Search for flavour-changing neutral currents in processes with a single top quark in association with a photon using a deep neural network at the ATLAS experiment at $\sqrt{s}$ = 13 TeV

In this thesis, a search for flavour-changing neutral currents in processes involving a singly produced top quark and a photon is presented. In this search, proton-proton collision data are used which were collected by the ATLAS experiment at the LHC and correspond to an integrated luminosity of 81 fb$^{-1}$. The top quark is assumed to decay into a $b$ quark and a $W$ boson, which is supposed to decay leptonically. Events are selected whose final state contains exactly one photon, one charged lepton, one $b$-tagged jet and a certain magnitude of missing transverse momentum. The background contributions, that arise from events with either an electron or a jet misidentified as photon, are estimated using data-driven techniques. The signal is modelled in an effective field theory. The signal coupling is assumed to be either left- or right-handed and to involve either an up quark or a charm quark. Assuming only one of these signal couplings being present at a time, a discriminant is constructed using a deep neural network which classifies events into signal and background candidates. A profile likelihood fit is performed to improve the background prediction and to estimate the signal strength. Since no signal contribution is observed in data and the data are found to be consistent with the background expectation, upper exclusion limits are set on the branching ratios $\mathcal{B}(t\rightarrow u\gamma)$ and $\mathcal{B}(t\rightarrow c\gamma)$. The observed limits at a confidence level of 95 $\%$ on the branching ratio are $\mathcal{B}(t\rightarrow u\gamma, \textrm{left-handed}) < 2.8 \times 10^{-5}$, $\mathcal{B}(t\rightarrow u\gamma, \textrm{right-handed}) < 6.1 \times 10^{-5}$, $\mathcal{B}(t\rightarrow c\gamma, \textrm{left-handed}) < 22 \times 10^{-5}$ and $\mathcal{B}(t\rightarrow c\gamma, \textrm{right-handed}) < 18 \times 10^{-5}$. These limits are interpreted as limits on the cross section for the production of a top quark and a photon via a flavour-changing neutral current, and as limits on the absolute value of the sum of the Wilson coefficients $C_\mathrm{uB}^{(ij)*}$ and $C_\mathrm{uW}^{(ij)*}$ for the left-handed and $C_\mathrm{uB}^{(ij)}$ and $C_\mathrm{uW}^{(ij)}$ for the right-handed couplings.