An international collaboration today reported first indications of the production of electron neutrinos from muon neutrinos, raising the prospect that it may be possible for future neutrino experiments to test for violation of an important symmetry of nature; that between matter and antimatter.
The results from the Tokai to Kamioka neutrino experiment (T2K), reported at a press conference in Tsukuba, Japan, fall short of the level of statistical significance needed to claim a definitive discovery. “The journey to a discovery takes a long time, so usually when an experiment sees an indication of something that’s very exciting,” says Chang Kee Jung, a physicist at Stony Brook University in New York who is international co-spokesman for the 500-member T2K collaboration.
Neutrinos are light, flighty particles that a growing body of evidence has suggested can oscillate between three flavors, electron neutrino, muon neutrino, and tau neutrino. Previous experiments, such as the Sudbury Neutrino Observatory in Canada, measured the number of electron neutrinos coming from the Sun, and produced evidence that some were “missing,” presumably because they had oscillated into other flavors. The KEK to Kamioka (K2K) experiment in Japan, similarly found that muon neutrinos from a controlled beam “go missing,” as they appear to oscillate. But T2K is the first experiment to start with a controlled beam of muon neutrinos and actually detect electron neutrinos produced by the oscillation, thus providing more direct evidence for the phenomenon.
The muon neutrino beam was produced by colliding high-energy protons from the Japan Proton Accelerator Research Complex in Tokai with a graphite target to produce muon neutrinos that were sent 295 kilometers to the Super-Kamiokande detector in Kamioka, which is capable of detecting electron and muon neutrinos. The new results come from a dataset spanning from January 2010 to March 11, 2011, when the Japanese earthquake damaged the lab and infrastructure at Tokai, taking the accelerator offline. The data include the detection of just six candidate electron neutrinos at Kamioka. In the absence of neutrino oscillation, the level expected would be 1.5 background events, which is a result at the 2.5 sigma level of statistical significance, or 0.7% probability that the signal is due to chance.
Because of the earthquake damage, T2K is now expected to be offline until around January 2012, so that it will be more than a year before the signal can be confirmed. If it is confirmed, Jung says that will be good news for plans to compare the rate of muon to electron neutrino oscillation to the rate of conversion for the particles’ antimatter partners; from anti-muon neutrinos to anti-electron neutrinos. Such a comparison could provide a direct test of the symmetry between matter and antimatter. An abiding mystery of our universe is why there is so much more matter than antimatter, and a violation of this symmetry could help to explain that.
The Main Injector Neutrino Oscillation Search (MINOS) experiment at Fermilab in Batavia, Illinois, is also searching for direct evidence of muon to electron neutrino conversion, but has yet to report this finding.
Update June 15: A copy of the paper is available.
Credit: Kamioka Observatory, ICRR, Univ. of Tokyo