Stefan Schael

A high resolution scintillating fiber tracker with SiPM readout

Using thin scintillating fibers with Silicon Photomultiplier (SiPM) readout a mo dular high-resolution charged-particle tracking detector has been designed. The fiber modules consist of 2 x 5 layers of 128 round multiclad scintillating fiber s of 0.250mm diameter. The fibers are read out by four SiPM arrays (8mm x 1mm) e ach on either end of the module.

Indirect dark matter search with the balloon-borne PEBS detector (15'+5')

A precision measurement of the cosmic-ray positron spectrum may help to solve the puzzle of the nature of dark matter. Pairwise annihilation of neutralinos, predicted by some supersymmetric extensions to the standard model of particle physics, may leave a distinct feature in the cosmicray positron spectrum. As the available data are limited both in terms of statistics and energy range, we are developing a balloon-borne detector (PEBS) with a large acceptance of 4000 cm 2 sr. A superconducting magnet creating a eld of 0:8 T and a tracking device consisting of scintillating bers of 250 μm diameter with silicon photomultiplier readout will allow rigidity and charge determination to energies above 100 GeV. The dominant proton background is suppressed by the combination of an electromagnetic calorimeter and a transition radiation detector consisting of eece layers interspersed with straw-tube proportional counters. The calorimeter uses a sandwich of tungsten and scintillating bers that are again read out by silicon photomultipliers. The design study, based on a detailed Geant4 simulation and testbeam measurements, will be presented along with an interpretation of the currently available positron data in the context of the mSUGRA model. The constraints that future precise measurements could put on this model will be discussed.

Discovery and characterization of a diffuse astrophysical muon neutrino flux with the iceCube neutrino observatory

The IceCube Collaboration has previously discovered a flux of high-energy astrophysical neutrinos whose measurement is based on events with interaction vertices contained within the IceCube detector. This thesis presents a complementary measurement of such a flux based on charged-current muon neutrino events whose interaction vertices can be outside the detector volume. Due to the large range of the induced muon the effective area is significantly larger but the field of view is restricted to the Northern Hemisphere. In this thesis IceCube data from 2009 through 2015 have been analyzed using a likelihood approach based on the reconstructed muon energy and zenith angle. The analyzed data consist of about 350, 000 muon neutrinos with a negligible background of atmospheric muons where the majority of these events are atmospheric neutrinos. The here presented analysis improves the statistics compared to previous analyses [1, 2] by almost an order of magnitude. At the highest neutrino energies between 194 TeV and 7.8 PeV a significant astrophysical muon neutrino flux has been measured. While the data is incompatible with a purely atmospheric neutrino flux at 5.6σ significance, the data are well described by an isotropic, unbroken power-law flux with a normalization at 100 TeV neutrino energy of ( 0.90 −0.27 ) × 10−18 GeV−1 cm−2 s−1 sr−1 and a hard spectral index of γ = 2.13 ± 0.13. The corresponding energy spectrum is harder compared to previous IceCube analyses with lower energy thresholds which may indicate a break in the astrophysical neutrino spectrum of unknown origin. The highest energy event observed has a reconstructed muon energy of (4.5± 1.2) PeV which results in a median muon neutrino energy of 8.7 PeV based on the best-fit neutrino energy spectrum. The probability of this event being of atmospheric origin has been estimated to be less than 0.005%, strongly suggesting an astrophysical origin of this neutrino. The arrival directions of this event and all other events with reconstructed muon energies above 200 TeV have been analyzed, but no correlation with known γ-ray sources was found. Besides the measurement of the astrophysical neutrino flux, the high statistics of atmospheric neutrinos enable to constrain the flux of prompt atmospheric neutrinos originating from heavy meson decays. Since no indications for such a signal was found, the corresponding flux needs to be below 1.06 in units of the flux normalization of the model in Enberg et al. [3].