Event categorization for narrow-width resonance searches in the H→ZZ→4l channel with the ATLAS detector

Thabede, Mzwandile
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The Higgs boson, which was discovered in 2012 by the ATLAS and CMS collaborations at the LHC, was the first scalar boson (elementary particle of spin zero) to be discovered in nature. This was the last missing piece of the Standard Model and its announcement marked an important milestone in particle physics research. Since then there has been much work done to measure the properties of the Higgs boson as well as to search for new physics beyond the Standard Model. A model was developed in 2015 to understand several features that were initially observed in ATLAS and CMS Run 1 data. This model predicts the existence of a heavy scalar boson with mass estimated around 270 GeV. Heavy scalar bosons considered here are produced mainly via the gluon fusion (ggF) and the vector boson fusion (VBF) production mechanisms. This means the search for these particles can be done in either of these production modes, provided that a method to separate ggF events from VBF events has been clearly defined. It is the scope of this dissertation to categorize signal events in theH→ZZ→4l channel into ggF and VBF categories using a cut-based method and ATLAS Monte Carlo with the total integrated luminosity of 140 fb−1. The method based on the signal selection efficiency and background rejection is used to separate ggF events from VBF events by taking advantage of the distinctive 2-jet signature of events in the VBF category. The VBF and ggF production samples with resonant masses from mH= 200 GeV to mH= 1000 GeV were used to optimize the di-jet invariant mass (mjj) and the pseudo-rapidity difference (∆ηjj) cuts. The new optimal VBF selection cuts, as obtained in chapter five, require events that satisfy mjj≥450 GeV and ∆ηjj≥3.0 in order to be classified into the VBF category. Comparison of this new selection with the currently used VBF selection cuts (mjj≥400 GeV and∆ηjj≥3.3) shows that the current standard selection is still good enough to be used for cut-based event categorization in the H→ZZ→4l channel
A dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Master of Science, 2020