Higgs mass measurement in the di-photon decay channel with the ATLAS experiment

This thesis work was carried out within the research group of elementary particle physics at the University of Milan, for the ATLAS experiment at CERN in Geneva . This experiment was designed to exploit the full potential of the hadron collider LHC going to explore regions of energy and luminosity never achieved until now. In my thesis I made the mass measurement of the Higgs boson in the di-photon decay channel using data from pp collisions collected in 2012 to center mass energy of $ \text{8TeV} $ corresponding to $20.3\,\text{fb}^{-1}$. This decay is characterized by a small branching ratio ( $ \sim 10^{ -3 } $ ) but has a very good mass resolution (FWHM $ \sim $ 3.6 GeV ) and a simple experimental signature corresponding to two isolated photons that release all their energy in the electromagnetic calorimeter. The signal of the Higgs boson ( cross section $ \simeq 3 \cdot 10^{-2} $ pb ) must be extracted from an irreducible background of QCD $ \gamma \gamma$ ( $30$ pb ) and a reducible background events $ \gamma $-jet ( $ \sim20 \% $ of the total fund ) and jet-jet ($\sim 0.03 \% $), where the jets are reconstructed incorrectly as photons.A first measurement in this decay channel is been carried out and yield public in March 2013 and has resulted: \begin{equation*} m_H=126.7\pm0.7\,\, \text{GeV} \end{equation*} In my thesis I have contributed to the refinement of the measurement and reduction of systematic experimental . The understanding of the operation of the calorimeter has a crucial impact on the determination of the Higgs mass and the relative systematic errors; An important effort was done for the restatement and re-evaluating of the calibration procedure of the calorimeter trying to reduce the systematic uncertainties related .