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MiniBooNE Results Inconsistent with Existence of "Sterile" Neutrinos

Photodetector array
Image courtesy of Fermilab
One neutrino anomaly has been resolved while another has sprung up, according to results from the MiniBooNE experiment at Fermilab. The results were officially announced the week before the APS April Meeting in Jacksonville, Florida, and there were several papers on the topic presented at the conference.

MiniBooNE is short for Mini Booster Neutrino Experiment, an international collaboration involving 77 physicists from 17 different institutions in the US and the United Kingdom. Its much-anticipated findings indicate that only three low-mass neutrino species exist: electron, muon and tau neutrinos. This in turn seems to rule out two-way neutrino oscillations involving a hypothetical fourth species of low-mass neutrino.

Several experiments had previously shown that neutrinos regularly transform from one species to another. Oscillating neutrinos are comprised of three different waves that combine in different ways as they travel through space. Small physical differences in mass lead to telltale interference effects. If, indeed, neutrinos oscillate–as seems to be the case per experimental results from Japan’s Super-Kamiokande collaboration announced in 1998–then they are not the massless particles assumed by the Standard Model.

About 10 years ago, the Liquid Scintillator Neutrino Detector (LSND) experiment at Los Alamos National Laboratory threw an unexpected wrinkle into the mix: the possibility of a fourth “sterile” neutrino that would only interact through gravity. The level of observed oscillations suggested very different values for neutrino masses than those inferred from prior studies of solar neutrinos and other accelerator-based experiments. MiniBooNE was conceived to test the results of the LSND experiment.

For the experiment, protons from Fermilab’s booster accelerator smashed into a fixed target, creating a swarm of mesons, which very quickly decayed into secondary particles, including many muon neutrinos. The MiniBooNE detector was placed 500 meters away. Although muon neutrinos might oscillate into electron neutrinos, over the short run from the fixed target to the detector, the scientists expected very few oscillations to occur.

The LSND and Fermilab detector both looked for electron neutrinos. Fermilab tried to approximate the same ratio of source-detector distance to neutrino energy, thereby setting the amount of likely oscillation. LSND used 30 MeV neutrinos observed after a 30-meter distance, while the earlier Fermilab experiment used 500 MeV neutrinos detected after a distance of 500 meters.

The trick 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 telltale signature–an 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 a proton. LSND saw a small but statistically significant (the team argued) number of electron neutrino events.

According to Heather Ray of Los Alamos, when analyzing MiniBooNE’s data, they took a “blind box” approach, meaning that as they were collecting the neutrino data, they didn’t even look at any of the data in the region of interest: the region where they would expect to see the same signature of oscillations as LSND. They didn’t “unblind” the data and open the box until three weeks before the official announcement.

Upon doing so, they found no telltale oscillation signature, contradicting the LSND findings from 1995. So MiniBooNE’s results rule out a fourth sterile neutrino, thereby verifying the current Standard Model with its three low-mass neutrino species.

However, a new anomaly presented itself. There were some electron neutrino events detected at low neutrino energies, and this tiny subset of data remains a mystery. More experiments are planned to explore this anomaly, this time using a beam of anti-neutrinos.

Project spokesperson Janet Conrad (Columbia University) said that the MiniBooNE data are robust and that, while some new physical effect cannot be ruled out, 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.

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