A strange discrepancy between theoretical predictions and experimental results in a major neutrino research project could be a sign of the elusive “sterile” neutrino – a particle so quiet it can only be detected by the silence that it leaves in its wake.
It’s not the first time the anomaly has been observed, adding to previous experimental data hinting at something strange in the world of neutrino research. This time it was detected during the Baksan Experiment on Sterile Transitions (BEST).
Unambiguous proof of the hypothetical sterile neutrino could provide physicists with a strong candidate for the Universe’s mysterious storehouse of dark matter. On the other hand, it could all just come down to a problem in the models used to describe the bizarre behaviors of old-school neutrinos.
Which would also be an important moment in the history of physics.
“The results are very exciting,” says Los Alamos National Laboratory physicist Steve Elliott.
“This definitely reaffirms the anomaly we saw in previous experiments. But what it means is not clear. There are now conflicting results on sterile neutrinos. If the results indicate that fundamental nuclear or atomic physics is misunderstood, that would be very interesting too.”
Although they are among the most abundant particles in the Universe, neutrinos are notoriously difficult to capture. When you have virtually no mass, no electrical charge, and only make your presence known through the weak nuclear force, it is easy to slip through even the densest materials unhindered.
The ghostly movement of the neutrino is not its only interesting quality. Each particle’s quantum wave transforms as it glides along, oscillating between characteristic “flavors” that echo their negatively charged particle cousins - the electron, muon and tau. .
Studies of neutrino oscillations at the US National Laboratory at Los Alamos in the 1990s noticed gaps in the timing of this flip-flop that left room for a fourth flavor, one that wouldn’t do as much of a ripple in the weak nuclear field.
Shrouded in silence, the sterile flavor of the neutrino would only be highlighted by a brief pause in its interactions.
BEST is protected from cosmic neutrino sources under a mile of rock in the Caucasus Mountains of Russia. It features a dual-chamber tank of liquid gallium that patiently collects neutrinos emerging from an irradiated chromium nucleus.
After measuring how much gallium had turned into an isotope of germanium in each reservoir, the researchers were able to work backwards to determine the number of direct collisions with neutrinos as they oscillated through their electron flavor.
Similar to the Los Alamos experiment’s own “gallium anomaly,” the researchers calculated between a fifth and a quarter less germanium than expected, hinting at a deficit in the expected number of electron neutrinos.
That’s not to say for sure that the neutrinos had briefly adopted a sterile flavor. Many other searches for the small pale particle prove fruitless, leaving open the possibility that the models used to predict the transformations are on some level misleading.
This in itself is not a bad thing. Corrections to the basic framework of nuclear physics could have far-reaching ramifications, potentially revealing flaws in the Standard Model that could lead to explanations for some of science’s remaining great mysteries.
If this is indeed the mark of the sterile neutrino, we might finally have proof of material that exists in huge quantities, but only forms a gravitational dimple in the fabric of space.
Whether this is the sum of dark matter or just a piece of its puzzle would depend on further experimentation on the most ghostly of phantom particles.
This research was published in Physics Review Letters and Physical examination C.