Calibration of the Liquid Argon Calorimeter and Search for Stopped Long-Lived Particles

This thesis dives into the three main aspects of today's experimental high energy physics: detector operation and data preparation, reconstruction and identification of physics objects, and physics analysis. The symbiosis of these is the key to reach a better understanding of the underlying principles of nature. Data from proton-proton collisions at a centre of mass energy of $\sqrt{s}=\,$13TeV collected by the ATLAS detector during 2015-2018 are used. In the context of detector operation and data preparation, the data quality assessment for the Liquid Argon calorimeter of the ATLAS experiment is improved by an adaptive monitoring of noisy channels and mini noise bursts allowing an assessment of their impact on the measured data at an early stage. Besides data integrity, a precise energy calibration of electrons, positrons and photons is essential for many physics analyses and requires an excellent understanding of the detector. Corrections of detector non-uniformities originating from gaps in between the Liquid Argon calorimeter modules and non-nominal high voltage settings are derived and successfully recover the homogeneity in the energy measurement. A further enhancement is reached by introducing the azimuthal position of the electromagnetic cluster in the calibration algorithm. Additionally, a novel approach to exploit tracking information in the historically purely calorimeter-based energy calibration for electrons and positrons is presented. Considering the track momentum results in an about 30% better energy resolution for low-$p_{\mathrm{T}}$ electrons and positrons. The described optimisation of the energy calibration is especially beneficial for precision measurements which are one way to test and challenge our current knowledge of the Standard Model of particle physics. Another path is the hunt for new particles, here presented by a search for stopped long-lived particles suggested by many theoretical models. This analysis targets gluinos which are sufficiently long-lived to form quasi-stable states and come to rest in the detector. Their eventual decay results in large energy deposits in the calorimeters. The special nature of the expected signature requires the exploration of non-standard datasets and reconstruction methods. Further, non-collision backgrounds are dominant for this search which are investigated in detail. In the context of simplified supersymmetric models an expected signal sensitivity of more than 3$\sigma$ for gluinos with a mass up to 1.2TeV and a lifetime of 100$\mu$s is achieved.