Novel Analysis Techniques and High-Speed Readout to Search for New Physics

The ATLAS experiment at the Large Hadron Collider is one of the most powerful tools for the measurement of the Standard Model properties and the search for new physics beyond it. This thesis describes a search for such a Beyond the Standard Model theory and also the development of the readout system of the future ATLAS Inner Tracker Pixel detector. Supersymmetric expansions of the Standard Model can solve some of the fundamental problems of it, but no sign of Supersymmetry has been found so far. This work presents the search for a Supersymmetry model that does not adhere to the Minimal Flavour Violation paradigm that is usually followed by simplified Supersymmetry searches. This opens the door to a possible mixing between the second and third generation supersymmetric partners of the quarks, the top-squark ($\tilde{t}$) and the charm-squark ($\tilde{c}$). In this Supersymmetry realisation, the decay of pair-produced $\tilde{t}_1$ can lead to final states with a $b$-jet, a $c$-jet and missing transverse momentum coming from the pair of neutralinos ($\tilde{\chi}_1^0$), which are the lightest supersymmetric particles in this model. I performed a search for this $tc+E_\text{T}^{miss}$ final state using simultaneous $b$- and $c$-tagging and also top-tagging techniques. In this context, I developed a Neural Network classifier to target compressed signal scenarios, where the jets have low transverse momenta. Limits on the cross section of generic new physics models and of the $tc+E_\text{T}^{miss}$ simplified model will be derived at the 95\% confidence level, if no significant deviation from the Standard Model will be found once unblinded. Models featuring light neutralinos of $m(\tilde{\chi}_1^0) = 1\ \text{GeV}$ are expected to be excluded up to $m(\tilde{t}_1) \sim 1050\ \text{GeV}$, while signal scenarios with $\Delta m(\tilde{t}_1,\tilde{\chi}_1^0)\sim m_t$ are excluded up to neutralino masses of 425 GeV. To enable more precise tests of the Standard Model and to expand the discovery reach for new physics, the Large Hadron Collider will be upgraded to drastically increase the collision rate in the context of the High Luminosity Large Hadron Collider project. The innermost part of the ATLAS detector will be replaced with the new Inner Tracker to cope with the higher requirements on radiation tolerance and the larger pile-up. The increased trigger rate and larger occupancy and number of readout channels call for a new readout system. This thesis describes the development and tests of the Optosystem, which is responsible for the recovery, aggregation and electrical-optical conversion of all Inner Tracker Pixel data links. The Optosystem successfully passed a preliminary design review and is scheduled to be installed in ATLAS in the next long shutdown during 2026.