Search for the Flavor-Changing Neutral Current, $t$ $\rightarrow$ to $q$$\gamma$, in Top Pair Events Using the ATLAS Detector

This dissertation presents the search for flavor-changing neutral currents in processes with top pairs where one top decays through the flavor-changing neutral current decay mode (to an up-type quark and a photon) and the other decays through the most common Standard Model mode to a b-quark and a W boson. The W boson then decays leptonically, defining the channels (electron+jets and muon+jets) searched for. This search uses the entire Run-2 dataset of √s = 13 TeV data collected using the ATLAS experiment between 2015 and 2018, corresponding to a total integrated luminosity of 139 fb$^{−1}$. Observation of the flavor-changing neutral current decays in top quarks would be an unambiguous sign of physics beyond the Standard Model. A neural network was developed to separate signal and background events using both low-level kinematic variables as well as physics-guided high-level variable combinations as inputs. The candidate signal events contain an isolated very high $_{pT}$ photon, a lepton (either an electron or muon), a b-tagged jet, at least one more light jet, and missing transverse energy. Various data-driven techniques were used to estimate contributions to the background from events with a hard scatter photon or a photon faked by either a jet or an electron. As there is no sensitivity for observation of the Standard Model flavor-changing neutral current process, upper limits on the observed (expected) branching ratio and cross section of these processes are set: BR($t$ → $q$$\gamma$)< 9.6 × 10$^{−5}$(11.0 × 10$^{−5}$) and σ($pp$ → $t$$\bar{t}$ → $bWq$$\gamma$) < 50(60)fb. This dissertation includes previously unpublished co-authored material.