A gateway to new physics: direct measurement of the top Yukawa coupling to the Higgs boson

The top quark has the largest Yukawa coupling in the Standard Model, is the main contribution to the Higgs mass $m_H$ corrections and defines the evolution of the Higgs effective potential with the energy, together with $m_H$. It can be directly measured through the associated production process $pp \to t\bar{t}H$. Two searches for this process are described in this thesis, using data collected with the ATLAS detector at the Large Hadron Collider. The fully hadronic analysis is performed with data corresponding to an integrated luminosity of $20.3\,\text{fb}^{-1}$ at a centre-of-mass energy of $\sqrt{s}=8\,\text{TeV}$ and uses a boosted decision tree algorithm to discriminate between signal and background: the dominant multijet background is estimated using a data-driven method. An upper limit of $6.4$ ($5.4$) times the Standard Model cross section is observed (expected) at 95% confidence level and a best-fit value of $1.6 \pm 2.6$ for the signal strength $\mu_{t\bar{t}H}=\sigma_{t\bar{t}H}^\text{obs}/\sigma_{t\bar{t}H}^\text{SM}$ is measured. The multilepton analysis uses data collected at $\sqrt{s}=13\,\text{TeV}$, corresponding to an integrated luminosity of $36.5\,\text{fb}^{-1}$. Events with exactly three leptons are selected and a boosted decision tree is also exploited. The major sources of background are estimated using a simultaneous fit technique, which determines their normalisations in three control regions. An upper limit of 2.3 (1.7) times the Standard Model cross section is observed (expected) at 95% confidence level and a best-fit value of $0.7 ^{+0.9} _{-0.7}$ for $\mu_{t\bar{t}H}$ is measured.