Experimental research in particle physics: Characterization of gas-avalanche THGEM particle detector and physics data analysis with the ATLAS experiment.

Modern experimental research in particle physics incorporates a broad range of scientic fields, from the development of the experimental tools, experiment assembly and running - to the final analysis of the data. My thesis covers both ends; a systematic study of physical processes in a novel particle-detector for future particle-physics experiments and data analysis from the LHC-ATLAS experiment. The thesis is made of three parts. The first part of the thesis is devoted to an advanced study of charging up processes occurring in gas-avalanche detectors based on Thick Gas Electron Multipliers (THGEM). The second part is devoted to detector performance studies - that of photon identication efficiency in ATLAS. The third part is dedicated to ATLAS data analysis - in particular related to the search for rare processes or hypothetical particles.