Measurement of the ZH(H $\rightarrow b\bar{b}$) associated production with Z$\rightarrow \ell^{+}\ell^{-}$ in $pp$ collisions at $\sqrt{s}$=13 TeV with the ATLAS Experiment

The signal strength measurement of the $pp\rightarrow VH(H\rightarrow b\bar{b}$) at a center of mass energy of 13 TeV is presented in this thesis. The data have been collected with the ATLAS detector in 2015, 2016 and 2017 data taking corresponding to an integrated luminosity of 79.8 fb$^{-1}$. The analysis has been performed in three different channels distinguished according to the number of charged leptons in the final state coming from the leptonical decay of the associated vector boson. The measured signal strength, with respect to the Standard Model expectation, is $\mu_{VH(H\rightarrow b\bar{b}$)=1.08$^{+0.27} _{-0.31}$ (from the combination of three analysis channels) with an observed significance of 4.9 $\sigma$, with respect to an expected value of 4.3. The iteration of the analysis with the full Run2 statistics (139 fb$^{-1}$) is also treated, with particular attention to the main innovations introduced with respect to the previous analyisis. A description of a phenomenological study of the total proton-proton cross section and $\rho$-parameter (defined as the ratio of the real to imaginary part of the elastic scattering amplitude in the forward direction) is also presented. LUCID-2 is the reference detector for online and offline luminosity measurements for ATLAS. It is described with particular attention to the PMT gain monitoring system where I developed a code for the analysis of the calibration data.