Search for pair-produced first and second generation scalar leptoquarks in $pp$ collisions at $\sqrt{s}=13$ TeV & work with the liquid argon purity monitoring system in the ATLAS experiment

The Large Hadron Collider (LHC) at CERN delivered data from proton-proton ($pp$) collisions up to the $13\ \text{TeV}$ center of mass energy so far. In the future the energy is even planned to grow. The collected data allows to make more precise assumptions on the Standard Model (SM) and the search of new physics. It is essential to make sure that this data was collected under the best conditions of the ATLAS detector. In this thesis two different topics are covered. On the one hand the search for pair-produced first and second generation scalar leptoquarks in $pp$ collisions at $\sqrt{s}=13\ \text{TeV}$ with the ATLAS detector is described. On the other hand the work with the liquid argon purity monitoring system of the ATLAS detector is summarized. The search for pair-produced first and second generation scalar leptoquarks in $pp$ collisions at $\sqrt{s}=13\ \text{TeV}$ has been carried out using data with an integrated luminosity of $36.1\ \text{fb}^{-1}$. Leptoquarks are featured in a number of extensions of the Standard Model and may provide an explanation for the similarities of the quark and lepton sectors in the Standard Model. They also appear in models adressing some of the recent $b$-flavours anomalies. The presented search used events with at least two jets and two charged same-flavour leptons. The expected amount of background from Standard Model processes has been estimated using Monte Carlo simulations and methods based on already recorded data. It has been compared to recorded data and the results were interpreted using the profile likelihood method. No significant excess above the SM background expectation is observed in any channel and exclusion limits have been evaluated. Within the minimal Buchmüller-Rückl-Wyler model and assuming a branching ratio for the decay into a charged lepton and a quark of 50%, leptoquarks with masses up to $1.20\ \text{TeV}$ are excluded with a confidence level of 95%. The liquid argon purity monitoring system consists of 30 monitors installed in the ATLAS detector, the software chain outside of the detector and several hardware components in between. In the most monitors Am and Bi ionization chambers are available. In order to fasten the visualization of the behaviour of the spectra from these ionization chambers the \glqq expert panel\grqq\ was designed from scratch and implemented to be ready to use instantly and directly on the production machine. Following by some indications seen on these spectra, the investigation on the high voltage mapping was performed with unexpected results which could be explained by the mismatching on the mapping. In order to better understand the behaviour seen on the spectra the so-called noise spectra were recorded under reduced detector conditions and their properties were studied. In the course of the ATLAS detector upgrade to be ready for the future Runs of LHC every subsystem including the LAr purity monitoring system has to be prepared for this. The first thoughts about the possible upgrade steps are summarized in this thesis.