Studies of new light resonances decaying into two hadronic jets produced in association with a radiated jet

Abstract: This report presents combinatorics studies in the search for a resonance of below 800GeVwhich decay product is a pair of hadronic jets from quarks. The resonance, predicted as a dark matter mediator, has couplings to dark matter particles as well as quarks and gluons. The resonance is created in association with a radiated object that can be either a photon or a jet. In the case of the radiation being a jet, the dijet from the resonance and the radiated jet cannot be distinguished. Choosing the wrong dijet to reconstruct the resonance decreases the sensitivity of the search. The study focuses on studies that help improve the search sensitivity. The signal samples used in this thesis are generated with Monte Carlo simulation and passed through the ATLAS detector simulation. Firstly, in an attempt to identify the jet from radiation, samples where a jet is radiated and a photon is radiated are compared. However, due to the differences in the sample generations, this attempt does not lead to a successful identification of the jet from radiation. Kinematic variable distributions of radiated jet signal samples at various resonance masses (250 - 1050 GeV) are studied in detail, leading to a better understanding of the event topology. As a next step, different variables are used to order the jets and choose two of them for the resonance reconstruction. The performances of these reconstruction methods are studied and presented. The correlations between the different ordering methods are also discussed. The most promising methods are chosen to be taken forward to a study of signal significance including the background. The results in this thesis indicate that the second and third leading jets ordered by pT form the best performing pair for the dijet in the mass range of interest (below 800 GeV) in this search. Jets that are close to the direction where most of the jet pT are aligned can also be acceptable candidates for dijet choices.