In the 1990s an experiment studying neutrinos saw something strange: too many particles showed up in its detector. In 2002 scientists began another experiment to figure out what happened. That trial also got surprising results—yet in a different way. Then came a third experiment in 2015. That one announced measurements last week that do not resolve either puzzle and only heighten the mystery.

The projects have all looked at neutrinos—nature’s most abundant particle, save for photons (particles of light). These tiny, chargeless specks stream out of the sun, as well as supernovae and other cosmic events, and about a trillion of them pass through your hand each second. They are known to come in three types, or flavors: electron, muon and tau neutrinos. But many scientists hope that a fourth type called “sterile neutrinos” will appear. If they exist, sterile neutrinos could help solve several mysteries in physics, such as why neutrinos have mass when theories predicted they should not and what the invisible dark matter pervading the cosmos is made of. The puzzling excess particles at the earlier experiments got researchers excited because they looked like possible signs of sterile neutrinos interfering with the normal neutrino flavors.

Such posited neutrinos are called “sterile” because they would only interact with other particles via gravity, whereas the known three flavors can do so through the weak force as well. But they could affect other neutrinos because of a weird property these particles all share: the ability to “oscillate,” or change flavor. A particle that starts off as an electron neutrino, for instance, can turn into a tau or muon neutrino, and vice versa. Usually, this transformation takes place while neutrinos travel a certain distance, but it seemed to be happening more quickly at the experiments—the Liquid Scintillator Neutrino Detector (LSND) at Los Alamos National Laboratory and its follow-up, the Mini Booster Neutrino Experiment (MiniBooNE) at Fermi National Accelerator Laboratory (Fermilab) in Batavia, Ill. Scientists thought that muon neutrinos might be oscillating into sterile neutrinos and then into electron neutrinos, a process that could happen faster than the simple muon-to-electron flavor switch.

Precision View

The latest findings come from MiniBooNE’s successor, the MicroBooNE experiment, also at Fermilab. Physicists there generate a stream of muon neutrinos and shoot them to a detector 470 meters away. The detector, a giant vat filled with 170 metric tons of pure liquid argon, waits to catch neutrinos in the act of slamming into the nucleus of one of the argon atoms. Such collisions are extremely rare, and the only signs of them are the secondary particles produced by the interaction.

Scientists announced the results from MicroBooNE on October 27, saying they saw no sign of the excess observed by MiniBooNE. “Yeah, this is a little strange,” says MicroBooNE co-spokesperson Bonnie Fleming of Yale University. The earlier experiments saw extra particles that looked like electrons or photons, though they could not confirm either of those possibilities. MicroBooNE, however, can look much more precisely at the direction particles travel in its detector and the energy they deposit. “It means we can resolve whether or not something is an electron or a photon,” Fleming says. “A real triumph of the experiment is that the technology works so well.” Yet the result is that MicroBooNE scientists are pretty sure there are no excess electrons or photons in all the places they looked, dampening hopes for certain versions of sterile neutrinos. If muon neutrinos could quickly turn into sterile neutrinos and then into electron neutrinos, electrons would have shown up at MicroBooNE. (The detector is not far enough from its source for the mundane muon-neutrino-to-electron-neutrino oscillation to occur.)

But if there are no extra electrons or photons, then what are the surplus particles that LSND and MiniBooNE saw? One option is that unexplained neutrino collisions were not actually taking place at either of those earlier experiments and that, in the case of MiniBooNE, researchers just missed some interference inside that experiment’s detector. “Perhaps there is something about the detector that is not fully understood,” says Joachim Kopp, a theoretical physicist at CERN, the European laboratory for particle physics near Geneva, and Johannes Gutenberg University Mainz in Germany. “But I consider that highly unlikely—these are highly skilled people running this.”

Others agree. “It’s very unlikely that there is some miscalibration of the detector,” says Northwestern University theoretical physicist André de Gouvêa. “There has to be a new source of either electrons or photons or something that looks like electrons or photons.” Perhaps, he says, something more complex is going on. Instead of a muon neutrino oscillating into a sterile neutrino and then into an electron neutrino, the neutrino source that provides the muon neutrinos could also produce heavier sterile neutrinos—or some other new particle. These particles might decay into other things—for instance, a regular neutrino and something exotic, such as a “dark photon” (a cousin to regular photons that has been theorized but never found). If this process were to occur, it would result in an electron and its antimatter partner, a positron, which would show up as a different signal than a plain electron. MicroBooNE has not searched for such pairs yet.

Sterile neutrinos remain an appealing prospect to physicists. They are a likely by-product of theories attempting to explain why neutrinos have mass at all. They could also help explain what dark matter is. Certain kinds of sterile neutrinos could be candidates for dark matter themselves, or they could be part of a “dark sector” in which a dark matter particle would be related to, or decay into, sterile neutrinos. “If you ask around [among physicists] and say, ‘Do you think a sterile neutrino exists?’ I think everybody would say it’s very plausible,” de Gouvêa says. “But the devil is in the details. Is it heavy or light, easy to see or hard to see?”

And figuring out what is going on at these neutrino experiments could be the first step to answering those larger questions. “It’s really interesting because all the obvious possibilities have been tested now,” Kopp says. “The good thing is: we have the tools to investigate this further and hopefully get to the bottom of it.”

What Lies Ahead

MicroBooNE is one part of a larger neutrino project at Fermilab called the Short-Baseline Neutrino (SBN) Program, which includes three liquid argon neutrino detectors spaced out at different distances from the neutrino source. The other two detectors are the Short-Baseline Near Detector (SBND), at just 110 meters from the source, and the farther ICARUS T600 detector, at a distance of 600 meters. MicroBooNE, in the middle at 470 meters, started collecting data first, and ICARUS and SBND will begin this year and in 2023, respectively.

“The plan is to do a measurement that is much more inclusive, looking in the possible regions of allowed parameters in a more global way,” says SBND co-spokesperson Ornella Palamara of Fermilab. If the right interpretation for the MiniBooNE results is sterile neutrinos, for instance, then, in addition to the appearance of electron neutrinos, scientists would see a corresponding disappearance of muon neutrino events at the far detector as the particles traveled farther from the neutrino source. “That’s the strength of the SBN multidetector program that you cannot do with a single detector,” Palamara says. “There’s a lot we can do, and these analyses from MicroBooNE are the beginning.”

And the technological sophistication of the recent MicroBooNE analysis bodes well for the prospects ahead, scientists say. “This is very exciting because MicroBooNE has achieved a level of knowledge in understanding these neutrino events that is unprecedented,” says Marcela Carena, head of the Theory Division at Fermilab. “This will enable future experiments to boost their physics searches.”

For now, the verdict on sterile neutrinos is still out. “I do not think the sterile neutrino idea is dead,” Carena says. “The search for sterile neutrinos continues.”