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"The bubble—a carbon nucleus in the experiment—deflects the neutrino 'bullet' by creating a particle from the vacuum," says Kevin McFarland. "This effectively shields the bubble from getting blasted apart and instead the bullet only delivers a gentle bump to the bubble." (Credit: Brian Dewey/Flickr)

neutrinos

Why neutrino ‘bullets’ don’t pop ‘bubbles’

In what they call a “weird little corner” of the already weird world of neutrinos, physicists have found evidence these tiny particles might be involved in a surprising reaction.

Neutrinos are famous for almost never interacting. As an example, ten trillion neutrinos pass through your hand every second, and fewer than one actually interacts with any of the atoms that make up your hand.

“After analyzing the results, we now have overwhelming evidence for the process.”

However, when neutrinos do interact with another particle, it happens at very close distances and involves a high-momentum transfer.

But new findings show neutrinos sometimes interact with a nucleus but leave it basically untouched, inflicting no more than a “glancing blow” resulting in a particle being created out of a vacuum.

“The bubble—a carbon nucleus in the experiment—deflects the neutrino ‘bullet’ by creating a particle from the vacuum,” says Kevin McFarland, professor of physics at University of Rochester.

“This effectively shields the bubble from getting blasted apart and instead the bullet only delivers a gentle bump to the bubble.”

Producing an entirely new particle—in this case a charged pion—requires much more energy than it would take to blast the nucleus apart—which is why the physicists are always surprised that the reaction happens as often as it does. Even painstakingly detailed theoretical calculations for this reaction “have been all over the map,” McFarland says.

“The production of pions from this reaction had not been observed consistently in other experiments,” he says. By using a new technique, researchers were able to measure how much momentum and energy were transferred to the carbon nucleus, showing that it remained undisturbed, and the distribution of the pions that were created.

“After analyzing the results, we now have overwhelming evidence for the process,” McFarland says.

‘Weird reaction’

McFarland is a scientific co-spokesperson with the international MINERvA collaboration, which carries out neutrino scattering experiments at Fermilab. The two members of the collaboration primarily responsible for analyzing the results are Aaron Higuera, postdoctoral student at University of Rochester at the time of the study, and Aaron Mislivec, a doctoral student in McFarland’s lab.

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“Our detector gave us access to the full information of exactly what was happening in this reaction,” Mislivec says. “Our data was consistent with the unique fingerprint of this reaction and determined how these interactions happen and how often.”

The researchers say the key to identifying the reaction was finding undisturbed carbon nuclei and then studying the two resulting particles—the pion, which is responsible for shielding the nucleus, and the muon.

Understanding this reaction, McFarland says, “is not going to make a better mousetrap, but it is exciting to learn that this weird reaction really does take place.”

The Department of Energy, the National Science Foundation, and partnering scientific agencies in Brazil, Chile, Mexico, Switzerland, Peru, and Russia provided funding for the work.

Source: University of Rochester

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