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The sterile neutrino hypothesis cannot account for why the neutrino flux seen from uranium-235 fission is lower than expected by theory, according to data from the French experiment STEREO. The STEREO team, on the other hand, thinks that the discrepancy results from theoretical challenges in simulating the decay process. The outcome represents the culmination of an 11-year investigation into a theory that was first put forth by a group of researchers that included two STEREO collaboration members.

One of the most important developments in particle physics throughout the 20th century was the Japanese Super-Kamiokande detector’s 1998 discovery of neutrino oscillations. This is because it demonstrated that neutrinos must possess a very small but discernible mass. As a result, electron neutrinos and their heavier siblings, muon neutrinos and tau neutrinos, oscillate quantum superpositions as neutrinos move through space. This clarified perplexing 1960s research in which physicists studying the Sun had discovered noticeably fewer neutrinos than expected. Many of these solar neutrinos were oscillating into neutrino flavors that the experiments were not intended to detect, which is what was happening.

According to 2011 research by David Lhuillier of CEA and colleagues in France, measurements of the neutrino flux from nuclear reactors as a whole revealed an abnormally low flux of neutrinos from uranium-235 in comparison to predictions of theoretical models. They also demonstrated that this anomaly might be accounted for by the neutrinos oscillating into “sterile” neutrinos that would not interact via the weak interaction and hence not be observed. The notion that sterile neutrinos might have been observed in neutrino oscillations was tantalizing since sterile neutrinos are hypothetical particles that are invoked by some theoretical extensions of the Standard Model of particle physics. STEREO, which stands for Search for Sterile Reactor Neutrino Oscillations, was installed by Lhuillier and colleagues in 2016 at the Institut-Laue-Langevin (ILL) research reactor in Grenoble.

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The thinking was, “Okay, maybe if we approach closer, we will be able to observe this first or second oscillation,” says Lhuillier. “Before, everyone was seeing a mean shortfall [in neutrinos].” He claims that although the putative oscillation wavelength was unknown, “the oscillation was definitely already smeared out after 100 m.” The ILL’s 40 cm-diameter, almost completely enriched uranium-235 nuclear fission reactor, which functions as a point source of uranium-235 neutrinos, is less than 20 meters from the six independent gadolinium-doped 1.8 m3 hydrocarbon-filled scintillators that make up the STEREO detector.

In one of the liquid scintillators, a neutrino striking a hydrogen atom triggers inverse beta decay, turning the proton into a neutron and the electron into a positron, freeing both particles. The positron’s slowdown causes immediate gamma ray production. A gadolinium nucleus is then likely to capture the neutron, stimulating it to a metastable state. The second, stronger gamma ray pulse that results from this following decay is what Lhuillier refers to as the “signal we are looking for”: “a small pulse from the positron and then, a few microseconds later, a massive pulse from the gadolinium.”

According to David Lhuillier, “If it’s not a sterile neutrino, the issue must be on the prediction side.” The researchers confirmed that there was in fact a shortfall in the neutrino flux compared to what was theoretically expected using their six consecutive detectors. The researchers came to the conclusion that this shortfall could not be explained by neutrinos oscillating in and of some undetected state because it appeared consistent in all six detectors. According to Lhuillier, “If it’s not a sterile neutrino, the problem must be on the prediction side.” “There are almost 800 different ways to split a uranium nucleus apart, so you need a vast amount of nuclear data to anticipate what neutrinos a reactor can release,” the author says.

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In that regard, the paper is helpful, according to theoretical physicist André de Gouvêa of Northwestern University in Illinois. This reactor neutrino anomaly is more likely to be related to our incorrect modeling of how neutrinos are formed in nuclear decays than it is to some fascinating new physics phenomenon, according to the trend of results we have been seeing. The paper’s title, “STEREO neutrino spectrum of 235U fission rejects sterile neutrino hypothesis,” is a touch overly optimistic, he continues. The possibility of sterile neutrinos explaining this anomaly is not disregarded in principle. “There are some exciting data [on that] coming out of the KATRIN experiment,” he says, adding “cosmological restrictions are the only thing that could rule out very heavy sterile neutrinos.”

“There has been this reactor antineutrino anomaly since 2011, and I think this finishes that chapter,” continues theoretical physicist Patrick Huber of Virginia Tech in the US. We now know that the cause of the discrepancy was faulty input data, and I think this is important moving forward when we consider possible applications of neutrino physics, such as nuclear security. Huber performed one of the neutrino flux calculations showing the tension with experimental observation. Their findings are the culmination of a broad community effort to comprehend why the computations from 2011 and the data did not concur, and why they now do. The scientific method is being used there.

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