Decoding the composition of cosmic-rays in the transition region from 10^17 -10^18 eV with Tunka-Rex

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One of the open mysteries in astroparticle physics is how nature accelerates atomic nuclei to energies of more than 10^20 eV. While cosmic rays at lower energies originate from inside our Milky Way, the highest energies most probably stem from the deeper universe. For energies around and above 10^17 eV the transition is expected. To find out at which energy and how the transition occurs, more accurate measurements of the cosmic-ray composition are needed.Currently, the techniques applied in this energy range are air-Cherenkov measurements and particle detection of air showers on ground. The first one limited to dark nights with good weather, the latter one suffers from uncertainties in interpreting the particle production in the atmosphere. An alternative technique operating 24/7 are radio measurements of air showers. Recent developments indicate that they can be competitive in precision, and maybe even better, when combined with the other techniques. Thus, the present proposal plans to combine simultaneous radio, air-Cherenkov, and particle measurements at the Tunka observatory close to Lake Baikal in Siberia.Since many years Moscow and Irkutsk State Universities perform air-shower measurements at this site. Since 2009 they operate an array of 133 photomultiplier detectors with 1 km² area named Tunka-133. Recently, an additional array of over- and underground scintillators was deployed for particle detection. In the next years, the site will emerge to one of the leading astroparticle physics centers in Russia by deploying additional detectors dedicated to gamma astronomy (TAIGA). Finally, the site features a digital radio array, Tunka-Rex (http://www.ikp.kit.edu/tunka-rex/), for which I am personally responsible.Tunka-Rex consists of meanwhile 44 antennas connected to the air-Cherenkov and scintillator arrays, and started data taking in 2012. The deployment and initial analyses were funded by a Helmholtz Russian Joint Research Group running until summer 2015, without possibility for reapplication - despite of its success. First, the already available data are valuable for analysis. Second, from now on continued hybrid measurements with the new scintillator array will exceed significantly the statistics of presently available measurements by Tunka-133 and other experiments.For this reason, the proposal requests funding of in total 433 kEUR for a period of three years, for one postdoc, one PhD student, and travels. At the beginning, the analysis will be performed with the already available Tunka-Rex events, and analysis methods will be optimized for a true hybrid analysis including the scintillators. Later, large statistics can be analyzed to reconstruct energy spectra for different mass groups in the transition region from galactic to extra-galactic cosmic rays. Interpreting the data and comparing them to theoretical predictions finally will reveal at which energy exactly and in which way extragalactic cosmic rays come into play.