The light at the end of the tunnel gets weaker: Observation and measurement of photon-induced $W^{+}W^{-}$ production at the ATLAS Experiment

This thesis presents the observation of photon-induced $W^{+}W^{-}$ production using proton-proton collision data at a centre-of-mass energy of 13 TeV and with an integrated luminosity of $\mbox{139 fb$^{-1}$}$, recorded by the ATLAS detector. Particular emphasis is given on data-driven corrections to pileup distributions in simulation and calculation of the signal efficiency to pass the exclusivity selection, which requires that no tracks be reconstructed within a $\mbox{$\pm1$ mm}$ region of the signal vertex. Studies into track multiplicity distributions due to multi-parton interactions are also presented, detailing corrections to leading-track transverse momentum. The fiducial $\gamma\gamma\to WW$ cross-section is measured to be \begin{equation*} \sigma^{\text{fid.}}_{\text{meas.}}=3.13^{+0.32}_{-0.31}~\text{(stat.)}^{+0.29}_{-0.26}~\text{(syst.)}~\text{fb}. \end{equation*} when considering the $e^{\pm}\nu_e\mu^{\mp}\nu_{\mu}$ decay channel and the background hypothesis is rejected with a significance of 8.4 standard deviations. The $\gamma\gamma\to WW$ process is then measured differentially in various kinematic distributions and an unfolding procedure is presented to remove detector effects from the data. Finally, prospects for future $\gamma\gamma\to WW$ analyses at the High-Luminosity LHC are presented. The analyses are split into different categories, depending on whether selections are made on forward protons measured with the ATLAS Forward Proton detector. It is concluded that reconstructing tracks below nominal kinematic thresholds is an important area of research and development for such analyses to be fruitful in the future of LHC data-taking. A novel analysis method of matching reconstructed forward protons to vertex timing information is presented and shown to possibly improve signal to background ratios relative to an analysis in which no requirement on forward proton tags is made. Detector performance work on the inter-plane alignment of the ATLAS Forward Proton detector and forward proton reconstruction is also presented.