The IceCube-lab (foreground) has two stories. Cables from the detector enter through the two pillars on either side to reach the computer cluster on the upper floor. Photo: Klas Hultqvist

The IceCube-lab (foreground) has two stories. Cables from the detector enter through the two pillars on either side to reach the computer cluster on the upper floor. Photo: Klas Hultqvist

IceCube is a neutrino telescope built in the ice at the geographic South Pole, next to the American Amundsen-Scott Station. Over 5,000 optical sensors have been installed at
depths of 1,500–2,500 m, giving an instrumented ice volume of 1 km3. Sensor installation work began in 2004 and was completed in December 2010. The completed detector has been collecting data continuously since May 2011.

The detector exploits the full performance capability of the instrument, and the data analysis has been highly intensive. In 2012, the collaboration published 13 scientific articles, while an additional four have been accepted and another four submitted. The results have also been presented at numerous conferences. The detector performance and data analysis methods have exceeded expectations in a number of areas. Plans call for data collection to continue for many years, in order both to achieve ultimate sensitivity to the neutrino fluxes and to monitor time-dependent sources in the universe.

Two events in which just over 1 PeV (1015 eV) of energy were imparted to the detector were among the spectacular results that came to light in 2012 (see figure). These events were of the cascade-type, but were discovered while searching for high-energy track-type events. Another filtering for events that began inside the detector was consequently performed, and we found an additional 26 events over two years, although they were somewhat lower in energy. The origin of these events remains a mystery. They could be the first signs of high-energy astrophysical neutrinos, although there are other possibilities. If these events are caused by neutrinos of astrophysical origin, they will provide information about the sources of cosmic radiation and the development of objects in the universe.

Two events with energy in excess of 1 PeV that were captured by the complete IceCube detector. The vertical dotted lines correspond to the detector’s strings of optical sensors. Modules that emitted signals during the event are marked with coloured spheres; their size indicates the amount of light in the sensor, while their colour indicates the relative time (red–blue => early–late). These events appear to occur inside the detector, with no tracks from any incoming charged particles, making them good candidates for neutrino events.

Two events with energy in excess of 1 PeV that were captured by the complete IceCube detector. The vertical dotted lines correspond to the detector’s strings of optical sensors. Modules that emitted signals during the event are marked with coloured spheres; their size indicates the amount of light in the sensor, while their colour indicates the relative time (red–blue => early–late). These events appear to occur inside the detector, with no tracks from any incoming charged particles, making them good candidates for neutrino events.

In 2012 we also succeeded in demonstrating that the deep, denser portion of the detector known as the Deep Core can measure low-energy neutrinos (on the order of tens of GeV) from all directions. The large surrounding mass of IceCube serves as a veto shield, isolating the Deep Core from the flux of muons arriving from above, which is many millions of times greater. Deep Core has enabled us to set limits with regard to how much dark matter is accumulated in the centre of the sun. The dark matter accounts for much of the matter in the universe, but its nature remains a mystery. Deep Core has been funded in large part by contributions from the Knut and Alice Wallenberg Foundation.

Deep Core’s ability to measure low-energy neutrinos has allowed us to observe that neutrinos oscillate, i.e., alternate between different “flavours”. An even denser sensor array, PINGU, is planned for the primary purpose of measuring the mass differences between the different types of neutrinos, i.e., the “neutrino hierarchy”.