94 Annual Report 2008

Newest Results from The KASCADE-Grande Experiment by J.Zabierowski and P. uczak [I.9]

The KASCADE-Grande experiment [1], located acceleration and propagation mechanisms of cosmic on site of the Forschungszentrum Karlsruhe in rays [5]. Germany, is a multi-detector extensive air shower Currently, the properties of the models and not the (EAS) array. It combines the electron and muon understanding of the KASCADE data are limiting the detectors of the KASCADE array [2], a hadron analysis of air shower data. Therefore, testing the calorimeter, Muon Tracking Detector [3] and an array models by means of various EAS parameters is a very 2 2 of 37 scintillators (10 m each) spread over 0.5 km . important part of KASCADE-Grande activity. Its purpose is to study cosmic rays in the energy range In a recent update of the analysis [6] we applied 10 14.5 -10 18 eV and to find an explanation of the “knee” the unfolding method with a different low-energy feature in the cosmic ray spectrum, as well as the interaction model (FLUKA instead of GHEISHA) in energy region of the conjectured transition between the simulations. While the resulting individual mass galactic and extra-galactic cosmic rays. group spectra (Fig. 1) do not change significantly, the overall description of the data improves. In addition, data a larger range of zenith angles were analyzed. The new results are completely consistent, i.e. there is no hint of any severe problem in applying the unfolding analysis method to KASCADE data. The successful continuous operation of KASCADE-Grande and accumulation of data on showers above 10 17 eV will soon allow us to apply the unfolding method to this energy region and, hopefully, answer the questions about the energy spectrum there [7]. Exploiting the possibility given by KASCADE data, correlations of various observables are used for detailed consistency tests of hadronic interaction models. Recently [8], predictions of air shower simulations using the EPOS 1.61 model have been investigated, revealing that the predictions of EPOS are not compatible with KASCADE measurements. Most likely, EPOS does not deliver enough hadronic energy to the observation level and the energy per hadron seems to be too small. By that, the number of particles at ground are shifted to lower electron and higher muon numbers relative, e.g. to QGSJet, resulting in the reconstruction of the mass of cosmic rays with a much lighter composition. EAS measured by KASCADE-Grande have been studied with respect to the arrival times of electrons

Fig. 1 Comparison between QGSJet/FLUKA based results and and muons at the observation level [9]. It has been QGSJet/GHEISHA based results for the energy spectra of H, He shown that for core distances > 200 m particles of the and C (top) and Si and Fe (bottom). Shaded bands correspond to muonic shower component arrive, on average, earlier estimates of the systematic uncertainties for the QGSJet/GHEISHA solutions. than particles of the electromagnetic component. The difference increases with the core distance from In 2005, using the unfolding method, for the first (t) = (12.9 ± 0.2) ns at R > 200 m to (t) = (47 ± 1) time the energy spectra for five elemental groups of ns at R = 500 m, where the widths of the muonic and cosmic rays were reconstructed out of KASCADE electromagnetic shower disks are comparable. This data [4]. It was shown that the “knee” at 3-5 PeV is difference in arrival time is used to separate the caused by the steepening in the light element spectra electrons and muons dependent on the distance from and that none of the two hadronic interaction models the shower center. The potential of the method evolves used was able to describe the experimental data in a with increasing distance to the shower core and it is consistent way over the whole investigated energy thus especially interesting for large EAS arrays. range. This result, in agreement with findings of other experiments, had immediate impact on the views on ASTROPHYSICS, COSMIC RAYS & ELEMENTARY PARTICLE PHYSICS 95

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