New particle probably can’t explain nuclear reactor neutrino mystery

A puzzling neutrino shortfall seems to be due to faulty predictions, not a new particle.

In experiments at nuclear reactors, scientists have consistently found about 6 percent fewer antineutrinos, the antimatter form of neutrinos, than expected. That deficit could hint that the lightweight particles are morphing into undetectable new particles called sterile neutrinos (SN: 3/19/16, p. 14). But in a paper published online April 4 at arXiv.org, scientists with the Daya Bay experiment, located near a nuclear power plant in China, point to the calculations that underlie scientists’ predictions to explain the missing antineutrinos.

Inside nuclear reactors, multitudes of antineutrinos are born in radioactive decays of isotopes such as uranium-235 and plutonium-239. Scientists can predict how many antineutrinos each isotope should produce. If sterile neutrinos are the source of the disagreement, detectors should observe an antineutrino deficit from both isotopes. Instead, the researchers found that plutonium-239 agreed with predictions, but researchers detected fewer neutrinos than expected from uranium-235. That means there’s probably something funny with the uranium-235 calculations.

This isn’t the end for sterile neutrinos — there are other hints that they exist. If so, sterile neutrinos could constitute dark matter, an unknown invisible substance that pervades the universe.