facility north of Tokyo which is aimed at the massive Super-Kamiokande detector 295 km (185 miles) away, near the west coast of Japan. 'This T2K observation is the first of its kind in that an explicit appearance of a unique flavor of neutrino at a detection point is unequivocally observed from a different flavor of neutrino at its production point.' This result clears the way for CP-violation neutrino studies which could show that 'regular' neutrinos act differently than their antimatter counterparts, a phenomenon that so far has only been observed in quarks. If neutrino CP-violation is found, it could explain why there is such a large predominance of matter over antimatter in the universe."
It's only obvious in SU(5). The currently en-vogue SU(2)+SU(3) lie groups only imply it. But you knew that right?
Neutrinos haven't been found to be traveling at the exact speed of light, only at speeds indistinguishable from the speed of light. The error bars around such measurements are not able to distinguish between a particle traveling exactly at the speed of light and a very light particle with a lot of energy to make it travel almost at that speed.
Lets take the neutrino to have a mass near the upper bound of current estimates of 2 eV, and use for example the neutrinos from the 1987A supernova which had energy of many MeV, and just conservatively estimate that the neutrinos had a relativistic factor of about a million. Their speed would different from the speed of light by one part in a trillion. Even though that supernova happened 160,000 light years away, a one part in a trillion speed difference from the speed of light amounts to only a 5 second delay (assuming the neutrino and light were emitted at the same time, which is probably not true for a supernova).
Alternatively, if we consider that we were able to measure neutrino detection to an accuracy of 1 ns, relative to a concurrent light pulse from the source, a one part in a trillion speed difference would require the sources to be 300 million km away to capture the difference with that 1 ns accuracy.
So in other words, the measurements of speeds being at the speed of light are not inconsistent with the neutrinos having a very small amount of mass given current error bars. For comparison, the faster than light claims had neutrinos going faster than light by 50 parts per million, a much larger difference than parts per trillion, and many detectors are sensitive to neutrinos with energy in the GeV or TeV range or more, where their speed difference from the speed of light would be even smaller.
The whole significance of the original discovery of neutrino oscillation was that it was evidence the neutrinos were not going the speed of light and hence had to have some mass. On going work has not quantified that a lot more since then.
This explanation is insufficient. If neutrinos were indeed massive particles we'd see a wide distribution of their velocities, just like we can observe slow and fast protons, slow and fast electrons, slow and fast everything that moves slower than c. Yet, in 100% of experiments that have been done all neutrinos are propagating through space at the speed close to or exactly equal to c.
The reason for this is the extremely small mass of neutrinoes. The current experimentally-derived upper bounds on their mass is around 1 eV (in contrast, an electron has a mass of 511,000 eV). This means that any process that creates a neutrino will give it enough energy to send it off at ultrarelativistic speeds. Even something simple like neutron decay can impart 1 MeV of kinetic energy to a neutrino, which, as the grandparent calculates, means the neutrino is traveling at 0.999999999999*c. Only chemical reactions would release a small enough amount of energy to have non-relatvistic neutrinos. But, chemical reactions don't release neutrinos.
This is why we only see speed-of-light neutrinos. This is also why it's taken so long to discover that they have mass.