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Neutrino physics is filled with questions. Is the neutrino its own antiparticle? How are the known mass states of neutrinos ordered from lightest to heaviest? Of late, researchers have even asked: How many neutrinos are there? In addition to the three known types of neutrinos—electron, muon, and tau—experiments in the last decade have hinted at the existence of one or more “sterile” neutrinos. The hypothetical particles could partially explain discrepancies between theoretical predictions and experiments involving neutrinos from radioactive sources, particle accelerators and, most recently, nuclear reactors. Now, however, the collaboration behind the Daya Bay Reactor Neutrino Experiment in China has presented convincing evidence that links the reactor “anomaly” to uncertain theoretical models, not new particles. While weakening the case for sterile neutrinos, the new finding also opens the door to improving these models, which could impact reactor safety and the monitoring of reactors for unauthorized nuclear weapon development.

Nuclear reactors are used worldwide for power production and basic research. The enormous heat provided by these reactors is generated by the fission of heavy isotopes, usually uranium or plutonium, which split into lighter nuclei after absorbing a reactor neutron. The resulting fission products are radioactive and undergo beta decay (Fig. ). During this process, a neutron converts into a proton and emits an electron and an electron antineutrino, which share the energy released by the decaying nucleus. A typical reactor will emit about 102neutrinos per second.

The reactor anomaly first appeared in 2011 when a group re-calculated the precise energy spectrum of antineutrinos emitted from the fission of 235U, 239Pu , 238U, and 241Pu. These calculations, which were confirmed the same year, predicted a significantly higher antineutrino flux than what experiments had detected. That is, antineutrinos seemed to be “missing” from experiments. When researchers re-evaluated parameters that affect the calculated flux, the deficit only became worse, amounting to 6%, or 3 times the uncertainty in the calculated spectrum.

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