IceCube is a neutrino observatory located in Antarctica at the geographic South Pole. After six years of construction, it was completed in December 2010, and is the

 
IceCube
IceCube
 

world's largest neutrino detector. IceCube is run by an international collaboration with more than 250 physicists, in which the groups from Stockholm and Uppsala Universities play prominent roles.

A total of 5160 light sensors (optical modules) have been embedded in the three-kilometer deep glacier, to observe the light that is generated by particles passing through the ice as a result of neutrino interactions. The modules are at depths between 1450m and 2450m and the complete array spans a volume of one cubic kilometer (hence the name IceCube). In addition, there are 162 surface stations, with two optical modules in each, that are used to detect high-energy cosmic rays, which interact in the upper atmosphere. More than 1,000 of the modules in the IceCube detector have been built in Stockholm.

The illustration below on the left shows how the detector is situated in the ice. The second shows the installation of a camera system constructed in Stockholm and deployed in December 2010 to the bottom of the final IceCube hole.

During the 20th century, our view of the universe expanded from visible light to the whole electromagnetic spectrum, from radio wavelengths to very high energy gamma rays. A new and fascinating picture of the universe has emerged, and it is often revealed in sudden and dramatic events such as supernovae, gamma ray bursts and the outburst of neutron stars and black holes. In 1912 Victor Hess discovered that the Earth is also continually bombarded by electrically charged particles from space, called cosmic rays. Some of these particles have tremendous energies, but where they come from remains a mystery because their paths have been bent and turned by magnetic fields while traveling through space. More mysterious still is the stuff known as "dark matter": it is believed to make up about 80% of the mass of the universe, but so far its presence is only known through its gravitational influence on the way stars and galaxies move.

The goal of the IceCube project is to observe, for the first time, the universe through the messenger of high-energy neutrinos rather than light. We hope to gain new insights into the most energetic processes in the universe and locate the sources from which the cosmic ray particles emerge with such energy. (Low-energy neutrinos are observed continuously from the sun, and were once detected from a supernova in the Large Magellanic Cloud, in 1987). It is possible that there are events that are ''hidden'' and can not be seen in the normal way. Because neutrinos interact so rarely, and therefore easily pass through large amounts of matter (including the earth) there may be sources from which light is blocked but neutrinos can escape and reach us. We hope to discover the unexpected!

 
Norrsken är vanligt i polarområdena Foto Sven Lidström
Photo Sven Lidström
 

 

It is not easy to find astronomical neutrinos. First, because neutrinos interact so rarely with matter, one must have a huge detector volume (like a cubic kilometer) for there to be any chance of even a few such neutrinos interacting there. Second, the majority of neutrinos which IceCube detects (more than 100 per day) are not from deep in outer space, but from cosmic rays interacting in the atmosphere. These neutrinos help us to understand what kinds of cosmic rays reach us, but not about where they came from. Although the detector lies deep in the ice there is also a significant background of charged particles, muons, which are formed in the atmosphere and can penetrate several kilometers deep. However, muons cannot travel great distances through the earth. So in general, we can treat particles going downward through the detector as background, and focus on neutrino searches for upward-going particles, since only neutrinos can travel through the earth from the other side.

A neutrino signal could appear in many different ways. The clearest signal could be a neutrino pulse from a gamma-ray burst or supernova, because the exact time and direction are cross-checked with other observations of the burst. For a supernova in our galaxy, an enormous low-energy neutrino pulse occurs several hours before the first light will arrive. This can be used to give astronomers worldwide an alert so that the supernova can be observed by conventional telescopes just as the first glow in light appears.

Another signal may be the detection of a modest but steady neutrino flux from a particular source in the sky. A strong source of cosmic rays, either in our galaxy or in a nearby galaxy, might provide a detectable signal over time of a few dozen neutrinos from the same direction, which would stand out strongly against the background of atmospheric neutrinos.

 

 
Sommar på norra halvklotet innebär vinter på södra. Bild Sven Lidström
Summer in the northen hemisphere means winther in the southern hemisphere.
Picture Sven Lidström

 

           IceCube is also looking for a diffuse neutrino flux from the sum of a variety
of sources scattered across the universe. To distinguish such a flux from the background of atmospheric neutrinos one searches for an excess of higher energy neutrinos from across the sky, compared with the lower energies that the atmospheric neutrinos typically correspond to.

A signal that the Stockholm group is especially interested in is related to the nature of the dark matter. The leading theories of what the dark matter particle is predict that these particles will be more concentrated deep inside massive regions like the center of the galaxy or the center of the sun. At high concentrations, the dark matter particles will slowly annihilate each other, and out of the annihilation will come "normal" particles, including neutrinos. If this indeed happens in the solar center, the neutrinos will be the only evidence that escapes from the dense interior of the sun. The IceCube Observatory is uniquely capable of searching for this clue to the identity of dark matter.

Neutrinos from dark matter annihilation are predicted in most theories to be at the lower energy region of IceCube's sensitivity. To improve this sensitivity, the

 
På väg till jobbet i - 70 grader. Foto Sven Lidström
Walking to work in - 70 degrees.
Photo Sven Lidström
 

original IceCube plan of 80 strings was expanded at the initiative of the Stockholm and Uppsala groups and with the support of the Wallenberg Foundation. The expanded plan added six additional strings of sensors at the center of detector, deployed close together in the deepest, clearest part of the ice. These strings and their neighboring strings form a dense sub-detector called the IceCube DeepCore detector and is optimized for lower energy neutrino studies.

 

With the detector completed, the Stockholm group is interested now in the flood of new data which will be arriving. Opportunities to work with other telescopes and experiments are growing, each of which increases the chances of discovering the unexpected. we are pursuing joint searches with gravitational wave detectors such as LIGO, Virgo, and GEO, to look for highly energetic, transient events that can emit neutrinos and gravitational waves. We are also participating in the planning of future upgrades to the IceCube detector and new endeavors at the South Pole Station.
 
Text: Klas Hultqvist