Discovery of A New Boson in the Search for A Standard Model Higgs Boson in the Diphoton Final State with the ATLAS Detector

The search for a Standard Model (SM) Higgs boson in the diphoton final state is performed with 4.8~fb$^{-1}$ data taken at $\sqrt{s} = $7~TeV in 2011 and 5.9~fb$^{-1}$ data taken at $\sqrt{s} = $8~TeV from March to June in 2012. An excess of diphoton events at around 126.5~GeV, with a statistical significance of 4.5~$\sigma$, is observed. This excess, combined with excesses observed in the search for SM \hzz~and the search for SM \lvlv, leads to an excess at around 126~GeV in the combination of SM Higgs boson searches with a statistical significance of 5.9~$\sigma$ that establishes the discovery of a new boson announced by the ATLAS collaboration in July, 2012. With the full LHC data sample taken in 2011 and 2012, corresponding to 4.8~fb$^{-1}$ data at $\sqrt{s} = $7~TeV and 20.7~fb$^{-1}$ data at $\sqrt{s} = $8~TeV, the statistical significance of the diphoton excess reaches 7.4~$\sigma$. The observation in the diphoton final state by itself can establish the discovery of the new boson. The mass of the new boson measured in the diphoton final state is 126.8 $\pm$ 0.4(stat.) $\pm$ 0.8(syst.)~GeV, and the signal strength of this excess is 1.57 $^{+0.32}_{-0.28}$ times the SM Higgs boson expectation at 126.8~GeV. The mass of the new boson measured from a combination of the \hgg~channel and \hzz~channel is~125.5 $\pm 0.2$ (stat) $^{+0.5}_{-0.6}$ (sys) GeV, and the signal strength of \hgg~measured at this mass is $1.55 ^{+0.33}_{-0.28}$ times the SM Higgs boson expectation.