Neutrinos are a notorious thorn in physicists' sides, as the majority of them go missing while traveling from the Sun to our detectors on Earth. We're alsomiscounting their antimatter counterparts, which could mean big things for our understanding of physics.
The original neutrino mystery was solved back in 2001. Here's the short version: in the 1960s, physicist Ray Davis determined that only about a third of the predicted amount of solar neutrinos ever actually reached Earth. Decades later, Canada's Sudbury Neutrino Observatory found the missing neutrinos by realizing that there are actually three flavors of the particle, and Davis had only been detected the smallest variety, the electron neutrino. (For more background info on neutrinos, check out our field guide.)
But now we've got another neutrino-based mystery on our hand, albeit a much more subtle one. Researchers at the French atomic energy commission recently used the Double Chooz neutrino detector to check the predictions for antineutrinos - the antimatter counterparts of neutrinos - against the actual amount produced in nuclear reactors. These production rates were first calculated in the 1980s, and subsequent experiments had shown that they're more or less correct.
Except we now have far more accurate tools to make the measurements, and the French researchers discovered the rate of production is actually 3% more than predicted. This is where it gets tricky - if the initial prediction was wrong but the subsequent results were all correct, then this somehow means the nuclear reactor experiments were under-producing antineutrinos by about that 3%. We now have to account for those missing particles.
So where are all the particles? The most interesting possibility is that antineutrinos - and, by extension, neutrinos - don't just oscillate between the three flavors electron, muon, and tau. Instead, they can very occasionally oscillate into a fourth flavor known as sterile, so-named because neutrinos and antineutrinos in this state don't interact with ordinary matter.
Of course, "particle that doesn't interact with ordinary matter" is more or less the working definition for dark matter. Could sterile neutrinos and antineutrinos be what we're looking for to explain the universe's hidden mass? It's a definite possibility - this latest find follows previous hints of sterile neutrinos from experiments at the Los Alamos National Laboratory and Fermilab - although it might just be one of a bunch of different particles that make up dark matter.
We shouldn't get ahead of ourselves - this is definitely an interesting result, but it's not significant on its own. Physicists need to look for similar neutrino disparities in other experiments. After all, it's possible that the researchers themselves are the ones in error, so there's still plenty of work left to do to figure out whether sterile neutrino and antineutrinos deserve to cross over from the theoretical to the proven.