Measurement and phenomenology of azimuthal correlations in Z+jet events in CMS at the center of mass energy of 13 TeV

The production of Z+jets events at high p${}_T$ in proton-proton collisions serves as a significant test for Quantum Chromodynamics (QCD). This thesis explores the azimuthal correlation of Z+jets production from both a phenomenological and an experimental perspective. We compare theoretical predictions of azimuthal correlation in Z+jets production with dijet production in the same kinematic range. It is observed that the azimuthal correlation Δ𝜙 for Z+jets productionappears steeper than in dijet production when p${}_T^{max}$ > 200 GeV. However, their behaviorsconverge when p${}_T^{max}$ > 1000 GeV. The reason is that when p${}_T$ is around 200 GeV, the initial state of dijet process is dominated by the gg and gq channels, while the Z+jet process's initial state is dominated by the gq channel. Given that gluons radiate more than quarks, a larger decorrelation effect is observed in dijet production compared to Z+jet production. As p${}_T^{max}$ exceeds 1000 GeV, the initial states' compositions in both productions become similar, leading to similar azimuthal correlation distributions. Furthermore, we measure Δ𝜙 between the Z boson and the leading jet in Z+jets events in the CMS experiment. The data used are obtained at a centre-of-mass energy of 13 TeV in 2016, corresponding to an integrated luminosity of 36.3 𝑓b${}^{-1}$. The experimental results align with theoretical predictions when accounting for uncertainties.