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UW-Madison: Icebound detector reveals how ghostly neutrinos are stopped cold

CONTACT: Francis Halzen, (608) 262-2667, francis.halzen@icecube.wisc.edu

IMAGES AND VIDEO LINKS: https://uwmadison.box.com/v/IceCube-earth-absorption

MADISON - Famously, neutrinos, the nearly massless particles that are a fundamental component of the universe, can zip through a million miles of lead without skipping a beat.

Now, in a critical measurement that may one day help predict new physics beyond the Standard Model of particle physics - the model that seeks to explain the fundamental forces of the universe - an international team of researchers with the IceCube Neutrino Observatory has shown how energized neutrinos can be stopped cold as they pass through the Earth.

The new measurement is reported today (Nov. 22, 2017) in the journal Nature by the IceCube Collaboration, an international consortium of scientists using the observatory to explore the neutrino and what it can tell us about matter and the nature of the universe.

Neutrinos are among the most abundant particles in the cosmos. With almost no mass and no charge, they rarely interact with matter. Tens of trillions of neutrinos course through our bodies every second.

Every once in a while, however, high-energy neutrinos interact with protons or neutrons and are absorbed. Theory predicts that at high energies - higher than can be generated by any earthbound particle accelerator - neutrinos can be expected to interact with matter and be absorbed in the Earth instead of continuing to transit the cosmos.

"We always say that no particle but the neutrino can go through the Earth," explains Francis Halzen, a University of Wisconsin-Madison professor of physics and the IceCube principal investigator. "However, the neutrino does have a tiny probability to interact, and this probability increases with energy."

That probability, Halzen adds, is what scientists call the neutrino cross section.

The new measurement determines the cross section for neutrino energies between 6.3 TeV and 980 TeV, energy levels more than an order of magnitude higher than previous measurements. (One TeV or teraelectronvolt is the energy of a proton's circulation in the Tevatron, a now-shuttered particle accelerator at Fermilab, that once propelled protons around the four-mile circumference of the accelerator's ring at nearly the speed of light.) The most energetic neutrinos studied so far from earthbound accelerators are at the 0.4 TeV energy level.

Catching neutrinos in the act of being absorbed as they collide with other particles in nature requires a massive detector such as the National Science Foundation-supported IceCube Observatory, an array of 5,160 basketball-sized detectors embedded in a cubic kilometer of crystal clear ice a mile beneath the geographic South Pole. IceCube does not see neutrinos directly, but detects and records a fleeting burst of Cherenkov radiation - a streak of blue light - that results when the occasional neutrino crashes into another particle.

Analyzing a year of IceCube data gathered between May 2010 and May 2011, the collaboration put 10,800 neutrino interactions under the microscope, paying closest attention to the most energetic neutrinos that course through the Earth from all directions. Neutrinos are generated in a variety of phenomena, ranging from the sun and nuclear reactors to clusters of galaxies and the Earth's atmosphere as cosmic rays interact with nitrogen and oxygen.

READ MORE AT https://news.wisc.edu/icebound-detector-reveals-how-ghostly-neutrinos-are-stopped-cold/

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-Terry Devitt, (608) 262-8282, trdevitt@wisc.edu


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