Study of the ATLAS Hadronic Calorimeter Response and J/ψ and ψ(2S) Mesons Production Cross Section Measurement in pp Collisions at √s = 13 TeV

In this thesis are included the results obtained within ATLAS experiment physics program. The first part is devoted to study of the ATLAS hadronic calorimeter response. The Large Hadron Collider (LHC) Phase–II upgrade aims to increase the accelerator instantaneous lumi- nosity by a factor of 5-10. Due to the expected higher radiation levels, aging of the current electronics and the need to provide the capability of coping with longer latencies of up to 35μs by the trigger system at such high pile-up levels, a new readout system of the ATLAS Tile Calorimeter (TileCal) is needed. A prototype of the new Phase-II upgrade electronics, called ATLAS TileCal Demonstrator module has been tested using the particle beams from the Super Proton Synchrotron (SPS) accelerator at CERN. Data were collected in 2015–2018 and 2021–2022 with beams of muons, electrons and hadrons at various incident energies and impact angles. The muon data allow to study the dependence of the response on the incident point and angle through entire volume of the detector. The electron data are used to determine the linearity of the electromagnetic energy measurement. The hadron data allows to study the en- ergy response and resolution of the detector and also to tune the modelling of the calorimeter response to pions and kaons with the purpose of improving the reconstruction of the energy of jets. Using the test beam data, new electronics noise threshold was evaluated and afterwards layer response uniformity was studied using the muon data. The data recorded by Demonstrator show a layer response uniformity within 1%. An offset of max 1.4% is observed for Data/MC. Energy response uniformity is observed cell by cell within uncertainties. Three spare modules of the ATLAS Tile Calorimeter were exposed to hadron beams. The energy response and resolution of the detector to positive pions and kaons and protons with energy in the range 16 to 30 GeV were measured. The results obtained using experimental and simulated data agree within the uncertainties. In the second part of this thesis is described the study of the heavy quarkonium (J/ψ and ψ(2S)) production, using the ATLAS detector data. Studies involving heavy quarkonia provide a unique insight into the nature of Quantum Chromodynamics (QCD) near the boundary of perturbative and non-perturbative regimes. However, despite the long history, the investi- gation of quarkonium production in hadronic collisions still presents significant challenges to both theory and experiment. In high energy hadronic collisions, charmonium states can be produced either from the short-lived QCD sources (referred to as ‘prompt’ production), or from long-lived sources such as decays of beauty hadrons (referred to as ‘non-prompt’ production). These can be separated experimentally by measuring the distance between the production and decay vertices of the quarkonium state. Effects of feeddown from higher charmonium states contributes to produc- tion of J/ψ mesons, whereas no significant contribution occurs for the ψ(2S) meson. While the theoretical calculations within the framework of perturbative QCD have been reasonably suc- cessful in describing the non-prompt contributions, a satisfactory understanding of the prompt production mechanisms is still to be achieved. It is hence increasingly important to broaden the scope of comparison between theory and experiment by providing a broader variety of experimental information on quarkonium production in a wider kinematic range. Double-differential production cross-section of J/ψ and ψ(2S) charmonium states through their decays to dimuons in pp collisions at √s = 13 TeV was measured, using the data collected by the ATLAS detector at the LHC during Run 2. For each of the two states, the cross-sections are measured separately for prompt and non-prompt production mechanisms. The non-prompt fractions for each state are also measured, as well as the production ratios of ψ(2S) to J/ψ. In case of J/ψ, the results cover the rapidity range |y| < 2 and the trans- verse momentum range between 8 GeV and 360 GeV. In case of ψ(2S) the rapidity range is the same, but the transverse momentum range is between 8 GeV and 140 GeV. In both cases, the transverse momentum range goes well beyond the values reached so far, which may help discriminate various theoretical models. The results show similar pT-dependence for prompt and non-prompt differential cross sections. In low pT region non-prompt fraction shows steep increase and in high pT region the fraction is close to constant for both J/ψ and ψ(2S). The results for non-prompt production are compared with the predictions of the theoretical model (FONLL) with default set of parameters. These predictions are consistent with the present measurement at the low end of the pT range, but exceed the experimental values at large transverse momenta.