An anomaly previously seen in experiments has been confirmed by some new work, adding to the evidence that something is wrong with the Standard Model of particle physics. The work might provide an idea of what exactly is wrong.
The results come from an experiment searching for the hypothetical sterile neutrino: a fourth electrically-neutral and low-mass particle hypothesized to exist beyond the standard three neutrinos we know to exist.
As reported in the journals Physical Review Letters and Physical Review C, the experiment confirms the previous findings. The setup is made of irradiated chromium-51 disks (which don’t form naturally), a major source of electron neutrinos. The disks are within two tanks made of gallium, and the bombardment of electron neutrino turns some of the gallium atoms into germanium-71 atoms.
However, the measured rate of this reaction is between 20 and 24 percent lower than the theoretical modeling. One possibility for this deficit of electron neutrinos is that there is a fourth neutrino which is known as a sterile neutrino. Alternatively, the theory is wrong.
“The results are very exciting,” Steve Elliott, lead analyst of one of the teams evaluating the data and a member of Los Alamos’ Physics division, said in a statement. “This definitely reaffirms the anomaly we’ve seen in previous experiments. But what this means is not obvious. There are now conflicting results about sterile neutrinos. If the results indicate fundamental nuclear or atomic physics are misunderstood, that would be very interesting, too.”
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 aspects of neutrinos is that they oscillate, meaning that they turn from one flavor to another as they move about the universe.
They can move for a really long time. Having such a tiny mass and being electrically neutral means that they interact very little. Every second, 100 trillion neutrinos pass through your body like you aren’t even there.
The Soviet-American Gallium Experiment (SAGE), which started in the late 1980s, indicated a possible deficit of electron neutrinos. The Baksan Experiment on Sterile Transitions (BEST), whose results we are discussing here, confirms this.
One possibility is that the electron neutrino is oscillating into a sterile neutrino, which could explain the reduced production of germanium. Alternatively, the cross-section of the electron neutrino – the probability that a specific particle interaction would take 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 humanity, allowing us to predict particles long before they were discovered, such as the Higgs Boson. But it is also limited, and physicists are now hitting those limits hoping to reveal what lies beyond.