Study of a highly boosted Higgs boson decaying to b-jets final states with the ATLAS experiment at CERN

The observation of the decay of the Higgs boson to a $b \bar{b}$ pair in 2018 confirmed the Yukawa interaction and provided the first measurement of a Yukawa coupling to down-type quarks. The most sensitive channel in the $H \rightarrow b \bar{b}$ measurement is given by the associated Higgs boson production with a vector boson $V$ decaying leptonically. The main focus is now the measurement of the Higgs boson production cross section as a function of its transverse momentum ($p_T$). This study is motivated by a wide range of Beyond the Standard Model (BSM) theories predicting an enhancement of $VH(H \rightarrow b \bar{b})$ events at high transverse momentum with respect to the SM expectations. At such high energy regimes, the Higgs boson is highly Lorentz-boosted and the $H \rightarrow b\bar{b}$ decay is reconstructed in the detector as a single object with a two-prong substructure. This thesis describes the measurement of $VH(H \rightarrow b \bar{b})$ in the boosted regime performed inclusively, as well as in bins of the vector boson transverse momentum, for $p_T^V \in [250,400[$ GeV and $p_T^V \in [400,\infty[$ GeV. The interpretation of these measurements using the Effective Field Theory (EFT) framework is also discussed. The results are based on the full LHC Run 2 dataset collected by the ATLAS experiment, corresponding to an integrated luminosity of 139 fb$^{-1}$. A very important component of this study is the correct identification of $b$-jets. A new calibration of the $b$-jet tagging algorithm at very high transverse momentum, which reduced the uncertainties for high-$p_T$ $b$-jets by 50\%, is also included.