Search for light CP-odd Higgs bosons with ATLAS and integration of ALTI boards for the ATLAS Liquid Argon calorimeters

Two major topics of modern particle physics are explored in this thesis, namely, physics analysis and the operation and upgrades of particle detectors.
Within the realm of physics analysis, the search for a light CP-odd scalar resonance with a mass of $20\,\mathrm{GeV}$ to $110\,\mathrm{GeV}$ is presented. The analyzed data comprises an integrated luminosity of $140\,\mathrm{fb}^{-1}$. The data were collected with the ATLAS detector at the Large Hadron Collider (LHC) proton--proton collisions at $13\,\mathrm{TeV}$ during LHC Run 2 from 2015-2018. The analysis assumes a resonance that is produced via gluon--gluon fusion and decays into a $\tau^{+}\tau^{-}$ pair. Its decay into the fully leptonic final state with exactly one electron, one muon, and the four accompanying neutrinos is analyzed.
No significant excess of events above the background prediction by the Standard Model of particle physics is observed. In a model-independent interpretation, upper limits are set on the production cross-section for a CP-odd Higgs boson that is created via gluon--gluon fusion and decays into a $\tau^+\tau^-$ pair, with the limits ranging from $2.4\,\mathrm{pb}$ to $67\,\mathrm{pb}$ at $95\%$ confidence level.
In addition, the results are also interpreted within the flavor-aligned two-Higgs-doublet model. An upper limit is given within this model on the absolute value of the up-type quark coupling parameter $|\zeta_\mathrm{u}|$ between 0.074 and 0.47 at $95\%$ confidence level.
In the context of detector operations, this thesis presents the commissioning of the ALTI (ATLAS Local Trigger Interface) boards for the Liquid Argon calorimeter of the ATLAS detector. These boards are responsible for the control and distribution of timing and trigger signals between different parts of the ATLAS detector. They have been installed during the Long Shutdown 2 of the LHC in 2021 and are now in operation for the ongoing Run 3 of the LHC.