| Sažetak | Studying 93 Vector Boson Scattering is crucial for understanding the electroweak symmetry breaking mechanism and it provides a complementary tool for measuring Higgs boson couplings to vector bosons. In addition, using the effective field theory (EFT) framework, one can probe the Beyond Standard Model physics through modifications of certain quartic gauge couplings. This thesis reports the first evidence, with the CMS detector, of electroweak (EW) production of leptonically decaying Z boson pair accompanied by two hadronic jets with a vector boson scattering topology. The study analyses 137fb1 of proton-proton collisions produced at CERN Large Hadron Collider (LHC) at 13 TeV centre-of-mass energy. Additionally, a prospective study is presented on the longitudinal scattering in the same channel at High-Luminosity (HL) and High-Energy LHC (HE-LHC) conditions, corresponding to 14 and 27 TeV centre-of-mass energy, respectively, with full event kinematics simulated. Although this channel is characterised by a fully reconstructable final state, the small cross section of EW signal compared to the QCD-induced background makes it challenging to measure. Efficient identification of final state leptons is essential since efficiencies on their measurement enter the analysis with a power of four. Measurement of electron selection efficiencies and derivation of scale factors for 2016, 2017 and 2018 data-taking periods was performed. Electron identification is done at CMS using the multivariate approach with a multivariate classifier retrained, for all three periods, using the ExtremeGradient Boost software and with electron isolation included in the training. Uncertainties on both electron selection efficiencies and scale factors were reduced across the pT spectrum with special care towards reducing the uncertainties at low pT .The EW signal was extracted at 13 TeV using the Matrix Element Likelihood Approach (MELA) and the performance was cross-checked with the boosted decision tree (BDT) classifier. The EW production of two jets in association with two Z bosons was measured with an observed (expected) significance of 4.0 (3.5) standard deviations. The cross sections for the EW production were measured in three fiducial volumes and is 0:33(+0:11)(stat)(+0:04) (syst) fb in the most inclusive volume, in agreement with the Standard Model (SM) prediction of 0:275 _ 0:021 fb. Limits on the anomalous quartic gauge couplings were derived in terms of EFT operators T0, T1, T2, T8, and T9. The extraction of the longitudinal component of the Z bosons at the HL- and HE-LHC was performed using two multivariate approaches. A combined-background BDT was trained to separate the ZLZL signal from the mixture of ZLZT , ZTZT and QCD-induced backgrounds. In addition, a more complex approach, referred to as the 2D BDT, was designed to increase signal sensitivity. Two BDTs were trained simultaneously to separate the ZLZL signal from the QCD-induced backgrounds and the ZLZL signal from the mixture of ZLZT and ZTZT backgrounds. The effect on signal significance when increasing electron acceptance from j_j = 3 to j_j = 4 was studied as well. With an increased electron acceptance, the longitudinal component is expected to be measured with a significance of 1.4 standard deviations at 14 TeV and with an integrated luminosity of 3000fb1. A measurement of the longitudinal scattering in the ZZ channel is expected at 27 TeV, corresponding to an integrated luminosity of 15000fb1, with a signal 128 significance of 4.6 standard deviations. With the extended electron acceptance, the first observation is expected with a significance of 5.4 standard deviations. Hence, this study demonstrates a significant benefit of further energy increase at the LHC for understanding the EW sector of the SM. |
