Thesis in preparation under the guidance of  Prof. Alexander Milov

Collisions of Relativistic Heavy Ions at the LHC energies is the QCD laboratory, where scientists can improve our understanding of the nature of the strong force interactions. To effectively analyse the data one needs good understanding of what physics processes contribute to the data sample and to relate measured observables to the initial collision geometry, as well as to other geometry-related characteristics generally called the collision centrality. Centrality determination is done with the so-called Minimum Biased event sample, typically including non-diffractive inelastic processes.

The Minimum Biased event sample obtained by the ATLAS experiment in the 2010 data taking is investigated. By using additional information from the ATLAS Zero Degree Calorimeters and by performing the analysis of the rapidity gap intervals in the ATLAS Calorimeter System, I will demonstrate that the fraction of the non diffractive events in the Minimum Biased sample constitutes 98%. This number is consistent with initial estimates. More precise knowledge of this fraction allows improving the centrality determination procedure in ATLAS. The implemented procedure is based on the 2010 data is considered as a part of the Minimum Biased event selection for the data analysis and for the centrality determination in the upcoming LHC Run-2 with lead ions.