Exploring the diphoton final state at the LHC at 13 TeV: searches for new particles, and the Higgs boson mass measurement with the ATLAS detector.

New high-mass states decaying into two photons are predicted in many extensions of the Standard Model (SM). The diphoton final state provides a clean experimental signature with excellent invariant mass resolution and well-known smooth backgrounds. This document presents a search for new particles with the diphoton final state at the Large Hadron Collider with the ATLAS detector. The $pp$ collision data used were collected during 2015 and 2016 runs with a center-of-mass energy of $\sqrt{s}$=13 TeV. The total corresponding luminosity is 37 fb$^{-1}$. In this thesis, I show my contribution to the search of scalar particle. The studies of signal modeling for different mass and width hypothesis will be described in details. The estimation of selection efficiencies and statistical interpretations of results are performed. The data are consistent with the Standard Model background-only hypothesis. Limits on the production cross section times branching ratio to two photons of such resonances as a function of the resonance mass and width are presented. The Liquid Argon electromagnetic calorimeter plays a crucial role in the diphoton analysis. The excellent energy resolution allows to reconstruct objects with high precision. The contribution to operation of LAr calorimeter and its online software will be discussed. The calibration of the electron and photon energy measurements with the electromagnetic calorimeter is performed. The systematic uncertainties related to energy response are one of the largest contribution limiting the precision measurements of the Standard Model Higgs boson mass. An approach to improve the energy response taking into account the lateral shower shape development is applied in the calibration procedure.