Don't toss out your particle physics textbooks just yet. Last Friday, a team of particle physicists announced results that could point to an exotic new particle called a sterile neutrino, triggering a cascade of feverish headlines. But the situation is more ambiguous than some reports suggest. Although the new data bolster one argument for the sterile neutrino, other evidence has weakened significantly in recent years.
Nearly massless, neutrinos interact only through the incredibly feeble weak nuclear force and, thus, barely interact with other matter. They come in three known types—electron, μ, and τ—depending on the specific particle interactions that create them. One type of neutrino can morph into another through so-called oscillations, which physicists study by firing streams of the particles through Earth to huge detectors hundreds of kilometers away.
A sterile neutrino would be a fourth type that would be even more elusive because it wouldn't directly interact with other matter at all. It could emerge only when an ordinary neutrino oscillates into it, and it would disappear in the same way. The first hint of sterile neutrinos came in the 1990s, from physicists with the Liquid Scintillator Neutrino Detector (LSND) at Los Alamos National Laboratory in New Mexico, who reported evidence for a particle about 1/500,000 as massive as an electron—light, but heavier than the other neutrinos. For technical reasons, LSND researchers actually studied μ antineutrinos—the antiparticles of μ neutrinos—traveling just 30 meters to their detector. They detected dozens of electron antineutrinos in the beam—too many to be produced over the short distance by the usual oscillation mechanism. The team suggested that the μ antineutrinos could be morphing into sterile neutrinos, which in turn morphed into electron antineutrinos.
Now, the 47 physicists working with the Mini Booster Neutrino Experiment (MiniBooNE) at the Fermi National Accelerator Laboratory in Batavia, Illinois, report similar and far more statistically significant evidence of such "extra" oscillations. They fired slightly higher energy μ neutrinos and antineutrinos 541 meters into their much larger detector and spotted 460 electron neutrinos or antineutrinos, they report in a paper posted on the arXiv preprint server. The MiniBooNE reported similar, less significant results in 2013.
However, the case for the sterile neutrino has long rested on other lines of evidence, and some of them have been weakening. For example, theorists have for years noted that nuclear reactors appear to produce about 6% fewer electron antineutrinos than standard theory predicts, suggesting that some of them were oscillating into sterile neutrinos. However, in April 2017, physicists with the Daya Bay Reactor Neutrino Experiment near Shenzhen, China, reported that the entire deficit could be explained if theorists had simply overestimated the number of antineutrinos produced by one component of the complex fuel, uranium-235.
In addition, scientists with the European Space Agency's Planck spacecraft in 2013 released an exquisitely precise measurement of the afterglow of the big bang, the cosmic microwave background (CMB). By studying variations in the microwaves across the sky, cosmologists can estimate the number of types of neutrinos. Whereas earlier CMB studies left room for a fourth neutrino type, the Planck data indicate that only the three known types exist. To find a viable model of sterile neutrinos, theorists will have to explain those seemingly negative observations, too.
As for the new MiniBooNE result, other particle physicists are reacting with caution. The excess events pile up in the low end of the detector’s energy range, notes Tommaso Dorigo, a particle physicist with Italy's National Institute for Nuclear Physics in Padua, on his blog. That’s also where the "backgrounds" from various other types of particles pile up. So if MiniBooNE researchers underestimated their background, Dorigo explains, then they may have essentially mistaken background events for a signal.