Search for Standard Model Higgs in Two Photon Final State at ATLAS

The Standard Model of particle physics describes very precisely the nuclear strong and weak forces and the electromagnetic interaction, by the exchange of vector bosons. It also describes all matter as composed of quarks and leptons and predicts their interactions. The Higgs boson is the last missing piece of the Standard Model, yet to be observed. The search for the Higgs particle is one of the most important goals of the ATLAS experiment at the Large Hadron Collider (LHC). The ATLAS electromagnetic (EM) calorimeter is a crucial subdetector system of the ATLAS detector in searching for the Higgs boson, in particular its final states that include high $p_{T}$ photons or electrons. To be able to detect the rare Higgs signals, the EM calorimeter must be not only be able to precisely measure the energy and direction of electrons and photons, but also identify electrons and photons against the overwhelming background from hadronic jets that mimic these particles. The discrimination against these background can be achieved by measuring the detailed shape of the EM showers. The shower shape variables characterizing an EM showers in the calorimeter are correlated. The covariant matrix technique, or H-matrix method, takes advantage of these correlations for electron and photon identification. This thesis presents the electron and photon identification algorithms for ATLAS, based on the covariant matrix technique and their performance f or single, isolated electrons and photons as well as for photons and electrons from several signal physics processes, along with the rejections against highly electromagnetic jets. The process Z → μμγ is studied as a possible signal in the upcoming ATLAS data to calibrate the photon identification tools. This channel appears promising in terms of statistics given the large integrated luminosity expected at the LHC and could provide a pure sample of photon. This thesis presents the feasibility of the calibration of photon identification using this channel. Prior to the LHC collisions, high-energy bremsstrahlung photons produced by cosmic ray muons passing through the ATLAS calorimeter provide valuable data that can used to validate the Monte Carlo simulation modeling of the ATLAS detector. The shower shape variables measured in the calorimeter with the cosmic ray data are compared with the prediction from the Monte Carlo simulation. The Higgs decaying to two photon final state is one of the cleanest discovery channels for the Standard Model Higgs boson in the low mass range 115 < mH < 150 GeV/c2. This thesis also presents the prospects for observing H → γγ and the significance of this signal, when using photon identification algorithm based on covariant matrix.