Quantum Mechanics

Physicists Caught A Rare Neutrino Interaction With The Smallest Detector Ever

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Of all the particles in the world of particle physics, neutrinos are by far the shyest. They don't like to socialize with other particles, and generally keep to themselves. A hundred trillion neutrinos pass through you every second, but it would take a week for just one to interact with a single particle in your body. That's too bad, because neutrinos are important for our understanding of the universe — they make up a lot of the energy from supernovas, for one thing, and knowing more about them might teach us important things about the mysterious substance scientists call dark matter. But physicists are on the case, and in August 2017, a team announced that they had detected a rare neutrino interaction. Even better, instead of using a building-sized detector buried deep underground, the researchers did it with a device the size of a fire extinguisher.

Prof. Juan Collar led a team of UChicago physicists who built a lightweight, portable neutrino detector to observe the elusive interactions of the ghostly particles.

The Antisocial Particle

The Standard Model of particle physics defines the tiniest building blocks that make up everything in the universe. It includes four fundamental forces that let those particles interact: gravity, electromagnetism, the strong force, and the weak force. The strong force is, well, strong; it binds the particles that make up an atom's nucleus and gets even stronger the further apart the particles get. The weak force is what makes radioactive materials decay, and only works at very short distances. One problem with neutrinos is that they only interact via the weak force. That makes them very hard to detect.

The Standard Model

But we manage to do it. Labs put huge detectors made up of tens of thousands of tons of liquid argon underground, where high-energy neutrinos can strike protons in the argon atoms that result in a flash of light when the proton decays. That proton interaction is the main way we know neutrons exist. But in 1974, MIT theoretical physicist Daniel Freedman mused in a paper that at low energies, neutrinos could probably also bump into whole nuclei, and if so, that interaction is probably way more common. That's because protons are to atomic nuclei as a gumball is to a gumball machine — they're one ingredient of a much bigger whole. But Freedman didn't think we'd ever see one, mostly because that interaction between a tiny neutrino and a massive nucleus would be too faint to detect. "Our suggestion may be an act of hubris," he wrote. Not so fast, Dr. Freedman.

Researchers Bjorn Scholz (left) from the University of Chicago and Grayson Rich of the University of North Carolina with the world’s smallest neutrino detector at Oak Ridge National Laboratory in Tennessee.

Confirming A 40-Year-Old Theory

Oak Ridge National Lab in Tennessee has an experiment that pumps bursts of neutrons down a beam line and just so happens to expel a lot of neutrinos as a byproduct. A research collaboration known as COHERENT decided that was just the place to try their luck at detecting one of these neutrino-nucleus interactions, called CEvNS (Coherent Elastic Neutrino-Nucleus Scattering). After loading their handheld detector with perfect crystalline material to act as a target and hiding out in a basement hallway to avoid all the extra neutron "noise," they set the detector loose. It worked. The detector picked up faint blips of light every time a neutrino bumped a nucleus. The lab's beam line pumped neutrons about 60 times per second, and that's just what they saw — a visible, timed rise and fall in the detection of those light blips. A 40-year-old theory proven.

The biggest benefit to this result is the detector itself. It costs a lot of money to make an underground neutrino detector the size of a building, and the cheaper something is to research, the more researchers can participate. The fact that the detector is portable could also help officials monitor nuclear reactors to make sure people aren't secretly creating fuel for atomic weapons. But most exciting of all is the discovery of the CEvNS phenomenon itself. Because those interactions are so common, our ability to detect them will really help research into supernovas — which release 99 percent of their energy as neutrinos — and dark matter, which some researchers think could rely on weakly interacting particles.

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