The idea of a mysterious fourth neutrino supported by new experiments
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An anomaly previously seen in experiments has been confirmed by new work, adding to the evidence that something is wrong with the Standard Model of particle physics. The work might give some idea of what exactly is wrong.
The results come from an experiment looking for the hypothetical sterile neutrino: a fourth electrically neutral, low-mass particle thought to exist beyond the three standard neutrinos we know exist.
As reported in Physical Review Letters and Physical Review C, the experiment confirms previous findings. The facility consists of irradiated chromium-51 discs (which do not form naturally), a major source of electron neutrinos. The discs are in two gallium tanks, and electron neutrino bombardment transforms some of the gallium atoms into germanium-71 atoms.
However, the measured rate of this reaction is 20-24% lower than theoretical modelling. One possibility for this electron neutrino deficit is that there is a fourth neutrino known as the sterile neutrino. Alternatively, the theory is wrong.
“The results are very exciting,” Steve Elliott, a senior analyst on one of the teams evaluating the data and a member of the Los Alamos Physics Division, said in a statement. “This definitely reaffirms the anomaly we’ve seen in previous experiments. But it’s not clear what it means. There are now conflicting results on sterile neutrinos. If the results indicate that fundamental nuclear or atomic physics is misunderstood, that would also be very interesting.
Neutrinos come in three flavors – a surprising technical term that has nothing to do with the sense of taste. There are electron neutrinos, muon neutrinos and tau neutrinos. One of the most fascinating things about neutrinos is that they oscillate, which means they change from one flavor to another as they move through the universe.
They can move for a very long time. Having such a small mass and being electrically neutral means that they interact very little. Every second, 100 trillion neutrinos pass through your body as if you weren’t even there.
The Soviet-American Gallium Experiment (SAGE), which began in the late 1980s, indicated a possible deficit of electron neutrinos. The Baksan Experiment on Sterile Transitions (BEST), whose results we discuss here, confirms this.
One possibility is that the electron neutrino oscillates into a sterile neutrino, which could explain the reduced germanium production. Alternatively, the electron neutrino cross section – the probability of a specific particle interaction taking place – might not be what the theory says it is.
The Standard Model of particle physics is one of the finest sets of ideas ever created by mankind, allowing us to predict particles long before they are discovered, like the Higgs boson. But it’s also limited, and physicists are now reaching those limits in hopes of revealing what lies beyond.