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A physics puzzle has been solved with a conclusion that is roughly as stunning as “the butler did it.” Physicists have debated for a decade why nuclear reactors produce fewer neutrinos than anticipated. Some hypothesized that the elusive particles might be transforming into stranger, undetectable “sterile” neutrinos. Instead, these findings confirm what previous experiments suggested: that theorists overestimated the number of neutrinos a reactor should create. “This is not an unexpected conclusion, but it is significant,” says Georgia Karagiorgi, a particle physicist at Columbia University who was not involved in the research. It is nonetheless something I desired to see.

Neutrinos are of three sorts, depending on how they are generated: electron, muon, and tau. Electron neutrinos are emitted by the Sun, muon neutrinos fall from the sky when cosmic rays contact the atmosphere, and tau neutrinos are produced by the decay of tau particles, which can be produced using atom smashers. The virtually massless particles change type during flight, so an electron neutrino from the Sun can transform into a different type before reaching Earth.

A few investigations have shown the existence of a fourth neutrino that is sterile. Only when an ordinary neutrino undergoes a metamorphosis into it could it interact with ordinary matter. Anomalous data suggested the existence of a sterile neutrino with a mass of around 1 electron volt (eV), at least 10 times that of regular neutrinos. In 2011, two groups of theorists determined that nuclear reactors produced 6% fewer electron neutrinos, or electron antineutrinos, than predicted by theory.

The results indicated that the missing neutrinos were mutating into sterile neutrinos. It was a challenging argument. In the core of a nuclear reactor, uranium and plutonium nuclei break in a chain reaction, and antineutrinos are produced by the radioactive “beta decay” of the lighter nuclei that remain. A neutron in a nucleus decay into a proton together with the emission of an electron and an electron antineutrino. To anticipate the entire flux of antineutrinos, physicists had to take into consideration the quantities and decay rates of a vast array of nuclei. In 2017, physicists from the Daya Bay Reactor Neutrino Experiment in China called into question the accuracy of this accounting.

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They investigated antineutrinos from six commercial reactors that burned fuel containing 4% uranium-235 atoms, which are capable of sustaining a chain reaction, and 96% uranium-238 atoms, which are incapable of doing so. As uranium-235 is consumed, neutrons from its fission convert uranium-238 into plutonium-239, which furthers the chain reaction. As the amount of uranium-235 decreased, Daya Bay physicists discovered that the antineutrino deficit decreased, indicating that theorists had overstated the flux of antineutrinos originating from uranium-235.

Now, physicists at a modest French research reactor have proven this hypothesis. The reactor of the Laue-Langevin Institute (ILL) generates an abundance of neutrons for material research. Moreover, it employs fuel containing 93% uranium-235. Researchers utilizing the neutrino detector STEREO could therefore determine the flux of antineutrinos from uranium-235 by analysing its antineutrinos. The detector consists of six identical oil-filled segments aligned like teeth and extending 9 to 11 meters from the core of the reactor. Rarely, a proton in oil will absorb an electron antineutrino and transform into a neutron while emitting a positron, which is similar to beta decay in reverse.

As the positron streaks through the oil, it emits light proportional to the initial neutrino’s energy. As the distance from the core rose, the spectrum of energies of electron antineutrinos remained unchanged, according to STEREO researchers. This discovery contradicts the theory that some neutrinos are transforming into sterile neutrinos, as neutrinos with lower energy should transform faster than those with greater energy, altering the spectrum as the neutrinos go. As reported today in Nature, STEREO researchers demonstrated that the total flow of antineutrinos emitted by uranium-235 was less than that predicted by theorists’ predictions.

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David Lhuillier, a neutrino physicist at France’s Atomic Energy Commission and spokesperson for the 26-person STEREO team, states that the observations put an end to the reactor antineutrino shortage as proof for a 1-eV sterile neutrino. “Can it be explained by a sterile neutrino of around 1 eV mass?” “The response is no.”

Lhuillier observes that other trials, such as the PROSPECT study at Oak Ridge National Laboratory, have found similar conclusions. According to Bryce Littlejohn, a neutrino physicist at the Illinois Institute of Technology and a PROSPECT coauthor, the new STEREO report contains smaller uncertainty and presents the case in a unified manner. “Rather than a defining moment, I see it as a great summary of what we’ve learned.”

Scientists have a strong suspicion as to how they overestimated the flux of antineutrinos produced by uranium-235. In the late 1980s, scientists at ILL exposed foils of plutonium-239 and uranium-235 to neutrons from the reactor in order to break atoms. Then, they counted the beta decay electrons and measured the energy spectrum of the beta decay electrons. Two decades later, theorists utilized these data to extrapolate the spectrum of antineutrinos that must have emerged alongside electrons.

Patrick Huber, a theoretical physicist at Virginia Polytechnic Institute and State University and one of the theorists who discovered the so-called reactor antineutrino anomaly, says that multiple lines of evidence now suggest that those experiments may have overestimated the total number of electrons coming from the uranium-235 sample. Simply, the input data we have been utilizing has been incorrect.

Northwestern University’s Zahra Tabrizi, a theoretical particle physicist, argues that the answer of the reactor neutrino issue does not rule out the possibility of the 1-eV sterile neutrino. She states, “There are still abnormalities in neutrino physics that we cannot explain.” Huber adds that when all neutrino studies are considered, the evidence for the sterile neutrino is not particularly strong: It is not a good fit to the data globally.

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