Late last week, the Daya Bay experiment in China released a new set of measurements of the neutrinos produced by the nuclear reactors on the site. The new data provides further examples of these strange particles refusing to act like we'd expect them to. This evidence further supports strange behavior that some have interpreted as evidence of the existence of particles beyond the Standard Model, but the new data doesn't bring evidence up to the level of significance required to announce discovery.

For good measure, there's also evidence of an entirely different anomaly—one that could be anything from an indication of new physics to a sign that our experiments were fundamentally misguided.

Any flavor you like

Last year's Physics Nobel Prize went to the people who discovered that neutrinos are less a single particle and more of an identity-shifting family of particles. Neutrinos come in three types, or flavors: electron, muon, and tau. But the identity of any given neutrino isn't fixed; instead, it can shift among these identities over time. Thus, even if you started with a population of pure electron neutrinos, you'd find a few muon neutrinos in the mix as well, given sufficient time.

This change in identity is called a flavor oscillation, and its confirmation was key to the discovery that neutrinos have mass.

We can measure the probabilities of these flavor oscillations by creating a relatively pure beam of one flavor of neutrino and sending it to a detector a specific distance away. Depending on the distance, a specific fraction is expected to change identity. If you measure the number of neutrinos that do, then you can see whether the flavor oscillations align with theoretical expectations—and whether the total of all three probabilities adds up to one.

Somewhat disturbingly, it hasn't. A variety of experiments, conducted using different sources and different detector technology, all indicate that there seems to be a deficit. If we expect a certain fraction of our beam to oscillate into electron neutrinos, we always detect fewer of them than we'd predict. While this has been seen in a number of experiments, the deficit has never become statistically robust enough that physicists were willing to hand out more Nobel Prizes.

Daya Bay is now the latest experiment to see this sort of deficit. The detector, based in China, is located near a nuclear power plant with six large reactors. Each second, those reactors collectively produce 3,500 billion electron antineutrinos. Even though detecting neutrinos is rare, these numbers ensure that the Daya Bay experiment is able to pick them up regularly. In a 217-day run, the detectors observed more than 300,000 neutrinos.

Once again, however, the discrepancy is below the five-sigma standard of significance that physicists expect before a discovery could be declared.

Signs of sterility?

There's one possible explanation for the missing neutrinos: if there are more than three flavors, then some of the missing ones could have oscillated into an identity we're unable to detect. This is exactly what you'd expect if there were a class of particles that physicists have been wondering about for decades: sterile neutrinos.

The three types of neutrinos we know about take part in particle interactions through the weak force, which is involved in radioactive decay. Any further neutrinos would not interact in the same manner, meaning their only option for interactions is through gravity. This apparent indifference to most physical forces is what gave them the name "sterile."

It also would make them even harder to detect than a regular neutrino. In fact, at the meeting of the American Association for the Advancement of Science, physicist Carlo Giunti said that the only direct way of detecting their presence is through the sorts of anomalies seen at Daya Bay—strange numbers coming out of experiments designed to track flavor oscillations.

Cosmologist Olga Mena, however, said that this may be some evidence of neutrinos that we could see due to their gravitational effect. Studies of the Cosmic Microwave Background indicate that we've already discovered all the neutrinos there are. The experimental errors of those measurements, however, would allow a sterile neutrino if it's relatively rare, she said. Provided they were sufficiently massive, however, they would act as what Mena termed "warm dark matter."

Most of the large-scale structure of the Universe can be explained through cold dark matter—dark matter that's moving slowly enough to easily form gravitational aggregates. But this doesn't explain a few anomalies that appear at smaller scales, like the relative paucity of dwarf galaxies orbiting the Milky Way. One possible explanation for this deficit is that we're not especially good at spotting dwarf galaxies.

An alternative, according to Mena, is that there is also some warm dark matter around—matter moving fast enough that it's harder to get it to slow down and aggregate. Mena said this would suppress the formation of dwarf galaxies while leaving the cold dark matter to do the heavy lifting of forming the large-scale structures of the Universe. And a heavy sterile neutrino would make an excellent candidate for warm dark matter.

Anomalies on top of anomalies

The second Daya Bay oddity, however, may throw a monkey wrench into all of this. It comes through measurements of the energy carried by the neutrinos when they slammed into the detector. This energy comes from the nuclear decay itself: some of it goes into a photon, and the rest imparts momentum to the neutrino.

Berkeley's Kam Biu Luk, a spokesman for the Daya Bay experiment, told Ars that it's possible to measure that energy based on the amount of energy the neutrino deposits in the detector. The number of neutrinos at various energies can then be compared to the number you'd predict you'd see. That later number relies on a combination of our understanding of nuclear decays—along with the knowledge of all the isotopes present in an active nuclear reactor as well as the rate at which they're all decaying.

When the Daya Bay team did this comparison, the curve of the measurements largely traced the calculated expectations for a while. But at energies around 5 Mega-electronVolts, there was a discrepancy. It's not quite at the five sigma standard, but it's quite large, and Luk seemed to expect that the next run of the detector should be able to tell whether it's real.

What would it mean if this discrepancy is real? Here's where interpretations can differ. It could be that there's something odd happening in either nuclear decays or our understanding of the environment inside a nuclear reactor. Either of these could be potentially interesting, but such conclusions might not tell us anything dramatic about the nature of the Universe.

If it's the former—a lack of understanding of nuclear decays—it could mean that we're mistaken in our understanding of how many neutrinos should be produced in these reactions. In which case, our expectations for the number of neutrinos we should be seeing is off, and the apparent deficit we see is nothing more than an indication of our flawed understanding. Sterile neutrinos would safely remain in the realm of theoretical oddity.

And the last possibility is that the result is real and has nothing to do with the presence or absence of sterile neutrinos. In which case, neutrinos could once again be hinting at new physics.

The Daya Bay facility will continue taking data, and Fermilab's Peter Wilson told the audience at the meeting of Fermilab's efforts to build a neutrino source and set of detectors that should be able to nail down whether the neutrino deficits are real in a few years. For now, this is one of those mysteries that we'll just have to wait patiently to see resolved.