Study of the irreducible background for the $X \rightarrow WZ \rightarrow$ $l$ $v$ $ll$ resonance search performed in ATLAS at 13 TeV

There is a need to answer the questions that the Standard Model theory SM fails to explain. Various strategies are used to look for new physics beyond the SM and obtain the missing answers. The resonance search is a direct investigation of unexpected bumps over an other- wise well known background in the invariant mass spectra, those bumps might represent new particles. This document focuses on the study of the irreducible background for the X→ WZ →ℓ 𝜈 ℓℓ resonance search performed in ATLAS at 13 TeV. The data was collected by ATLAS during the full Run 2 (2015-2018). In this thesis the modelling of the main background for the WZ resonance search is considered, in particular in a topology called the Vector Boson Fusion (VBF). The main contribution to the WZ production is the WZ QCD. This creates an opportunity to perform a deep study and optimization of WZ QCD production in the VBF control region. A part of this thesis also focuses on a comparison between two different Monte Carlo generators which can carry out the WZ production.