Physicists Capture the Elusive Neutrino Smacking Into an Atom's Core

March 20, 2019 Off By HotelSalesCareers

Every second of every day, trillions of tiny particles called neutrinos are raining down on your head. But unlike raindrops, hailstones, or bird poop, these elementary particles go right through your body—and through Earth’s crust, mantle, and core—at nearly the speed of light. After they sail through the entire planet, they fly silently back into the cosmos with scarcely a hello. It’s almost as if they never existed. “They’re the most mysterious type of particle we know of,” says Juan Collar, a physicist at the University of Chicago.

But neutrinos do leave fingerprints—if you know what to look for. In a study published Thursday in Science, Collar’s group observed a new type of neutrino interaction: a neutrino bumping into an atomic nucleus, a process known as coherent elastic scattering. At Oak Ridge National Laboratory in Tennessee, they fired a beam of neutrinos at a toaster-sized detector made of cesium iodide crystals. When the neutrino interacted with a cesium or iodine nucleus, the crystal would emit about 10 photons’ worth of dim light, cracking a window into the personality of the shyest particle. Understanding this collision could help physicists study weirder properties of neutrinos—and complicate their search for dark matter.

Neutrinos and nuclei are quantum mechanical particles, which means they don’t knock into each other quite like marbles on a sidewalk. They just get close, and then the neutrino transfers a tiny bit of energy to a neutral particle called a Z boson. “The neutrino kind of tosses the Z boson to the nucleus,” says physicist Hirohisa Tanaka of the University of Toronto, who wasn’t involved in the research. When the nucleus “catches” the Z boson, it recoils slightly, like the feeling after you catch a medicine ball, and then it emits the photons.

And it’s really difficult to detect these gentle interactions. Collar’s group bombarded their detector with trillions of neutrinos per second, but over 15 months, they only caught a neutrino bumping against an atomic nucleus 134 times. To block stray particles, they put 20 feet of steel and a hundred feet of concrete and gravel between the detector and the neutrino source. The odds that the signal was random noise is less than 1 in 3.5 million—surpassing particle physicists’ usual gold standard for announcing a discovery. For the first time, they saw a neutrino nudge an entire atomic nucleus.

Neutrino experiments like this one are a piece of a much larger puzzle. Physicists want to fix the current formulation of the laws of physics known as the Standard Model—a model that describes everyday phenomena perfectly well. But physicists have found that it gets some things wrong, especially about neutrinos. For example, in 1998, physicists found that neutrinos have mass—but the Standard Model predicted that they wouldn’t. By studying neutrinos in more detail, researchers like Collar and Tanaka hope to uncover unusual behavior that might illuminate what exactly is wrong in the Standard Model.

Alas, this measurement won’t help them rewrite any physics textbooks. The neutron-nucleus collision occurred exactly as the Standard Model predicted, says physicist Gerald Garvey of Los Alamos National Laboratory, who wasn’t involved in the research. In fact, physicists first predicted that a neutrino should interact with a nucleus in this way over 40 years ago. This time, the Standard Model will live to see another neutrino experiment.

But Garvey points out that Oak Ridge could adapt the experiment to study some exotic Standard Model-defying neutrino phenomena. Like, say, the sterile neutrino. Physicists have observed three types of neutrinos—electron, tau, and muon—but some hypothesize that there’s a fourth, the sterile neutrino, which is even more invisible than its cousins. It’s possible that regular neutrinos change into sterile ones as they fly through space. Researchers could move the detector such that the neutrinos have to fly a longer distance through the air, then count whether the same number of collisions occur. If they count fewer collisions, that could be evidence of sterile neutrinos.

Counterintuitively, though, scientists will find this new information useful by tuning it out. Neutrino collisions aren’t the only weak signal in the universe—far from it. Dark matter, the elusive particles that physicists think make up a quarter of the universe’s mass and energy, also barely interact with detectors. Nobody knows what it actually is yet, but some proposed dark matter particles should cause nuclei to emit light just like a neutrino-nucleus collision. In those cases, neutrino collisions would be the background noise that needs to get filtered out. And though current dark matter detectors aren’t sensitive enough to be swamped by neutrino collision signals, they should be in the near future. “The saying goes, yesterday’s signal becomes today’s background,” Tanaka says. Current technology can’t filter out the neutrino noise, and they may have to develop an entirely new type of dark matter detector. Even the quietest particle sometimes needs a mute button.

Physicists Capture the Elusive Neutrino Smacking Into an Atom's Core

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