According to the standard model of particle physics, the neutrino is one of the elementary particles of our universe. There are three matter flavors of neutrino, one each to accompany the electron and its two heavier relatives, the muon and tau particles. Of course there are also three anti-matter versions, which accompany the positron and its more massive brothers. However, the latest results from the MiniBooNE experiment not only seem to directly confirm that the symmetry between matter and anti-matter is broken, but that there is also an extra neutrino flavor [Aguilar-Arevalo et al., (2010) Phys Rev Lett 105, 181801].
One bizarre property of neutrinos is their ability to spontaneously flip between individual flavors. While a neutrino may start out as a muon-neutrino, it can easily become an electron-neutrino. By studying the number of muon- and electron-neutrinos which reach a detector from a muon-neutrino source, it is possible to check whether the predictions of the standard model hold up. In MiniBooNE the neutrinos are incident on a detector composed of a 40-foot sphere containing 800 tons of pure mineral oil. The few neutrino reactions that occur produce muons or electrons which result in flashes of light thanks to Cherenkov radiation. This light is then detected by one of over 1500 photomultiplier tubes which are situated within the detector.
But this isn’t the first time an experiment like this has been performed. In the 1990s the Liquid Scintillator Neutrino Detector (LSND) at Los Alamos National Lab found evidence for the sterile neutrino, when more anti electron-neutrinos reached the detector than they expected. This suggested that there may be an extra neutrino flavor, and thus forces beyond our current understanding. Co-author of the new paper, and spokesman for the MiniBoone experiment, Prof. Van de Water explains “Sterile neutrinos are the right handed partner of normal (left handed) neutrinos (also called active neutrinos). The left handed neutrinos can interact via the weak interaction, but the right handed do not. Thus the name sterile”. “The existence of sterile neutrinos does not imply a fourth family of leptons. Given the three neutrino families, we might expect sterile neutrinos to exist in three similar states as well, but that is only speculation. There could be only one, or a zoo of sterile states.” However, the LSND result was controversial as other experiments failed to detect any such deviation from the standard model.
The LSND experiment found an excess of oscillations from anti muon-neutrinos to anti electron neutrinos. Initial results using neutrinos in MiniBoone failed to detect the same excess. However, now that MiniBoone is operating in anti neutrino mode, the new results seem to confirm the LSND result. This is not only startling because of the implications on the number of possible neutrino states, but also because it represents a direct confirmation of charge-parity (CP) symmetry breaking. CP symmetry tells us that matter and anti-matter are completely equivalent, aside from being opposite. If correct, this result reveals that this is not the case.
However, Prof. Byron Roe makes it clear that the story is far from over. “Our result is not yet at the stage of proof, but at the stage of strong indications. If the effect is real it would imply CP violation”.
Experiments continue at MiniBooNE, as well as around the world. Experiments using different types of detector have been scheduled at Fermilab and proposed for CERN. Hopefully one of these experiments will provide the final evidence and resolve any doubt. According to Roe, “The Fermilab device may be too small to verify this, but the CERN device is much larger and should be able to fully settle the matter”.
Stewart Bland