Recherche et découverte du boson de Higgs avec son mode de désintégration en paire de photons avec l'expérience ATLAS au LHC : une introduction

In the context of particle physics, within the ATLAS experiment at LHC, this document presents the discovery of a particle compatible with the Higgs boson by the combination of several decay channels, with a mass of the order of $126\ GeV$. With an increased statistics, that is the full set of data collected by the ATLAS experiment at LHC during the years 2011 ($L=4.5\ fb^{-1}$ at $\sqrt{s}=~\hspace{-0.15cm}7\ TeV$) and 2012 ($L=20.3\ fb^{-1}$ at $\sqrt{s}=8\ TeV$), the particle is also discovered individually in the channel $H\rightarrow\gamma\gamma$ with an observed significance (respectively expected) of $5.2\ \sigma$ (respectively $4.7\ \sigma$). The analysis dedicated to the measurement of the mass ($m_H$) and signal strength ($\mu$) in the channel $H\rightarrow\gamma\gamma$ gives the measurement : \begin{eqnarray} m_H &= &125.98\pm 0.42\ (stat) \pm 0.28 (syst)\ GeV \mu &= &1.29\pm 0.30 \end{eqnarray} and the constraint on the width $\Gamma_H$ of the Higgs decay of $5.0\ GeV$ at $95\ \%\ CL$.\\ The mass combination of 2011 and 2012 campaigns for the $H\rightarrow\gamma\gamma$ and $H\rightarrow 4l$ channels gives the measurement : \begin{equation} m_H=125.36\pm 0.37\ (stat) \pm 0.18 (syst)\ GeV \end{equation} The analysis dedicated to the measurement of the production modes in the channel $H\rightarrow\gamma\gamma$ gives the measurements : \begin{eqnarray} \mu_{ggF}= &1.32\pm 0.32\ (stat)\ ^{+0.13}_{-0.09}\ (syst)\ ^{+0.19}_{-0.11}\ (th)\\ &1.32\pm 0.38\\ \mu_{VBF}= &0.8\pm 0.7\ (stat) ^{+0.2}_{-0.1}\ (syst) ^{+0.2}_{-0.3}\ (th)\\ &0.8\pm 0.7\\ \mu_{WH}= &1.0\pm 1.5\ (stat) ^{+0.3}_{-0.2}\ (syst)\ ^{+0.2}_{-0.1}\ (th)\\ &1.0\pm 1.6\\ \mu_{ZH}= &0.1^{+3.6}_{-0.1}\ (stat)\ ^{+0.7}_{-0.0}\ (syst)\ ^{+0.1}_{-0.0}\ (th)\\ &0.1^{+3.7}_{-0.1}\\ \mu_{ttH}= &1.6^{+2.6}_{-1.8}\ (stat)\ ^{+0.6}_{-0.4}\ (syst)\ ^{+0.5}_{-0.2}\ (th)\\ &1.6^{+2.7}_{-1.8} \end{eqnarray} The spin study rejects the hypothesis of spin $2$ at $99\ \%\ CL$. The odd parity (spin-parity~$0^-$ state) is excluded at more than $98\ \%\ CL$. Within the theoretical and experimental uncertainties accessible at the time of the analysis, all results : channels showing the excess with respect to the background only hypothesis, measured mass and signal strength, couplings, quantum numbers ($J^{PC}$), production modes, fiducial and differential cross-sections, are compatible with the Standard Model Higgs boson.