[en] In this thesis, I will show my own contribution to the search and observation of the scalar boson within the ATLAS detector in the channel when it decays into a pair of photons. A brief review of the history of the spontaneous symmetry breaking mechanism is presented in Chapter 1 as well as the expected production and decay of the standard model scalar boson at the LHC. Chapter 2 presents the statistical methods used at the LHC. A description of the test statistic used for establishing a discovery or setting an exclusion limit is given. Chapter briefly describes the LHC machine and then details the ATLAS detector focusing mainly on the description of the inner detector and the electromagnetic calorimeter. Chapter 4 explains the calibration of the electrons and photons in ATLAS in 3 different steps: the electronic calibration, the Monte-Carlo based calibration and the in-situ calibration. Chapter 5 describes the reconstruction and the identification of the photons. Chapter 6 presents the evolution of the analyses in the H → γγ channel from 2010 to 2012. The systematic uncertainties on the signal yield and on the mass resolution are detailed. Chapter 7 recalls the results for the H → γγ search from 2010 to 2012. An excess over the background is observed in this channel with a local significance of 4.5σ at a mass of 126.5 GeV while the expected significance is about 2.5σ. The maximum observed local significance is 5.9σ for a mass of 126.5 GeV while the expected significance is 4.9σ. Chapter 8 summarizes briefly the H → γγ search within the CMS experiment

[en] The use of tracking information at the trigger level in the LHC Run II period is crucial for the trigger and data acquisition system and will be even more so as contemporary collisions that occur at every bunch crossing will increase in Run III. The Fast TracKer is part of the ATLAS trigger upgrade project; it is a hardware processor that will provide every Level-1 accepted event (100 kHz) and within 100μs, full tracking information for tracks with momentum as low as 1 GeV . Providing fast, extensive access to tracking information, with resolution comparable to the offline reconstruction, FTK will help in precise detection of the primary and secondary vertices to ensure robust selections and improve the trigger performance

[en] Nearly 50 years ago, theoretical physicists proposed that a field permeates the universe and gives energy to the vacuum. This field was required to explain why some, but not all, fundamental particles have mass. Numerous precision measurements during recent decades have provided indirect support for the existence of this field, but one crucial prediction of this theory has remained unconfirmed despite 30 years of experimental searches: the existence of a massive particle, the standard model Higgs boson. The ATLAS experiment at the Large Hadron Collider at CERN has now observed the production of a new particle with a mass of 126 giga-electron volts and decay signatures consistent with those expected for the Higgs particle. This result is strong support for the standard model of particle physics, including the presence of this vacuum field. The existence and properties of the newly discovered particle may also have consequences beyond the standard model itself. (authors)

[en] This Letter presents a search for magnetic monopoles with the ATLAS detector at the CERN Large Hadron Collider using an integrated luminosity of 2.0 fb^-1 of pp collisions recorded at a center-of-mass energy of √s=7 TeV. No event is found in the signal region, leading to an upper limit on the production cross section at 95% confidence level of 1.6 /ε fb for Dirac magnetic monopoles with the minimum unit magnetic charge and with mass between 200 GeV and 1500 GeV, where is the monopole reconstruction efficiency. The efficiency is high and uniform in the fiducial region given by pseudorapidity |η| ≤ 1.37 and transverse kinetic energy 600-700≤E^kin sinθ≤1400 GeV. The minimum value of 700 GeV is for monopoles of mass 200 GeV, whereas the minimum value of 600 GeV is applicable for higher mass monopoles. Therefore, the upper limit on the production cross section at 95% confidence level is 2 fb in this fiducial region. Assuming the kinematic distributions from Drell-Yan pair production of spin-1/2 Dirac magnetic monopoles, the efficiency is in the range 1%-10%, leading to an upper limit on the cross section at 95% confidence level that varies from 145 fb to 16 fb for monopoles with mass between 200 GeV and 1200 GeV. This limit is weaker than the fiducial limit because most of these monopoles lie outside the fiducial region. (authors)