Signatures of SUSY dark matter at the LHC and in the spectra of cosmic rays

Over the past decades it has become evident that luminous matter amounts only to a small fraction of the energy density in the universe. More than 75% is accounted for by what is called the dark energy, and about 20% must exist in the form of some kind of non-relativistic dark matter. Among the candidates for the constituents of dark matter, the supersymmetric (SUSY) neutralino is one of the most promising. This thesis discusses the search for supersymmetry at the future Large Hadron Collider (LHC) and the ongoing construction of one of the four large LHC experiments, the Compact Muon Solenoid (CMS), and focuses on the detection of signals from the annihilation of supersymmetric dark matter in the spectra of cosmic rays. CMS relies on the excellent performance of its components and thus requires strict quality control before their assembly. The final steps of assembly of 1061 silicon microstrip detector modules for the CMS tracker endcaps are performed at the 1. Physikalisches Institut B at the RWTH Aachen. A laser test facility for these modules was developed and is described in this thesis. In contrast to test procedures based only on the evaluation of pedestal and noise data, the test facility relies on the generation of signals in the silicon sensors by infrared laser illumination. Subsequent analysis of the signals allows reliable detection of module defects. The fully automatic test facility provides high throughput and easy operation for the series production of the modules. Its performance is validated by investigating a reference module with artificially prepared defects of three types: open wirebonds, short-circuited strips and pinholes. It is shown that all defects are clearly detected. In addition to defect detection, an indication for the type of defect is provided. In a further validation step, nine modules from a prototype series are investigated with the laser test facility. Confirming the earlier results on the reference module, defective strips are reliably identified. The results are in agreement with those from other test facilities using different techniques. Measurements of cosmic ray antiparticles, such as positrons, can impose strong constraints on the nature of new physics beyond the Standard Model. However, cosmic ray positron measurements are experimentally very challenging due to the vast proton background. This thesis describes a novel approach of positron identification with the space-borne AMS-01 experiment, namely through the detection of bremsstrahlung conversion in a silicon microstrip detector. In contrast to earlier single-track analyses, this approach involves the selection and reconstruction of multi-track events. Subsequent to an introduction to cosmic ray physics and a description of the AMS-01 experiment, the discussion of the signal process shows that bremsstrahlung from protons is suppressed by a factor of more than 3*10^6 with respect to positrons. The background to the positron sample can largely be suppressed using the topological and geometrical properties of the events. In order to obtain the highest positron selection efficiency possible, novel combinatorial track finding algorithms were developed, particularly optimized for the signature of converted bremsstrahlung. By applying restrictions on the invariant mass of particles the background to the positron sample is largely eliminated. The remaining background contamination is determined from large samples of Monte Carlo data taking into account the effects of the geomagnetic field. It amounts to 26% of the positron counts and is corrected for. In order to remove atmospheric secondaries from the positron and electron samples, a precise method involving trajectory backtracing in the magnetic field of the Earth was developed and is applied individually to all positron and electron candidates. The results of the positron measurement show that the bremsstrahlung approach extends the sensitivity range of AMS-01 to positron momenta up to 50 GeV/c, which is far beyond the original scope of the experiment. The precision of the positron measurement is statistically limited by the small number of particle counts. The positron fraction e+ / (e+ + e-) is calculated for particle momenta in the range from 1 to 50 GeV/c. For momenta up to 8 GeV/c it is found to be in good agreement with model predictions for background from purely secondary positron production, while at higher momenta there is indication for a positron overabundance. Therefore, the AMS-01 data lend further weight to the hints of a positron overabundance seen in the data from earlier experiments. In addition to the positron fraction, the absolute fluxes of positrons and electrons are calculated from the event samples of the present analysis. For this purpose, a method was developed which allows the determination of the geomagnetic transmission as a function of momentum and direction of incidence with high accuracy. The results of the flux calculation are found to be in very good agreement with earlier data, confirming the good performance of electron and positron selection with the bremsstrahlung approach. Finally, the positron fraction results from this analysis have been combined with results from earlier experiments. In the combined results, the significance of the positron overabundance with respect to the background expectation for purely secondary positron production increases to 5.3 standard deviations, which would be reduced to 4.2 standard deviations without the results of the present analysis. Therefore, a statistical fluctuation causing the positron overabundance in the data with respect to the background-only expectation can now be excluded.

Grantor dc:publisher

Publikationsserver der RWTH Aachen University

Year dc:date

2007

Author dc:creator

• Olzem, Jan

Contributors dc:contributor

• Schael, Stefan

• info:eu-repo/classification/ddc/530
• Physik
• Dunkle Materie
• Kosmische Strahlung
• LHC
• Supersymmetrie
• CMS
• Siliziumstreifendetektor
• Positronen
• Positronenanteil
• silicon strip detector
• Positrons
• positron fraction

Statement dc:rights

• info:eu-repo/semantics/openAccess

Language dc:language

eng

Identifier

https://publications.rwth-aachen.de/search?p=id:%22RWTH-CONV-123363%22

OAI identifier oai:identifier

oai:publications.rwth-aachen.de:61729