OPERA Bags Fifth Tau Neutrino
By sifting through data collected between 2008 and 2012, researchers working on the OPERA experiment at the Gran Sasso National Laboratory in Italy have identified the fifth incidence of a tau neutrino in a beam of muon neutrinos. Taken together with the four incidences previously announced, the finding brings OPERA’s detection of muon neutrinos transforming into tau neutrinos to a statistical significance of “5 sigma.” This level of significance is a big deal in particle physics: it means that the result qualifies as a discovery, not as mere evidence.
Neutrinos are nearly massless, neutral particles that rarely interact with other particles. They come in three types: electron, muon, and tau. And they are believed to change, or “oscillate,” from one type to another. OPERA’s tau-neutrino candidate events were observed in a beam of muon neutrinos that traveled through Earth’s crust from CERN, near Geneva, Switzerland, to the Gran Sasso lab located 730 kilometers away in Italy’s Gran Sasso mountain. The fact that OPERA detected five tau neutrinos in this beam means that five muon neutrinos changed to tau neutrinos on their way to Gran Sasso. The OPERA researchers’ data analysis, which involved an improved estimation of the background noise from particles other than tau neutrinos, indicates that the odds that the detector would find five tau neutrinos by chance are less than one in a million. The result lends new support to the phenomenon of neutrino oscillations, evidence for which has been gathered from different neutrino sources and detectors.
This research is published in Physical Review Letters
–Ana Lopes