Measurement of Charged-Particle Distributions in Proton–Proton Interactions at $\sqrt{s} $ = 8 TeV with the ATLAS Detector at the LHC

The majority of proton–proton interactions at the LHC can be described as inelastic soft-QCD processes, characterised by a small momentum transfer between their constituents. These interactions cannot be calculated from first principles with perturbation theory, and must hence be described by phenomenological models that are improved by tuning their parameters against experimental data. Measurements of the hadronic final states, presented as distributions of stable primary charged particles that were produced in the hadron collisions, can be used as an input for the tuning. To this end, distributions of reconstructed events and tracks are extracted from the recorded collision data, which must be corrected and unfolded to account for detector effects such as selection and reconstruction inefficiencies, migration effects and background contaminations. Some of these corrections are derived from Monte Carlo simulations of the full detector response to the particles that were generated by the phenomenological models. The dominant source of systematic uncertainty in this measurement is found to be the uncertainty of the material description used in the simulation. This thesis describes the successful efforts to improve the detailed material description of the inner tracker of ATLAS, resulting in a more accurate simulation geometry with significantly better constraints on this dominant source of systematic uncertainty. Building upon this knowledge, the measurement of primary-charged-particle distributions is performed on data recorded in 2012 with the ATLAS detector at $\sqrt{s}$ = 8 TeV centre-of-mass energy, utilising a minimum-bias trigger and a special low-luminosity experimental setup. This study aims to achieve the highest precision of all ATLAS minimum-bias measurements from the Run-1 data-taking period of the LHC between 2009 and 2013. Results are obtained in five different regions of particle kinematics and multiplicity, with transverse-momentum thresholds of $p_T$ > 100 and 500 MeV. Among these, two high-multiplicity phase-space regions are measured by ATLAS for the first time, thus extending the scope of the measurements. Hadronic final-state distributions of primary-charged-particle multiplicity, pseudorapidity and transverse momentum are presented along with carefully evaluated total systematic and statistical uncertainties. The results are compared to predictions made by various phenomenological models and thus provide valuable constraints for their potential further improvement, while also contributing to a better theoretical understanding of the soft-QCD sector of particle physics.