Trigger Algorithms and Monte Carlo Event Generation for Dijet Searches in ATLAS and Beyond

Abstract: So far, the Standard Model (SM) has been very successful in describing non-gravitational interactions of fundamental particles. However, many of its theoretical and experimental shortcomings remain unresolved. In collider searches, Beyond the Standard Model (BSM) signatures are expected to manifest themselves as localised excesses of events in the invariant mass lineshape of their decay products. Alternatively, they may become apparent as deviations in the observed shape of angular distributions. In order to discern BSM phenomena, the expected SM background has to predicted with very high accuracy. This is achieved by use of Monte Carlo (MC) techniques. The ATLAS dijet Trigger Level Analysis (TLA) searches for processes indicative of new physics in final states comprising two jets. Due to the large SM multijet background and the limited bandwidth available to inclusive single-jet triggers, the low mass regime of this search signature is not well constrained by previous experiments. To alleviate the bandwidth limitation, a 20 GeV minimum threshold for the jet transverse momentum has been implemented to reduce the average event size, thus allowing to record events at a higher rate and ultimately resulting in an increased search sensitivity. In the second part of this thesis, kinematic and angular distributions of dijet topologies are studied using the MC event generator PYTHIA 8.219, including topologies with an additional associated jet that have not been considered in previous MC studies. Pure dijet topologies are corrected up to Next-to-Leading Order (NLO) using k-factors derived from the Matrix Element (ME) event generator NLOJET++. Topologies with associated jet production are studied using a reweighting procedure to account for the reduced strong interaction scale associated with the emission of soft jets, as well as under variation of the minimum angular separation between two jets.