Gallium is a fun material. It’s solid as long as you are not picking it up with your own hands, because it would turn liquid due to its relatively low melting temperature. For physicists, one of the two stable isotopes of gallium also has another use: it can be employed to study solar neutrinos. And for this reason, physicists have stumbled upon a mystery that could shake up all of modern physics.
The problem is not with gallium. The Standard Model of Particle Physics is probably one of the finest theories ever produced by human minds, and yet it’s limited. We are aware of it – we are just not sure where the limits actually are and what’s beyond. It is here that the sometimes-liquid metal comes in.
Meet the three flavors of neutrinos
If an atom of gallium-71 interacts with electron neutrinos, it decays into germanium-71 and an electron. Unlike gallium, germanium-71 is not stable and it decays with a half-life of 11.4 days back into gallium, a process that is both convenient and measurable.
Neutrinos are a weird little particle. There are three known types: electron neutrino, muon neutrino, and tau neutrino. These are known as flavors. They have no electric charge and such a small mass that it was long thought that they had zero mass. They can fly across intergalactic distances undisturbed. There are 100 trillion neutrinos passing through your body every second, but you can’t feel it. An interaction with neutrinos is rare.
They are produced in a myriad of physical events, including in the fusion reaction at the center of the Sun. But as they travel something happens. They change flavors. They go from being electron neutrinos, to muon neutrinos, to tau neutrinos or vice versa. The ratio of this oscillation is well predicted by observations and theory.
And this is when it gets complicated. Because gallium experiments are not making enough germanium.
Experiments say the theory is missing something
Knowledge that there was something afoot comes from the results of the Soviet–American Gallium Experiment, which was started in 1989 and ran for decades. Just two years later another experiment called GALLEX was started in Italy. Both indicated a deficit in the amount of germanium produced. This is the gallium anomaly.
To misquote Shakespeare, the fault is not in our star, but in our choice of element. A follow-up study called BEST – Baksan Experiment on Sterile Transition – subjected gallium to neutrinos using radioactive chromium (which is an intense source of neutrinos). The idea was to see if maybe the problem was in the solar expectations. Once again, the germanium-71 yield was lower than expected, between 20 and 24 percent lower.
Another possibility was the fact that maybe the interaction between the neutrinos and the gallium was not fully understood. This was refined in 2023 without solving the anomaly. Another alternative was the half-life of germanium-71 – maybe the 11.4-day measurement was not correct. But this too ended up being refined, remaining in agreement with previous experiments. The anomaly persisted.
Enter: the sterile neutrino
We are not implying that there is a secret menu of particles in the restaurant that is the Standard Model. As far as we can tell, the flavors are three. But we mentioned the limitations. We suspect that there are other particles out there, and a version of a neutrino that only interacts through gravity might be one of them. The existence of a sterile neutrino with a mass of about one-five-hundredth of an electron’s own has been rejected by other experiments. But the results from BEST actually put the sterile neutrino’s mass higher than that.
This doesn’t mean that the sterile neutrino is the right answer. There could be some other features in the experiments that scientists have misunderstood. Or there could be a completely different explanation that still requires physics beyond the Standard Model. We just don’t know yet, but it is really fascinating that such a fun metal might hold the key to a whole new way of understanding the universe.
