New antineutrino results from China don't yet have an explanation, which is maybe a good thing.
Correction: An earlier version of this article identified the Daya Bay research as originating in Japan, while the researchers and institutions involved are from China. The post has since been updated. Motherboard regrets the error.
The past couple of weeks have seen much jubilation following the observation of gravitational waves after years of frustrated efforts and one notable "just kidding!" discovery in 2014. It's well-deserved, but results from a different lab, this one collecting data on antineutrino detections in China, remind us of the other side of things: being wrong.
What the Chinese results indicate is that the flux and energy distributions of observed antineutrinos do not match with our theoretical predictions. Our models might just be incorrect, or there might be a whole new, so-far unknown neutrino variety. But the results, which are reported this week in the Physical Review Letters, are far from being "bad." They're exciting. Being wrong is exciting.
The data behind the current paper was collected via six of the eight detectors that make up the Daya Bay Reactor Neutrino Experiment. These detectors look for antineutrinos likely to be produced in the nearby Daya Bay Nuclear Power Plant and the Ling Ao NPP. Antineutrinos make up a substantial fraction (around 4.5 percent) of the total energy released in a nuclear fission reaction.
The Chinese results are the product of 217 days of data collection. The energies of the antineutrinos were measured to within 1 percent uncertainty, which the physicists claim are the most precise antineutrino measurements to date. The Daya Bay group's data revealed a conspicuous "bump" in the energy levels of detected particles at around 4–6 MeV (megaelectronvolts).
Two similar antineutrino detection experiments, Double Chooz in France and RENO in Korea, have found excesses in the same energy range. While the spike is interesting, the really confounding thing is that the total excess in the 1 to 7 MeV range found across the three experiments is lower than expected by around 6 percent. It would seem that there are missing particles.
The new results offer a statistical significance of 4-sigma. This isn't the 5-sigma required for a proper discovery, but it's also very promising. The prior detections were 3-sigma (Double Chooz) and 3.5-sigma (RENO). It would seem that something interesting and unexpected is afoot.
Because of this incompleteness, any hints of "new physics" offer hope for a more accurate picture of the subatomic world.
A fair question to ask is what in the hell is an antineutrino in the first place? These antiparticle versions of neutrinos are produced naturally during the process of nuclear beta decay, e.g. when a neutron falls apart into an electron, proton, and antineutrino. This is why we hunt for them by nuclear reactors, where beta decay is taking place continually as the fragments leftover from fission reactions try to find a stable state to end up in. We've been measuring fission products for five decades now, but it's only been in the past couple of years that the anomalous bump and antineutrino deficiency have been apparent.
Crucially, these anomalies aren't accounted for in the Standard Model of Physics, the table of fundamental particles that's assumed to be incomplete, but has been kept around because it still does a decent job and there hasn't been anything better to come along. Because of this incompleteness, any hints of "new physics" offer hope for a more accurate picture of the subatomic world.
The new physics hinted at by the Daya Bay results might include an altogether new version of neutrino; one possibility is the theorized "sterile" neutrino. But more likely is that we need to further refine our understanding of nuclear decay in reactors or even how the detectors themselves work, as Thierry Lasserre of the CEA Saclay physics research institute in France tells Physics World. What being wrong in this case means is that we're going to learn something new about the universe. Eventually.
"Daya Bay now provides the most precise data, and this is a great result, but we don't have yet any solid explanation of what it means exactly," he says. "We need new experiments dedicated to search for sterile neutrinos, and several of them are currently being realized. We may expect new results within the next three years."
An open-access version of the results is available at the arXiv pre-preprint server.