Exploring Extended Higgs Sector through Search for Heavy Neutral Higgs Bosons Decaying to Tau-Lepton Pairs with the ATLAS Detector

This thesis presents two main contributions to the ATLAS experiment at the Large Hadron Collider. The first part focuses on the Tile Calorimeter Phase-II upgrade, where a monitoring framework was developed and validated for the new FELIX-based readout architecture. The framework performs event-by-event comparisons between the legacy and upgraded readout systems, ensuring data integrity during the detector transition. Validation using test-beam data demonstrated excellent agreement in reconstructed amplitudes and pulse shapes, confirming the readiness of the upgraded readout for future ATLAS operation. The second part presents a search for heavy neutral Higgs bosons decaying into pairs of tau leptons in the fully hadronic final state using the complete Run 2 dataset and the currently available Run 3 data. The analysis employs improved object reconstruction, including RNN-based tau identification and the GN2 b-tagging algorithm, together with dedicated data-driven methods to estimate the dominant fake backgrounds. Expected exclusion limits are derived using a profile likelihood fit and interpreted within the MSSM framework. The results extend previous ATLAS searches, while studies indicate that future developments, such as Graph Neural Network-based tau identification and the full Run 3 dataset, will further improve the sensitivity to heavy Higgs bosons.

Santiago Grisolia grant fro predoctoral