Study of QCD and Diffraction with the ATLAS detector at the LHC

The thesis is devoted to the study of diffractive physics with the ATLAS detector at the LHC. After a short introduction to diffractive physics including soft and hard diffraction, the Jet-Gap-Jet production at the LHC, which is particularly interesting for testing the BFKL theory, is discussed. Using the signal selection cuts and a gap definition based on tracks reconstructed in the ATLAS Inner Detector, a clear signal of Jet-Gap-Jet events in the data was observed. Starting from the half-gap size of 0.8 the data cannot be properly described using only the Non-Diffractive Jet sample. Furthermore, the DPE JGJ production, with both protons tagged in the AFP stations, was demonstrated to provide a significant test of the BFKL theory, once the 300~pb$^{-1}$ of integrated luminosity is collected. In the last part of the thesis, the processes of Central Exclusive Jet and Exclusive $\pi^+\pi^-$ production was discussed. After the postulated data selection, the signal to background ratio was found to be of about 5/9 (1/13) for $<\mu> = 23$ (46). For the collected integrated luminosity of $40(300)$ fb$^{-1}$ (for pile-up of $23(46)$) this measurement should deliver ten times better constraints on the theoretical models than the most recent ones. An additional measurement of exclusive pion production, relying on the use of the ALFA stations, allows to constrain the exclusive models. It was shown that a data sample collected by the ALFA detectors should be sufficient to measure the cross section and to study properties of various distributions, especially the invariant mass of the produced pion system.