Cosmic neutrinos detected for the first time

Detector IceCube registers 28 tracks of energetic "ghost particles" from space

One cubic kilometer in size, 5, 160 detector balls and the whole thing under the South Pole: Building IceCube. © IceCube / NSF
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We are constantly being bombarded by neutrinos from the distant universe - that's what the theory has been saying for a long time. But only now has the proof of these energetic, but invisible and almost massless particles succeeded. The particle detector IceCube under the Antarctic ice has for the first time registered some traces of these "ghost particles". This could start a whole new era of "neutrino astronomy", states the IceCube collaboration in the journal "Science".

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Neutrinos are the most common elementary particles and the most mysterious. Every second, 100 trillion of these tiny particles race through our bodies at nearly the speed of light - without us even feeling the slightest bit of it. Because these elementary particles are invisible, have almost no mass and only very rarely interact with other matter. And that is exactly the problem: To prove one of these rare events, gigantic detectors are needed.

5, 160 detector balls in the Antarctic ice

The largest of these neutrino observatories is IceCube - a detector distributed over a cubic kilometer of Antarctic ice. 86 cables, each with 60 spherical sensors, reach deep below the ice of the South Pole. Each of these 5, 160 basketball-sized optical modules is calibrated to register the tiny flashes of light that occur when a neutrino collides with one of the atomic building blocks of the ice.

However, there is also a problem here: Neutrinos not only flow from space through the Earth, they also arise from high-energy particle reactions in the atmosphere. The researchers first have to filter out these weaker neutrino signatures in order to find what they are looking for: the trail of the ghost particles traveling with the cosmic rays. display

"Ernie" and "Bert" made the beginning

In April 2012, a first breakthrough came: two events in the detector left an unusually high-energy track. The values ​​were at a tremendous 1, 000 Teraelektronvolt (TeV) - and thus several orders of magnitude above what atmospheric neutrinos can produce. It had to be cosmic neutrinos, she suspected.

"After watching hundreds of thousands of atmospheric neutrinos, we finally found something else - that's what we've been waiting for, " says IceCube Project Manager Francis Halzen of the University of Wisconsin-Madison. The IceCube researchers lovingly christened the two neutrino signatures Ernie "and Bert". But the two alone were not enough to finally prove the covert cosmic neutrinos.

In the meantime, however, this has changed. Because the researchers have discovered 26 other high-energy events - albeit with just over 30 TeV not quite as fierce as "Ernie" and "Bert". The majority of these signatures produced a chill-like pattern in the detector, but a few, but few, particle reactions that led to relatively narrow tracks. All 28 neutrino signatures together differ so significantly from the atmospheric variant that the researchers are now certain: "This is the first indication of very high-energy neutrinos coming from beyond our solar system, " Halzen reports.

Schematic view of some detector balls in ice Jamie Yang / IceCube Collaboration

Where do the ghost particles come from?

Based on the traces in the detector, the scientists next tried to find out where the cosmic particles had come from in space. For it is clear that the neutrinos, similar to the rest of the cosmic radiation, are generated in extremely high-energy events - in a kind of cosmic particle accelerator. However, several candidates come into question: gamma-ray bursts, supernovae, but also the massive particle jets that send out the supermassive black holes in the heart of active galaxies.

To narrow this field, the researchers tried to trace the trajectory of their 28 neutrino signals. If they come in a particular region of the universe, this could help to assign them to one of the potential sources. Unfortunately, this did not succeed. The analysis revealed a rather random spatial and temporal distribution of events, as the researchers report. With just 28 signals, the number is simply too small to identify calls.

"We are now working hard to increase the significance of our observation, " explains IceCube spokeswoman Olga Botner from Uppsala University. With increasing detection rates, the scientists hope to be able to identify individual sources of high-energy neutrinos in the cosmos. (Science, 2013; doi: 10.1126 / science.1242856)

(Science / IceCube Collarboation, 22.11.2013 - NPO)