For decades, physicists have known that particles called neutrinos, which have almost no mass and barely interact with other matter, come in three types—electron, muon, and tau. And for just as long, some theorists have argued that there could exist a fourth, sterile neutrino that would be even weirder and more inert than its familiar cousins. But the case for the sterile neutrino just took a hit, as physicists working on an experiment in China report data that undermine one of three key pieces of data for its existence.
The different types of neutrinos are born of different particle decays and interactions. For example, an electron neutrino—more precisely, an electron antineutrino—emerges when an atomic nucleus such as tritium undergoes a type of radioactive decay called “β decay” and turns into a slightly less massive helium-3 nucleus while spitting out an electron and an antineutrino. Similarly, a muon neutrino can emerge from the decay of a particle called a muon, which is commonly found in cosmic rays. And a tau neutrino emerges in the decay of a massive particle called a tau that can be produced with an atom smasher.
Since 1998, physicists have also known that neutrinos can change type as they zing along at near light-speed, so that a muon neutrino can become an electron neutrino, and so on. A sterile neutrino would be a fourth type that couldn’t be born in the decay of any known particle or even interact with ordinary particles. Instead, it could only arise if one of the known neutrinos morphed into it.
For 20 years, various experiments have hinted at sterile neutrinos with a mass of about 1 electron volt, about 10 to 100 times as massive as the other neutrinos are thought to be. For example, from 1993 to 1998 physicists with the Liquid Scintillator Neutrino Detector at Los Alamos National Laboratory in New Mexico studied a beam of muon neutrinos and found tantalizing clues that they might be morphing into sterile neutrinos. Another indication comes from a pair of experiments started in the 1990s in Russia and Germany that was designed to sense electron neutrinos from the sun. Both experiments used detectors made of gallium, and when researchers calibrated them with radioactive sources, they counted too few electron neutrinos, suggesting they were quickly morphing into sterile ones.
The latest evidence for sterile neutrinos emerged in 2011, when a team of theorists argued that various experiments that detect electron antineutrinos from nearby nuclear reactors saw fewer antineutrinos than they should. Dubbed the reactor antineutrino anomaly, that deficit bolstered the case for a sterile neutrino, as it suggested that the antineutrinos were morphing into the undetectable sterile form. In fact, the reactor antineutrino anomaly rekindled interest in the idea of sterile neutrinos, says Patrick Huber, a theorist at Virginia Polytechnic Institute and State University in Blacksburg and a collaborator on the Daya Bay Reactor Neutrino Experiment near Shenzhen, China.
Now, however, physicists with Daya Bay report data that support a much simpler explanation: Scientists are merely overestimating the number of neutrinos born from the various radioactive nuclei produced in the fission of one component of standard nuclear fuel.
The Daya Bay experiment comprises six detectors in three clusters, all within 1.9 kilometers of six working nuclear reactors. Physicists study antineutrinos from the reactor cores, and in 2012 they reported the measurement of a key parameter in the morphing of neutrinos.
A nuclear reactor derives power from the fission of four different atomic nuclei: uranium-235, uranium-238, plutonium-239, and plutonium-241. These nuclei split randomly to make myriad lighter nuclei. So, for example, uranium-235 can split to make krypton-89. Neutron-rich krypton-89 will then β decay repeatedly to form rubidium-89, strontium-89, and yttrium-89, spitting out an antineutrino at each step. So each type of fissionable nucleus gives rise to myriad other nuclei that spit out antineutrinos. And physicists have measured the total spectrum of antineutrinos originating with each of the four key isotopes.
Crucially, the relative amounts of the four fissionable isotopes change as a reactor consumes its fuel. The fuel starts out as a mixture of uranium isotopes, and the plutonium isotopes are “bred” in place. So over the life of the fuel—about 18 months—the amount of uranium-235 decreases. By measuring the spectrum of the antineutrinos and knowing the fractions of uranium-235 in the cores, Daya Bay physicists were able to show that the supposed deficit in the number of antineutrinos goes up and down with the amount of uranium-235, they report in a preprint posted to the arXiv server.
That makes sense if physicists are underestimating the number of antineutrinos from uranium-235 decays, Huber says. But it wouldn’t make sense if the effect were being caused by electron antineutrinos morphing into sterile ones, in which case the deficit should be constant over time, he explains. “The Daya Bay result is clearly not favoring the interpretation of a sterile neutrino.”
The new results do leave physicists with a mystery, however: Why are their estimates for the antineutrinos coming from uranium-235 so bad? “That is certainly the million-dollar question,” says Kam-Biu Luk, a physicist at the University of California, Berkeley, and co-spokesperson for the Daya Bay team.
Huber says he’s not quite ready to give up on the idea of a sterile neutrino. In fact, he says, most theoretical ideas about how neutrinos get their tiny, but not-quite-zero masses assume that sterile neutrinos have to exist—however, they could be hugely massive and thus nothing like the sterile neutrino hinted at so far. “I’m still on the fence” about sterile neutrinos, Huber says. “Every 2 years a piece of evidence comes along that points one way or the other, but it’s never decisive.”