The sterile neutrino, if it truly exists, only answers to gravity.
Physicists are spelunking the complex findings from an experimental particle reactor found a mile below the surface in the mountains of Russia. What they found has the potential to send an earthquake through the bedrock of the standard model of physics itself: the results could confirm a new elementary particle, called a “sterile neutrino,” or demonstrate a need to revise a portion of the standard model.
The research comes from New Mexico’s Los Alamos National Laboratory in collaboration with the Baksan Neutrino Observatory near the Georgia border in far southwestern Russia. The scientists outlined their findings in two new papers published last month in the journals Physical Review Letters and Physical Review C.
To understand the team’s findings, we need to talk about neutrinos, the most common and least massive of the massive particles (the particles that have any mass at all). They were first theorized decades ago and only interact through gravity and the “weak force” of the standard model of physics, which means that, like dark matter, neutrinos can just pass through us and our planet and space however they want; they interact with almost nothing. Over the decades, scientists have developed ways to measure neutrinos by tracing their effect on what’s around them.
Then, over 20 years ago, scientists first detected something called the “gallium anomaly.” In a special experiment prepared to measure solar neutrinos—particles ejected from the heart of the sun that pervade the matter all around us—the scientists combined a synthetic isotope called chromium 51 with a large source of gallium. Gallium is a peculiar metal that melts at human body temperature and then looks kind of like mercury. Together, the two produce an isotope called germanium 71.
(Remember that while elements are fundamentally themselves, atoms are just equations of subatomic particles. Changing the formulation by adding or subtracting particles like electrons changes what element that atom is. Nuclear energy in general is based on the principle that you can pry open the nucleus and futz around in there, making other things come out).
But during that experiment at Baksan, called the Soviet-American Gallium Experiment (SAGE), scientists observed something peculiar. There were 20–24 percent fewer germanium 71 atoms than there should have been based on the gallium and chromium 51 supplies available and the nature of the experiment. Scientists at the time, and to this day, aren’t sure why there wasn’t enough germanium 71. All of this has come to be known as the “gallium anomaly.”
There are two main schools of thought to explain the discrepancy, and you’re not really going to like either one. The first is that we could just have a misunderstanding of the fundamentals of the standard model of physics, meaning this is a brand-new thing we have no paradigm for at this time, and that could shake up the whole institution. The second explanation is that this is a special kind of neutrino that doesn’t even answer to the weak force—it only answers to gravity itself.
In the new research, scientists from Los Alamos begin to unpack data from the latest experiment to be built at Baksan: the Baksan Experiment on Sterile Transitions (BEST). Continuing to iterate on the research tradition that started with SAGE, BEST includes a spoiler in its name. It’s the idea of the sterile transition, where “sterile” is the technical term for the neutrino that would only be affected by gravity. “The rates at each distance were found to be similar, but 20 percent–24 percent lower than expected, thus reaffirming the anomaly,” the researchers report in one paper.
What’s cool about the BEST experiment and its data so far is that it has fine-tuned the experimental setup in play so that the results will start to point toward or away from sterile neutrinos in particular. That could help scientists either confirm or eliminate this possibility as they continue to study the gallium anomaly. It’s hard to design experiments to test for sterile neutrinos, because gravity is the weakest of the fundamental forces and neutrinos are so, so tiny and light; it’s like trying to catch one leaf on the tree making the tiniest motion in a paltry puff of wind.