Pair production of Higgs bosons in the bbττ decay channel for pp collisions at √s=13 TeV in the ATLAS detector

Rare processes such as HH (Higgs boson pair) production are essential for testing the current understanding of fundamental interactions. The HL-LHC (High Luminosity Large Hadron Collider) will play a crucial role in improving the sensitivity to these and other precision studies, as well as in searches for new physics. The increased collision rate, precision and radiation hardness that this upgrade demands calls for the replacement of many components of the LHC, the ATLAS ITk (Inner Tracker) being among the most important ones. This thesis presents work done using the FE-I4 ITk Demonstrator, from chip scans to study their performance, to DCS (Detector Control System) software development to facilitate and improve operations, with the then planned RD53A ITk Demonstrator in mind. In addition, two tau triggers, based on those used in the Run 2 HH production analysis for the HH→bbττ decay channel in the hadronic decay mode, were chosen for efficiency optimisation, with the aim to implement them in Run 3. Work is also presented regarding the development of tau trigger online monitoring tools for real time performance control. Furthermore, a full Run 2 HH→bbττ search for pp collisions at √s=13 TeV using 139 fb-1 is partially mentioned to introduce the framework needed for a study on trigger recommendations, while an updated search using 140 fb-1 is presented alongside the work done to validate the fake background estimation methods used in the hadronic decay mode. Lastly, DNNs (Deep Neural Networks) have been studied and shown to discriminate slightly better signal from background when compared to the methods used in the updated HH search, while the use of dedicated categories is shown to enhance the signal significance.