Étude des états finals diphoton dans l’expérience ATLAS au LHC : Mesure de sections efficaces différentielles, découverte d’une nouvelle résonance dans la recherche du boson de Higgs et étude de ses propriétés.

Since 2010, the LHC performances are remarkable, more than 25 fb$^{−1}$ have been delivered and recorded by the ATLAS detector. The ATLAS detector is a multi-purpose experiment, one of its main aim was the discovery of the last Standard Model particle predicted : the Higgs boson. In the search for the Higgs boson in the diphoton channel, the non resonant photon pairs production corresponds to the irreducible background. The differential cross-section measurement of photon pair production provides then a deep understanding of the background in $H \rightarrow \gamma\gamma$ channel and it is also an excellent test for the pertubative QCD predictions. With the full dataset collected in 2011 at $\sqrt{s}$ = 7 TeV, corresponding to 5 fb$^{−1}$ , we performed the differential cross-section measurement of photon pairs production at LHC as a function of four variables, the diphoton mass, transverse momentum, azimuthal separation, and cosine of the polar angle of the leading photon. The background arising from misidentified hadronic jets and isolated electrons, is estimated with data-driven techniques and subtracted from the data samples. The results, dominated by the systematic errors, are then compared to the prediction of parton-shower (PYTHIA, SHERPA) and parton-level NLO (DIPHOX) and NNLO (2$\gamma$NNLO) generators. With 4.9 fb$^{−1}$ of data at $\sqrt{s}$ = 7 TeV and 5.9 fb$^{−1}$ at $\sqrt{s}$ = 8 TeV, a first observation of a new resonance in the diphoton channel has been made by ATLAS. With the full dataset, corresponding to ∼25 fb$^{−1}$ , the discovery in the $H \rightarrow \gamma\gamma$ channel alone is established. In order to assess any tensions with the Standard Model predictions, the error on coupling measurement of this particle to vector bosons must be reduced as much as possible ; this was done with the full statistics by isolating events representative of vector boson fusion (VBF) and associated WH and ZH productions. So far, no significative deviation from the Standard Model predictions has been observed.