One of the most high-energy neutrinos in space measured by the IceCube detector. (Photo: DESY, IceCube Collaboration)
At the South Pole, physicists catch glimpses of extragalactic energy hurlers - and start a new branch of astronomy.

One of the coldest places in the world is particularly hot: in the ice at the geographic South Pole. There, an international team of researchers built the largest neutrino detector in the world over several years - IceCube. With his help, the physicists now opened a new window to the cosmos. For the first time, they discovered - neutrally for many scientists - neutrinos with energies that surpass everything measured so far by many orders of magnitude. These almost light-fast elementary particles must come, at least in part, from millions of light years of distant parts of the universe. They bear witness to vehement energy storms - and researchers are puzzling what their source could be.

"This is the beginning of a new epoch of astronomy, " says Francis Halzen. The physicist from the University of Wisconsin in Madison is IceCube project leader. Now he is reaping the fruits of his years of effort to finance and realize the construction of this unusual detector. "We found the first evidence of very high-energy neutrinos outside the solar system - with energies more than million times the 1987 supernova in the Great Magellanic Cloud."

These neutrinos, released shortly before the explosion of a Blue Giant Star in our neighboring galaxy, are so far the only ones not from the Sun or the Earth. Therefore, it is not exaggerated when IceCube member Markus Ackermann from the Research Center DESY in Zeuthen celebrates: "We are just now witnessing the birth of neutrino astronomy."

The biggest experiment ever

IceCube is the largest physical experiment of all time in terms of volume. In this sense, the $ 270 million that the project has cost in total - 20 million come from Germany - is not much. For, for once, the evidence provides nature itself: one cubic kilometer of pure ice, the result of a 100, 000-year snowfall. In it, the almost massless, with matter only weakly interacting neutrinos produce characteristic fine flashes of light when they are scattered to electrons. These flashes can be registered by scientists using 5160 photomultipliers. And they have sunk them into the ice on 86 vertical cable strands at 1450 to 2450 meters depth, 125 meters apart, to form a hexagonal lattice structure of equilateral triangles (see diagram on page 30). display

Over 250 researchers from 40 institutions in 11 countries are involved in IceCube. From Germany, researchers from DESY mix with and from the universities of Aachen, Berlin, Bonn, Dortmund, Mainz, Munich and Wuppertal. IceCube measures around the clock. 90 gigabytes of data are transmitted daily via satellite for evaluation.

Meanwhile, the IceCube team has evaluated 998 days of measurement time. The analysis is very time-consuming: Every year the detector registers around 100, 000 neutrinos, with about half coming from "below" - that is, from the northern celestial sphere - and the whole earth has passed smoothly. These neutrinos, as well as almost all coming from "above", arise from the interaction of cosmic radiation with atoms of the Earth's atmosphere. IceCube can only detect high-energy neutrinos with energies above about 10 gigaelectronvolts and is therefore "blind" to the myriad of low-energy solar neutrinos.

However, IceCube measures not only neutrinos, but a total of about 85 billion events a year, due to the influence of cosmic rays. They generate interference signals that have to be eliminated - mainly from muons, the heavy siblings of the electrons. "This is a painful interference with the search for neutrinos, but a goldmine for the physics of cosmic rays, " Francis Halzen puts the double-edged down to the point.

The researchers initially expected a diffused signal of cosmic neutrinos within the "background" of atmospheric neutrinos, and then certain accumulations in it, so that a few years later they might locate their sources in the sky. But the development was the other way round: the scientists only detected the diffuse signal a few months ago - but already in 2012 they noticed two strong signals when they saw through the data since 2010. The signals had left a veritable fireworks in the sensitive electronics of photomultipliers deep in the Antarctic underground. Both events had an energy of over one petaelectron volt (1015 electron volts).

Ernie and Bert

The researchers baptized them as a joke Ernie and Bert, after the popular characters from the children's television series Sesame Street. And they decided to name conspicuous signals for more rag dolls, not only from Sesame Street, but also from the Muppet Show. Not everyone finds this funny - and in the scientific papers researchers prefer to use the vile acronym HESE (for "High Energy Stating Event") and a number. "I myself am a big fan of doll names, also because there are women's names among them and they represent many different types, " says Laura Gladstone of the University of Wisconsin.

After Ernie and Bert, the scientists have 26 other events in the slightly weaker energy range of 30 to 250 Teraelektronenvolt (1012 electron volts) aufgesp rt. When they published the 28 signals from the first two years of measurement in the November 2013 issue of the journal Science, their article was shortly thereafter proclaimed the "breakthrough of the year" in physics. However, the statistics were not quite enough to fulfill the requirement for a discovery in physics: 5 sigma or a statistical security of 1 to 3.3 million. That means: The 28 events could still have come from the atmospheric neutrino background.

A big bird

Feverishly, the IceCube team began evaluating another year of their data collection. And again the researchers came up with high-energy outliers altogether there were nine. One of them, found by young physicist Lisa Gerhardt at the Lawrence Berkeley National Laboratory, was called Big Bird. With about two Petaelektronenvolt he marks the most energetic known neutrino of all time. By comparison, in the case of the Large Hadron Collider at CERN, the most powerful particle accelerator ever, only eight teraelectronvolts of collision energy of two protons were possible a 250th of Big Bird.

With these now 37 events totaling more than 30 teraelectronvolts, the researchers broke the statistical significance limit: 5.7 sigma. Technically speaking, even though 8.8 plus / minus 4.2 of the events could be generated by muons and 5 to a maximum of 13 by atmospheric neutrinos, at least a dozen of the high-energy neutrinos are ambassadors from the depths of space. But what did she send?

