(Thesis defence) “Improvement on the texorpdfstring{H→γγ}{Lg} mass measurement by constraining the photon energy scale uncertainty and search for Higgs boson pair production in the texorpdfstring{bb¯γγ}{Lg} final state with the full Run 2 at texorpdfstring{s√=13}{Lg} TeV texorpdfstring{pp}{Lg} collision of LHC and the ATLAS experiment. “