One neutrino anomaly has been resolved while another has sprung up. A Fermilab experiment called MiniBooNE provides staunch new evidence for the idea that only three low-mass neutrino species exist. These results, reported over the past week at a Fermilab lecture and at the American Physical Society (APS) meeting in Jacksonville, Florida, seem to rule out two-way neutrino oscillations involving a hypothetical fourth type of low-mass neutrino.
Several experiments have previously shown that neutrinos, very light or even massless particles that only interact via gravity and the weak nuclear force, lead a schizoid life, regularly transforming from one species into another. These neutrino oscillations were presumably taking place among the three known types recognized by the standard model of particle physics: electron neutrinos, muon neutrinos, and tau neutrinos.
However, one experiment, the Liquid Scintillator Neutrino Detector (LSND) experiment at Los Alamos, provided a level of oscillation that implied the existence of a fourth neutrino species, a “sterile neutrino,” so-called because it would interact only through gravity, the weakest of physical forces. (For background see Physics Today, August 1995 and http://www.aip.org/pnu/1995/split/pnu239-1.htm and http://www.aip.org/pnu/1996/split/pnu269-1.htm)
From the start, this result stood apart from other investigations, especially since it suggested possible neutrino masses very different from those inferred from the study of solar or atomospheric neutrinos or from other accelerator-based neutrino experiments.
MiniBooNE (whose name is short for Booster Neutrino Experiment; the “mini” refers to the fact that they use one detector rather than the originally proposed two) set out to resolve the mystery.
The experiment proceeds as follows: protons from Fermilab’s booster accelerator are smashed into a fixed target, creating a swarm of mesons which very quickly decay into secondary particles, among them a lot of muon neutrinos. Five hundred meters away is the MiniBooNE detector.Although muon neutrinos might well oscillate into electron neutrinos, over the short run from the fixed target to the detector one would expect very few oscillations to have occurred.
The Fermilab detector, and the LSND detector before it, looked for electron neutrinos. Seeking to address directly the LSND oscillation effect, Fermilab tried to approximate the same ratio of source-detector distance to neutrino energy. This ratio sets the amount of likely oscillation. The Los Alamos experiment used 30 MeV neutrinos observed after a 30 m distance; the Fermilab experiment used 500 MeV neutrinos detected after a distance of 500 m.
The trick of doing this kind of experiment is to discriminate between the few rare events in which an electron neutrino strikes a neutron in a huge bath of mineral oil, thereby creating a characteristic electron plus a slow moving proton, and the much more common event in which a muon neutrino strikes a proton to make a muon and proton. LSND saw a small (but, they argued, statistically significant) number of electron neutrino events. MiniBooNE, after taking into account expected background events, sees none. Thus they see no oscillation and therefore no evidence for a fourth neutrino.
Actually it’s not exactly true that they see no electron neutrinos.
At low neutrino energy they do see events, and this tiny subset of the data remains a mystery, to be explored in further data taking now underway using a beam of anti-neutrinos. At the APS meeting, MiniBooNE co-spokesperson Janet Conrad (Columbia Univ) said that the low-energy data are robust (meaning that a shortage of statistical evidence or systematic problems with the apparatus should not be major factors) and that some new physical effect cannot be ruled out.
At the very least, the low-energy data do not undo the new assertion that the earlier LSND results cannot be explained by the existence of a fourth neutrino type. (Fermilab press release and figures, http://www.fnal.gov/pub/presspass/images/BooNE-images.html)