Neutrino sources are many objects and processes in space. Astrophysicists have already published several hundred articles on this subject. Where besides the question of where from one source or many? Galactic or extragalactic? which also needs to be answered by what: What were the particles accelerated, from whose decay or transformation, the neutrinos in the first place arise?

As galactic sources are mainly supernova remnants and Pulsarwindnebel eligible, so the relics of stellar explosions. But from the direction of the Crab-pulsar, which should be a particularly strong, because young and near Neutrinoquelle, so far, nothing has been measured. The interaction of Cosmic Radiation with the interstellar medium, that is, the atoms between the stars, is also the source: the neutrinos that are produced have at most one-hundredth of the energy of Ernie and Bert.

There is also no shortage of extragalactic candidates. Some researchers favored starburst galaxies. They are characterized by a high star formation rate and contain many supernova remnants. Newer model calculations showed, however, that their acceleration force is probably not sufficient to explain the measured neutrino energies. Even hypernovae and gamma-ray bursts are no longer convincing candidates, because IceCube has no temporal coincidence with such giant stellar explosions in distant galaxies aufgesp rt. Galactic collisions and high-energy processes in the environment of supermassive black holes continue to be considered. Speculations about exotic sources are also not yet off the table about the decay of unknown elementary particles that make up dark matter in space, or the interaction of hypothetical particles called leptoquarks.

"The origin of high-energy neutrinos is a new mystery in astroparticle physics, " says Kohta Murase of the Institute for Advanced Study at Princeton, New Jersey. "Most likely it's extragalactic, even though there may be some galactic neutrinos in it."

Traces of black holes?

Unfortunately, IceCube's angular resolution is relatively poor. Only 20 percent of the neutrinos could be located below one degree because their tracks in the detector were associated with muons, which produce a very accurate signal. The other events were determined only to 15 degrees exactly. As long as no more particles are measured, their place of origin can not be identified.

After all, it is already clear that the distribution in the sky is quite uniform. It follows that at least some of the neutrinos do not come from the Milky Way, but are of extragalactic origin - otherwise they would be clustered in the galactic plane. To assign them to a known source in space, the data is not statistically significant, as the IceCube researchers emphasize.

However, an analysis by Sarira Sahu and Luis Salvador Miranda at the State University of Mexico revealed that 7 of the 37 high-energy neutrinos originate roughly from the direction of the Galactic Center and another 10 events are consistent with the position of 9 blazars and the active galaxy Centaurus A.,

Blazars are among the active galactic nuclei. They are fed by the collapse of vast quantities of gas and dust, which tumble into a supermassive black hole. Such a gravitational costume is also found in Centaurus A and in the Galactic Center, where it is quieter. And the position of Blazar H2356-309 is consistent with three IceCube events. •

RÜDIGER VAAS, astronomy and physics editor of bild der wissenschaft, is the author of several books on particle physics and cosmology.

Sources of neutrinos

Neutrinos arise only in processes involving the weak interaction. Since the existence of the neutrinos in 1930 was theoretically predicted by Wolfgang Pauli and then in 1956 Clyde L. Cowan, Frederick Reines and their co-workers achieved the first experimental proof with the help of nuclear reactors, physicists have found various sources of neutrinos:

· Nuclear reactions in physical experiments

· Nuclear reactions in nuclear power plants

· Nuclear reactions in the radioactive decay of certain elements in the Earth's interior: These neutrinos were recently detected for the first time (see "Messages from the Earth's interior" from p. 40).

· Nuclear reactions in the collision of cosmic ray particles with atoms in the Earth's atmosphere

· Nuclear fusion processes in the center of the sun

· Supernovae: So far, only one of these stellar explosions has measured neutrinos - from SN 1987A in the Great Magellanic Cloud.

· Neutrinos of nuclear reactions in brute shock fronts in the Milky Way and in other galaxies: These include hypernovae, supernova remnants, and accretion disks around black holes, which are particularly turbulent in the centers of active galaxies. The IceCube detector has recently detected such neutrinos for the first time.

· Neutrinos from the Big Bang: They still fly through space all over the world, but so far can not be measured directly because of their low energy. However, they have left subtle indirect traces in the temperature distribution of cosmic background radiation that has been detected (bdw 9/2009, "Ghost Particles All-Around").

Good to know: neutrinos

According to the standard model of matter, there are three flavors of neutrinos: the electron, muon and tau neutrino. They are as it were the cousins ​​of the electron and its heavy siblings Myon and Tauon. Together with them they are counted among the leptons. In contrast to these neutrinos are not electrically charged and therefore are not subject to the electromagnetic force. All leptons are elementary particles and together with the quarks form the well-known ordinary matter.

Like every other particle of matter, every neutrino also has an antiparticle. These antielectron, antimony and antitau neutrinos are often simply referred to as neutrinos in the everyday language of physicists. Maybe antineutrinos and neutrinos are even identical - then neutrinos would be their own antiparticles. Also unknown is whether there are other neutrinos, such as "right-handed neutrinos" (related to their spin) or "sterile neutrinos", which are not subject to weak interaction but are predicted by speculative extensions of the standard model.

Compact

· The IceCube detector in the Antarctic has for the first time measured high-energy neutrinos from space.

· They testify to brute processes far outside the Milky Way - for example, from the environment of supermassive black holes.

© science.de - Rüdiger Vaas
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