Investigating Tau Lepton Pair Production with the ATLAS Experiment

This book presents a search for leptoquark particles in final states with two tau leptons and a cross-
section measurement of the tau lepton pair production, performed with the ATLAS experiment at 
the Large Hadron Collider. Measurements of B-hadron decays to D(*)τν have shown increased rates 
relative to the corresponding decays to lighter leptons, suggesting the existence of leptoquark 
particles more strongly coupled to third generation fermions. This makes the production of tau 
lepton pairs a privileged final state for their detection, as well as a relevant process to probe 
electroweak properties. The results presented exploit the Run 2 dataset (2015-2018) of pp collisions 
recorded by the ATLAS experiment at a centre of mass energy of 13 TeV. The search for leptoquark 
particles sets the most stringent constraints on such models in the non-resonant production mode, 
while the cross-section measurement of tau lepton pair production is the first one performed in high 
energy proton-proton collisions. One of the most compelling aspects of both analyses is the estimate 
of a sizable background of events with hadronic jets mis-reconstructed as fake tau leptons. This is 
addressed through a novel technique — the Universal Fake Factor method — extensively detailed in 
the text. This book also describes the upgrade of the Muon Trigger system of the ATLAS experiment 
in preparation for the High-Luminosity LHC program, with a focus on the author’s involvement in the 
design of the Sector Logic sub-system. This book is ideal for Ph. D. students and scientists with a 
background in high energy physics and provides introductory chapters which make it accessible also 
to newcomers in the field.