Thesis:

Performance of boosted identification algorithms for H → τ+τ− and HH → b¯bτ+τ− and new ITkStrip bytestream converter developments in ATLAS

The discovery of the Higgs boson by the ATLAS and CMS collaborations at the Large Hadron Collider (LHC) completed the particle content of the Standard Model, while opening new directions for precision measurements of Higgs boson properties. In particular, the study of Higgs boson production in the high transverse momentum (pT) regime, including Higgs boson pair production, poses significant challenges for object identification and triggering due to the highly collimated nature of the decay products. In parallel, the implementation of the High-Luminosity LHC requires major upgrades to the ATLAS detector, including the installation of a new Internal Tracker (ITk), as the current one is not designed to withstand HL-LHC conditions. This thesis investigates the performance of new boosted di-τ identification algorithms in boosted H → τ+τ− and HH → b¯bτ+τ− final states, and presents the development of a new trigger monitoring algorithm based on the new boosted di-τ trigger. In addition, a new ByteStream converter package for ITk Strip simulation detector is implemented, including encoding and decoding algorithms integrated into the ATLAS software framework.