Estimating the Sensitivity to New Resonances Decaying to Boosted Quark Pairs and Produced in Association with a Photon in the ATLAS Experiment

Particle physics' most solidified theory is the Standard Model, which includes three of the four known interactions in the Universe, namely the electromagnetic, weak and strong. Although the Standard Model's predictions are repeatedly tested to remarkable accuracy, there are eminent reasons to believe it is not a complete theory of Nature. These consist of the model's inability to incorporate gravity in its description of the fundamental forces, predict non-zero neutrino masses and account for the existence of Dark Matter. Numerous extensions known as beyond the Standard Model physics are proposed and explored by Large Hadron Collider experiments where bounds are set upon them. The study described in this thesis relates to mediators between Standard Model and Dark Matter particles and appraises novel techniques by which such resonances may be investigated with the ATLAS detector. The benchmark model proposes a leptophobic vector boson, $Z'$, charged under a new $U(1)'$ gauge symmetry which couples to Standard Model quarks and to Dark Matter Dirac fermions. Its interactions with Dark Matter are infinitesimally rare and thus searches involving such mediators primarily focus on their decays to Standard Model particles, identifiable by Large Hadron Collider detectors. This new type of hardonically-decaying particles remains illusive to current searches due to an extensive multijet background, saturating the trigger scheme. An alternative method is adopted in this thesis. The method uses events wherein an initial-state radiation photon is the trigger object whilst the recoiling resonance is reconstructed from the hadronic final state as a single large-radius jet. These jets are reconstructed with the Hadronic Calorimeter and their energy measurement is of an approximately 3% accuracy. Due to the recoil of the resonance against the photon, the resonance is highly Lorentz-boosted. Hadronically-decaying bosons with high Lorentz boosts can be tagged using substructure observables, targeting their two-pronged characteristic. Using Monte Carlo simulations, selection criteria are optimised. The usefulness of a machine learning tool based on jet substructure observables for tagging two-pronged resonances is tested. Following the optimised selection, Monte Carlo simulations and a control study are used to assess the sensitivity of the ATLAS experiment to the $Z'$ in this channel. Within the control study, $36.0 \, \textrm{fb}^{-1}$ data are compared with jet$+\gamma$ and $q\bar{q}+\gamma \rightarrow W/Z\rightarrow q\bar{q}$ simulations. Thereby, it is estimated that this scheme bears the potential to discover processes similar to a Standard Model vector boson production in association with a photon and a diquark final state at cross sections as small as 16% and 46% of the $W$ and $Z$ production cross sections, respectively. Finally, using Monte Carlo simulations of processes where $q\bar{q}+\gamma \rightarrow Z' \rightarrow q\bar{q}$ for a mass range $m_{Z'} \in [100, 220] \, \textrm{GeV}$, the sensitivity to the new resonance is estimated and provided in terms of cross section and coupling confidence level limits.