Search and discovery with the resonant $\gamma\gamma$ final state at ATLAS

A search for the Standard Model Higgs boson was conducted with the ATLAS Experiment at the LHC using the two-photon decay channel. Datasets with integrated luminosities of 4.8 fb$^{-1}$ and 5.9 fb$^{-1}$ were collected in 2011 and 2012 for $pp$ collisions with center-of-mass energies of $\sqrt{s}=7$ TeV and $8$ TeV, respectively. A statistically significant excess of events above the background-only expectation in the $\gamma\gamma$ decay channel of 4.5 standard deviations was observed. Combined with excesses in the $WW^{*}$ and $ZZ^{*}$ decays, this observation led to the discovery of a Higgs boson with a significance of 5.9 standard deviations at a mass of $126.0 \pm 0.4$ (stat) $\pm 0.4$ (sys) GeV. Searches for diphoton resonances during 2015 and 2016 with LHC data were motivated by extensions to the Standard Model that predict additional scalar particles. Data were analyzed from $\sqrt{s}=13$ TeV $pp$ collisions, corresponding to an integrated luminosity of 15.4 fb$^{-1}$, and no significant deviations from the Standard Model hypothesis were observed. 95\% \textit{CL} limits on the production cross-section times branching ratio to $\gamma\gamma$ were calculated for scalar models with masses between 200 GeV and 2.4 TeV and widths ranging from 4 MeV to $10\%$ of the scalar mass.