Search for dark matter in the Milky Way halo with the High Energy Stereoscopic System

An indirect search for the presence of dark matter particles in the halo of the Milky Way with data that were recorded with the High Energy Stereoscopic System (H.E.S.S.) is discussed in this work. For this, the prediction is used that besides other known particles also photons can be created in the decay or mutual annihilation of dark matter particles. For dark matter particles with TeV masses, this process can produce diffuse photon fluxes with TeV energies that can be detected with imaging Cherenkov telescopes like the H.E. S.S. array. The spatial proximity of the halo of the Milky Way leads to the expectation of the presence of larger high energy photon fluxes on the earth from the Milky Way halo than for other astrophysical objects that are investigated in this context like for example dwarf galaxies. However, the extension of the Milky Way halo is challenging for the observation with small field of view Cherenkov telescopes. In particular the need for a subtraction of background events that are mostly generated by cosmic ray events, which Cherenkov telescopes cannot always distinguish from high energy photon events, requires the application of special observation methods for the halo of the Milky Way. Three different methods (driftscan, On/Off and rotated pixel) to observe the Milky Way halo with Cherenkov telescopes are compared in this thesis. Data that were recorded with the H.E. S.S. telescopes in all considered observation modes are analyzed for this purpose. Of special importance in this context is the investigation and control of systematic effects that influence the subtraction of background events. One result of the consideration of the different systematic effects is that the observation of the Milky Way halo in the so called driftscan mode is not recommended. The sensitivity of all three investigated methods to search for the presence of dark matter in the Milky Way halo is compared. One result is that the sensitivity of the so called On/Off method is in general better than for the other considered methods. However, the sensitivity to search for the presence of dark matter in the Milky Way halo with Cherenkov telescopes depends strongly on the unknown distribution of dark matter in the Milky Way. For the most frequently assumed dark matter distributions, the sensitivity of all three investigated methods for the presence of dark matter in the Milky Way halo is at a comparable level. Upper limits on the velocity averaged annihilation cross section 〈σv〉 are derived from the recorded H.E. S.S. data in the framework of models for the distribution of dark matter in the Milky Way and the creation of photons in the dark matter annihilation. Depending on the observation method and the exposure, values of 〈σv〉 that are larger than 3 · 10−25 cm3/s for dark matter particles with mass 1.8 TeV can be excluded at 95% confidence level. Currently discussed models predict a value for 〈σv〉 that are about one order of magnitude smaller than the best exclusion limit that is derived in this work. Additionally, the possibility to search for the presence of dark matter with the planned CTA observatory which will be about one order of magnitude more sensitive than H.E. S.S. is investigated. The possibility that the increased sensitivity of CTA to high energy photon fluxes does not lead to a substantial improvement in sensitivity to search for the presence of dark matter particles in the Milky Way halo is discussed. An analysis strategy that can be used to search for a diffuse high energy photon flux at TeV energies from the galactic plane with Cherenkov telescopes is developed in this context based on data that were recorded with the H.E. S.S. array. Besides other processes, the presence of diffuse TeV energy emission