The DESY scientists also conduct research in astroparticle physics. Various kinds of particle from the cosmos constantly reach the Earth – particles that can provide insights into events happening in the depths of the universe. The DESY researchers use two of these cosmic messengers, neutrinos and high-energy gamma rays, to uncover the secrets of stellar explosions, cosmic particle accelerators – such as the surroundings of black holes – or dark matter.

The neutrino telescope IceCube

Neutrinos are extremely light particles that pass through everything in their path almost unhindered. They are created in a variety of ways, such as inside the sun or as a result of stellar explosions known as supernovae. Although around 60 billion neutrinos from the sun pass through every square centimetre of the Earth’s surface every second, they hardly react with their surroundings. That’s why the ghostly particles can only be detected with the help of elaborate experiments, such as large tanks in mines and measuring devices in lakes, oceans and the everlasting ice at the South Pole.

Neutrinos provide important information about events in the cosmos. In the same way that certain phenomena can be made visible with light, other phenomena can be “seen” with the help of neutrinos. Paradoxically, the fact that they very rarely interact with their environment makes neutrinos ideal cosmic messengers. That’s because they reach the Earth on a direct path, coming, for example, from the centres of distant galaxies, millions or even billions of light-years away. Light beams or gamma radiation from these galaxies easily get caught in nebulae on their way to Earth. Charged particles are deflected by cosmic magnetic fields, so that their actual source can no longer be determined. Neutrinos are not affected by nebulae or magnetic fields – which is why they can provide us with information from regions of space from where almost no other signal reaches the Earth.

Telescopes for neutrinos

Due to the vast distances involved, high-energy neutrinos from outer space only rarely reach the Earth. In order to track them down, therefore, researchers have to construct neutrino telescopes hundreds or thousands of times bigger than the detectors in shafts or tunnels used to detect solar neutrinos. Such gigantic instruments are submerged deep in water or ice. Together with colleagues from many other countries, DESY researchers are hunting for these elusive particles with neutrino telescopes in Lake Baikal and at the South Pole.

The neutrino telescope AMANDA is located in Antarctica, deep in the polar ice. It has now been extended to form IceCube, the world’s largest particle detector, which was completed in December 2010. The researchers’ objectives in detecting the neutrinos from the far reaches of space are to investigate the origin of cosmic rays and to plant the first flags on the as-yet blank map of the high-energy neutrino universe. They also plan to use the neutrino telescopes to observe the rare supernova explosions and to search for particles of dark matter or even more exotic particles such as magnetic monopoles.

Cosmic tracks in the eternal ice

The neutrino telescope IceCube consists of several thousand glass spheres with light sensors – called optical modules – that are strung on long cables like pearls, and melted almost 2.5 kilometres deep into the polar ice of the Antarctic. The thick ice shield screens the sensors from interfering signals. Thanks to the crystal-clear ice, the particles’ direction of origin can be determined. When a neutrino reacts with an atomic nucleus a muon is created. In water or ice, this particle emits what is known as Cherenkov radiation. The electronics in the glass sphere record the light cones of the Cherenkov radiation and thus the path of the muon. The direction of flight of the neutrino responsible for the event can be calculated from the sensor data sent to the measuring station on the surface.

Remarkably enough, neutrino reactions produce not only optical, but also acoustic signals. However, the tiny popping noises are only produced by extremely high-energy neutrinos. Whether or not the neutrinos at the South Pole can also be detected acoustically is currently being investigated using the SPATS South Pole Acoustic Test Setup.

IceCube will encompass a volume of one cubic kilometre, making it the world’s largest particle detector. It will be around 30 times as sensitive as its predecessor AMANDA, whose optical sensors have now become part of IceCube. In addition, the neutrino telescope is being enhanced by a further detector field known as IceTop. Measuring one square kilometre in area, IceTop will comprise 80 detector stations located directly above IceCube’s light sensors. The scientists intend to use IceTop to observe extended air showers triggered in the atmosphere by high-energy cosmic rays. One quarter of IceCube’s detector modules have been produced in Zeuthen. The Zeuthen-based researchers are also major participants in the analysis of the data from IceCube and IceTop.