Over the top : from tt¯ measurements to the search for the associated production of the Higgs boson and a top quark pair with the ATLAS detector

The validity of the Standard Model of particle physics received additional confirmation from the recent discovery of the so-called Higgs boson. The observation concluded a 40 year-long search for the `last piece' of the Standard Model description and opened a new direction for precise measurements of the particle’s properties. The production of a Higgs boson in association with a pair of top quarks (ttH) has not yet been observed. The expectation is that this is one of the rarest ways to produce the Higgs particle; a measurement of this process is crucial to determine the strength of the coupling of the Higgs boson to the top quark, the heaviest known elementary particle. This document describes the search for the described process using the 2012 LHC data collected by the ATLAS detector. The analysis is performed in the single lepton plus jets final state, specifically targeting the decay of the Higgs boson into a b-quark pair, by considering events with a large number of b-tagged jets. Compared with previous analyses, a substantially improved sensitivity has been achieved by the use of advanced analysis techniques such as multiple multivariate discriminants and profile likelihood fitting. No significant evidence for the signal has been found and this has led to one of the most stringent upper limit on the strength of this process. Using techniques similar to the ones described in this work, with a high probability, the ttH production mechanism will be observed in the upcoming LHC Run2 phase. The document also contains additional analyses performed during the early years of the LHC which paved the way for the searc