The IceCube Neutrino Observatory III: Cosmic Rays

(SNIC), and Knut and Alice Wallenberg Foundation, Sweden; German Ministry for Education and Research (BMBF), Deutsche Forschungsgemeinschaft (DFG), Research Department of Plasmas with Complex Interactions (Bochum), Germany; Fund for Scientific Research (FNRS-FWO), FWO Odysseus programme, Flanders Institute to encourage scientific and technological research in industry (IWT), Belgian Federal Science Policy Office (Belspo); University of Oxford, United Kingdom; Marsden Fund, New Zealand; Japan Society for Promotion of Science (JSPS); the Swiss National Science Foundation (SNSF), Switzerland; D. Boersma acknowledges support by the EU Marie Curie IRG Program; A. Groß acknowledges support by the EU Marie Curie OIF Program; J. P. Rodrigues acknowledges support by the Capes Foundation, Ministry of Education of Brazil; A. Schukraft acknowledges the support by the German Telekom Foundation; N. Whitehorn acknowledges support by the NSF Graduate Research Fellowships Program.

The Neutrino Observatory IceCube is a 1-km 3 detector situated in the ice of the geographic South Pole at a depth of about 2000 m. IceTop, the surface component of Ice-Cube, is an air shower array covering an area of 1 km 2 . The prime purpose of IceTop is the determination of the mass composition of primary cosmic rays in the energy range from about 10 14 eV to 10 18 eV. In the ‘knee’ region, at several PeV, the spectral index of the observed cosmic ray energy spectrum changes. Several experiments found this change to be accompanied by a change in the chemical composition of the primaries. However, details of the features are not well known. In particular, there may be at the high end of the IceTop energy range another change of the spectral index and an accompanying change of the composition, possibly indicating the transition from galactic to extra-galactic origin of cosmic rays. An improvement of the experimental situation in this energy rangebetween direct measurements with balloons and satellites and the highest energies tackled by experiments like HiRes and Auger -is one of the main goals of cosmic ray physics with IceCube. The mass determination from extended air showers (EAS) is notoriously difficult because the measurements are indirect and have to rely on models for the hadronisation processes. Observables sensitive to the primary mass composition are mainly the height of the shower maximum (measured through fluorescence, Cherenkov or radio emission) and the number of muons in a shower. Concerning the muon rate, the highest energy muons stemming from the first interactions in the higher atmosphere are most closely correlated to the mass of the primary nucleus. IceCube, in combination with IceTop, offers the unique possibility to observe these muons, typically with initial energies above about 500 GeV, in the deep ice in coincidence with the mostly electromagnetically deposited shower energy measured at the surface. This provides an exceptionally powerful method for the determination of the mass composition. To scrutinize the dependence on hadronisation models, several alternative methods for studying mass composition have been developed by the IceCube collaboration. Other mass sensitive observables are for example: the shower absorption in the atmosphere at different zenith angles, the number of dominantly low-energy muons in the surface detector, and other shower properties such as shower age and shower front curvature. The IceTop array has additionally been used to study highp T muons, PeV-gammas and transient events, such as the radiation effects of solar flares. It also serves as a veto for the detection of downward-going neutrinos with IceCube and for direction calibration.

The IceCube construction was completed in December 2010. The results presented here are based on data taken with smaller detector configurations.

IceCube: The main component of Icecube is an array of Figure 1: The IceCube Observatory with its components DeepCore and IceTop. of 1 km 3 at a depth between 1450 m and 2450 m (Fig. 1). In the lower part of the detector a section called DeepCore is more densely instrumented. The main purpose of IceCube is the detection of high energy neutrinos from astrophysical sources via the Cherenkov light of charged particles generated in neutrino interactions in the ice or the rock below the ice.

IceTop: The IceTop air shower array is located above IceCube at a height of 2832 m above sea level, corresponding to an atmospheric depth of about 680 g/cm 2 . It consists of 162 ice Cherenkov tanks, placed at 81 stations and distributed over an area of 1 km 2 on a grid with mean spacing of 125 m (Fig. 1). In the center of the array, three stations have been installed at intermediate positions. Together with the neighbouring stations they form an in-fill array for denser shower sampling. Each station comprises two cylindrical tanks, 10 m apart from each other, with a diameter of 1.86 m and filled with 90 cm ice. The tanks are embedde

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