¶You have to go big to go small. Neutrinos are notoriously difficult to detect, but each time we observe them from a new source, we seem to learn som

Issue No. 128 | The Orbital Index

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2021-08-04 16:00:04

¶You have to go big to go small. Neutrinos are notoriously difficult to detect, but each time we observe them from a new source, we seem to learn something fundamental about the universe. Recently, we observed one of these nearly massless particles with a gobsmacking kinetic energy of 6 PeV (that’s 6x1015 electron volts—about 100x the highest center-of-mass energy we’ve ever produced in an accelerator here on Earth). It was spotted by IceCube, a detector at the South Pole that instruments a gigaton of ice with photosensors to look for the radiation produced when a neutrino collides with a proton or neutron in the ice. Since neutrinos seldom interact, and get rarer at higher energies, this discovery needed the absolute largest detector we could build. But astroparticle physicists are greedy: we want more neutrinos, at ever higher energies. There are lots of monstrous cosmic accelerators out there in the Universe that could be making exavolt (1018 eV) neutrinos—flaring jets of pulsars, neutron star collisions, dying stars, or jets driven by supermassive black holes. On top of that, decades ago we observed the "Oh-My-God'' particle, a 1021 eV cosmic ray whose origin we still don’t know. Neutrinos can help us disentangle these puzzles, especially when combined with gravitational-wave observations of cataclysmic sources. Relativistic neutrino interactions with ice lead to flashes of light—detectable with optical sensors—and intense radio impulses that last <1ns. The Radio Neutrino Observatory in Greenland (RNO-G) uses this second effect to dramatically increase sensitivity in a cost-effective way. Since radio waves propagate over long distances (~1 km in ice), spacing detectors by that distance lets you affordably build an enormous detector. (With this method, lunar satellites might someday turn the entire far side of the Moon into a detector; we’re already doing it with balloons over Antarctica.) Pictured below is RNO-G’s first “discovery”: a physicist riding a snowmobile whose spark plugs emitted radio pulses that lit up the antennas 100 m below the ice surface. The planned RNO-G array is so huge that it will double as the largest-ever survey of ice flow in Greenland. Look for the rest of the 35 stations coming online in the next three years, and root for neutrino observations to follow soon after. — contributed by Stephanie Wissel, a physicist and astrophysicist at the Pennsylvania State University who finds new places to put neutrino detectors. She’s often found in weird places like Greenland, Antarctica, high-elevation mountains, or hiking with her family.

Left: Christoph Welling’s photo of antennas being lowered 100 m below the surface of the ice at an RNO-G station. Right: Cosmin Deaconu’s photo of the first RNO-G station (everything but the solar panels is buried in the ice).