Neutrino experiments have always been a source of the unexpected. When physicist Ray Davis and astronomer John Bahcall set up an underground detector at the Homestake mine in South Dakota in 1964, they were trying to intercept the constant stream of tiny ghostly particles emanating from the sun (1). Although the experiment was a success, the scientists were left scratching their heads. They observed only about a third of the neutrinos anticipated from their theoretical calculations.

Recent measurements of neutrino fluxes inside the Daya Bay Reactor Neutrino Experiment, pictured here, seemed to turn up significantly fewer neutrinos than theorists had predicted, potentially due to the existence of sterile neutrinos. Image courtesy of Interactions.org/Institute of High Energy Physics, Chinese Academy of Sciences.

Eventually explained as neutrino “oscillation” or change from one of the three known neutrino types to another, the result also implied neutrinos have mass, an astounding discovery that was recognized with the 2015 Nobel Prize in Physics.

Some experimentalists now think they’ve stumbled across another unexpected result: neutrinos might have an odder and even more ghostly cousin: the sterile neutrino. In the last two decades, detectors placed near nuclear accelerators, reactors, and radioactive isotopes seem to keep finding either more or fewer neutrinos than they should. The results could indicate that over short distances the three known neutrinos oscillate into a sterile type, which doesn’t interact with the weak force and therefore can’t show up in existing detectors. A worldwide effort is currently underway to figure out if this is in fact the case, with answers expected in the next few years. If discovered, the sterile neutrino would be the first particle found beyond the Standard Model and could potentially upend much of cosmology and particle physics.

“It’s probably one of the hottest but most speculative topics in neutrino physics right now,” says physicist Karsten Heeger of Yale University. “Some people are convinced they exist and some say it’s total nonsense, but the implications are huge.”

Anomalies Galore Neutrinos are very weakly interacting and at least six orders-of-magnitude lighter than any other massive particle in the Standard Model of particle physics. They can pass undisturbed through a mountain, so detecting them is tricky. Physicists generally gather an enormous mass of some substance and wait until a neutrino hits one of the molecules, generating some effect like a flash of light. The Liquid Scintillator Neutrino Detector (LSND), which ran from 1993 to 1998, used a 167-ton vat of mineral oil to watch if neutrinos emerging from an accelerator at Los Alamos National Laboratory in New Mexico were oscillating between the known types (at the time, a controversial idea). Neutrino oscillations would eventually be proven through experiments, such as Super-Kamiokande in Japan and the Sudbury Neutrino Observatory in Canada. But in the LSND data, scientists discovered a strange anomaly: a curious surplus of electron antineutrinos seemingly coming out of nowhere (2). Around the same time, researchers with the Gallium Experiment (GALLEX) at Gran Sasso National Laboratory in Italy and the Soviet–American Gallium Experiment (SAGE) in the Caucus Mountains of Russia were studying neutrinos coming from the sun. Both collaborations calibrated their detectors using the decay of radioactive isotopes of chromium and argon. But they counted fewer electron neutrinos than expected from these sources, as if some were disappearing before their eyes (3). Another team decided to get to the bottom of the LSND results in 2002 with the Mini Booster Neutrino Experiment (MiniBooNE), which also used mineral oil to search for neutrino oscillations at the Fermi National Accelerator Laboratory in Illinois. Whereas MiniBooNE initially seemed to rule out the LSND anomaly, it also found an odd excess of both neutrinos and antineutrinos that couldn’t be squared with the earlier results (4). A final experimental anomaly arose in 2011. Armed with an understanding of neutrino oscillations and their properties, some physicists recalculated the number of neutrinos they expected to see coming out of nuclear reactors and found they’d been underestimating the amount for decades. Although detectors placed at reactors had previously fit perfectly with theory, the recalculation suggested they were actually seeing a strange neutrino deficit. The shortfall from this recalculation was experimentally confirmed in February of this year using the Daya Bay experiment near a reactor in Hong Kong (5). What ties all of these results together is that they entail examining neutrino oscillations over short distances. Neutrinos streaming from the sun have 150 million kilometers to flip back and forth between their different forms. But something odd seems to happen when the distance between a neutrino source and detector is shrunk down to only a few tens or hundreds of meters. One possible explanation is that a small percentage of neutrinos are oscillating over meter-length scales into one or more sterile types. But many researchers remain skeptical, attributing the anomalies to background noise or systematic errors.