New Analysis At Nuclear Reactor Reignites Search For Mysterious ‘Sterile’ Neutrino

You’re probably aware that stuff is made from particles. But the second most abundant particle in the universe, the neutrino, refuses to be fully understood. This tiny and elusive speck only barely interacts with the other particles that make up us humans and our galaxy. Its mysteries continue to confound the public and get scientists talking, to this day.

Image: University of California, Lawrence Berkeley National Laboratory

There are three kinds of these neutrinos, the “electron”, “muon” and “tau” and they can switch identities through a process called neutrino oscillation. But lots of evidence from several experiments hints at another neutrino, even more difficult to detect than the other three, called the “sterile neutrino.” This past April, a new paper caused the media to cast doubt on the existence of this strange particle.

But yet another analysis shows the sterile neutrino is far from dead. And understanding the “reactor anomaly” fuelling this conversation can have important implications for the nuclear power industry.

“The general sentiment is that there is a need for new experiments,” Anna Hayes, first author on a new analysis posted on the arXiv preprint server from Los Alamos National Lab told Gizmodo.

The sterile neutrino has a long story. Experiments on nuclear reactors and particle accelerators have confirmed the strange neutrino identity-switching oscillation behaviour, which won scientists Takaaki Kajita and Arthur McDonald the 2015 Nobel Prize in physics. But an experiment called the Liquid Scintillator Neutrino Detector at Los Alamos saw extra-electron “antineutrinos” (in particle physics, every particle has an antiparticle that acts exactly the same but annihilates its partner should they ever meet) in the oscillation process. Another experiment called MiniBooNE saw some extra data in its own oscillations, too.

This extra data implied that maybe there’s some fourth neutrino, the sterile neutrino, that experiments can’t detect, one which the other three neutrinos can change into. That might feel like blaming unexplained cookies in the cupboard on ghosts, but that’s not necessarily true. There are four confirmed ways that particles communicate with one another: gravity, electromagnetism, the strong nuclear force and the weak nuclear force.

Regular neutrinos speak to other particles with the weak nuclear force and gravity, but perhaps these sterile neutrinos only speak via gravity, which would explain why experiments can’t see them.

Let’s say you’ve got a walkie talkie tuned to the weak nuclear force channel. Neutrinos are broadcasting their information with two walkie talkies: one on your channel and one gravity channel. But if your radio can’t pick up signals on the gravity channel and that’s the only place sterile neutrinos can broadcast, then you can’t hear them.

Recently, eyes have been on the Daya Bay nuclear power plant in China, where scientists have set up special neutrino detectors to catch the particles coming out of the reactor. Giant pools of a special liquid produce tiny flashes of light when a neutrino interacts with their molecules via the weak nuclear force. These flashes are amplified and turned into signals in the detectors.

The experiment has revealed what folks refer to as the “reactor anomaly.” Bryce Littlejohn from the Illinois Institute of Technology explained to Gizmodo in an email: “Reactor neutrino experiments,” those where neutrino detectors observe the radioactive elements at nuclear power plants, “see 6% fewer reactor neutrinos than what theorists predict they should see.

One hypothesis for this deficit is that some of the electron-type neutrinos produced in the reactors are changing into even-more-ghostly sterile neutrinos en route to the various reactor neutrino detectors.” The detectors can’t communicate with these sterile neutrinos.

But a new set of results from April got lots of coverage and cast doubt on whether sterile neutrinos explain the anomaly. Essentially, researchers found that the size of the anomaly changed as the elemental composition of Daya Bay’s radioactive fuel changed over time. Those results implied that maybe the detectors were simply misjudging the number of antineutrinos coming from one of the fuel’s uranium flavours, uranium-235.

“Maybe the U-235 discrepancy could explain the reactor anomaly,” Karsten Heeger, neutrino physicist at Yale University responsible for much of the United States’ contribution to Daya Bay’s design and assembly, told Gizmodo. Simply, people started saying maybe they hadn’t found evidence for sterile neutrinos after all.

Those headlines sparked lots of conversation, but Hayes’ new analysis shows that the uranium-235 results might not explain away the anomaly. “We would say the new Daya Bay results certainly question the origin of the anomaly, but they don’t rule out sterile neutrinos.” The researchers went back to nuclear power industry and other databases and looked at the neutrino fluxes and nuclear decay data again.

And while the size of the neutrino anomaly was smaller than the six per cent anomaly described by Littlejohn, it was still present and unchanging with the fuel, refuting the news reports published in April. Sterile neutrinos aren’t dead yet — though Hayes wouldn’t rule out the fact that maybe scientists just don’t know as much about nuclear physics as they thought.

Littlejohn pointed out that many neutrino physicists think there could be something wrong with some analysis somewhere. But “the only real way to address sterile neutrinos at reactors once and for all is to directly measure sterile neutrino transformations with a new type of reactor experiment,” he wrote. He and Heeger both work on such an experiment called PROSPECT. Rather than a mixture of isotopes like those found at Daya Bay, PROSPECT will observe antineutrinos from an Oak Ridge National Laboratory reactor that uses pure uranium-235.

“New experiments would go to a much smaller reactor and go very close, say less than ten meters,” said Hayes. Daya Bay’s detectors are located hundreds of meters from the reactor but PROSPECT is only seven meters from its reactor, allowing researchers to observe neutrinos much sooner after they’re emitted. Hayes pointed out that other experiments in Europe that will similarly attempt to explain the anomaly.

And if this all feels like some esoteric particle physics, it’s actually quite important for nuclear reactors in general. “Beta decaying,” the neutrino-producing process, “produces a lot of heat,” said Hayes. “If we aren’t counting the neutrino correctly, we’re not counting the amount of heat correctly,” which could have important implications in reactor safety.

Heeger and Hayes both pointed out the sterile neutrino problem transcends neutrino physics, calling in know-how from other particle physicists and nuclear physicists. “If this is a nuclear physics problem,” said Hayes, “for another community it’s equally important.”

[arXiv, h/t Dan Garisto!]