Posted on behalf of Nicola Nosengo.
Researchers from the Opera experiment at the Gran Sasso Laboratory in Italy, yesterday announced that they may have spotted their first oscillating neutrino.
Neutrinos, elementary particles with no electric charge that rarely interact with matter, come in three flavours: muon, tau and electron. For at least 15 years, researchers have been collecting evidence that they can change (or oscillate) from one flavour to another. Whereas neutrinos were originally thought to have no mass, such oscillations would imply that they do, a finding that would have significant consequences for cosmology and for the standard model of particle physics.
While analyzing data from 2008 and 2009, the Gran Sasso team found that one among billions of muon neutrinos beamed towards the lab from CERN, Europe’s particle-physics facility near Geneva, Switzerland, had turned into a tau neutrino by the time it reached the Opera detector.
Experiments like Opera take muon neutrinos from particle accelerators and beam them towards a distant detector, comparing the beam’s composition at their source and at their destination. With the same technique, the K2K experiment in Japan and the MINOS group in the US had already proved that muon neutrinos were disappearing from their beams. But this is the first time someone is claiming to have seen the “new” neutrino appearing at the other end.
In an email to Nature, MINOS spokeswoman Jenny Thomas said that the opera result is “the evidence needed to truly seal the fate of the neutrino oscillation hypothesis and tie together the evidence from the other experiments that neutrinos have small, but non-zero masses”.
Neutrino physicists, however, are trying to tidy up what is still quite a messy picture. Why is the mass of the neutrino so low? How low is it exactly, and how to rank the masses of the three flavors? What should we make of the hypothesis of a fourth flavour, a “sterile neutrino” that does not interact at all (see: Hunt for the sterile neutrino heats up)?
According to Opera spokesman Antonio Ereditato, confirming the muon-to-tau oscillation rules out the sterile neutrino hypothesis. But he pointed out that this is only Opera’s first observed event, with a relatively low confidence level: there is still a 2% chance that their observation is an experimental artifact. “I would bet some money on it, but neither my life nor my scientific reputation” he told Nature. Ereditato will feel much more confident once Opera, which will run until the end of 2012, has spotted another three or four tau neutrinos, he says. The event is so unlikely that scientists do not expect it to happen more than ten times during the experiment’s life.
A preprint paper should follow in a few days on Arxiv.org.
Image: A picture of the OPERA laboratory at Gran Sasso. Italy’s National Institute for Nuclear Physics (INFN)