This dissertation reports two analyses for Standard Model processes using proton-proton collisions at a centre-of-mass energy of √s = 13 TeV and proton-lead collisions at a centre- of-mass energy of √sN N = 8.16 TeV, recorded by the ATLAS detector at the Large Hadron Collider at CERN between the years 2015-2017 (pp) and 2016 (p-Pb). The first analysis describes the search for t ̄tH production in multilepton final states using proton-proton collisions at √s = 13 TeV with an integrated luminosity of 80 fb−1. Six final states, categorised by charged-lepton number and flavor, and 25 event categories are defined to simultaneously detect the t ̄tH signal and limit significant backgrounds. The t ̄tW background normalisation is not constrained in the statistical analysis, resulting in a higher than expected normalisation. An excess of events consistent with t ̄tH production is observed, corresponding to 1.8 standard deviations, compared to an expected 3.1 standard deviations. Assuming Standard Model branching fractions, the best-fit t ̄tH production cross section is σt ̄tH = 294+182 −162 fb, consistent with Standard Model predictions. The second reports the observation of top-quark pair production in proton-lead collisions in the ATLAS experiment at the Large Hadron Collider. The measurement is performed using 165 nb−1 of p+Pb data collected at √sN N = 8.16 TeV in 2016. Events are categorised in two analysis channels, consisting of either events with exactly one lepton (electron or muon) and at least four jets, or events with two opposite-charge leptons and at least two jets. In both channels at least one b-tagged jet is also required. Top-quark pair production is observed with a significance over five standard deviations in each channel. The top-quark pair production cross-section is measured to be σt ̄t = 58.1±2.0 (stat.) +4.8 −4.4 (syst.) nb, with a total uncertainty of 9%, which makes this measurement the most precise t ̄t cross-section determination in nuclear collisions to date. The measured cross-section is found to be in good agreement with a previous measurement by the CMS collaboration and with SM predictions. In addition, the nuclear modification factor is measured to be RpA = 1.090 ± 0.039 (stat.) +0.094 −0.087 (syst.).
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This dissertation is written as an annotated collection of selected articles. The dissertation focuses on the modelling of accretion disks in X-ray binaries with a black hole or neutron star. The main objective is to use advanced numerical methods to reveal the fundamental processes that influence the observed spectral and temporal features. Among the significant results of this modelling is the recognition of the puffy accretion disk, a novel type of accretion disk based purely on the results of numerical simulations. Furthermore, the work focuses on modelling the X-ray variability and quasi-periodic oscillations using analytical models of accretion disks, but an advanced description of the oscillations, and a discussion of the implications of this modelling for observational data.
vedouci: G. Torok.
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The work is focused on investigation of properties of the neutron rich light exotic nuclear systems. The studied superheavy hydrogen isotopes 7H and 6H were populated in the direct proton transfer 2H(8He,3He)7H and deuteron transfer reaction 2H(8He,4He)6H respectively. These the most neutron-rich systems with the biggest ratio of mass over charge relate to the modern trends to investigate the most systems as close to the neutron matter as possible. The 7H states have been observed in the missing mass spectrum at the energy of 2.2(5), 5.5(3), 7.5(3), 11 MeV relative to the 3H+4n threshold. The main novelty of the conducted experiment is the measurement of the triton emitted in the studied system decay, which allowed to reconstruct the momentum and angular distribution of the 3H fragment with good accuracy. The analysis of the reconstructed center-of-mass angular distributions and correlations of the 7H decay products allowed to identify the ground and the first excited states at 2.2(5) and 5.5(3)MeV respectively. The 5.5(3) MeV state was interpreted as the 5/2+ member of the 7H excitation doublet, built on the 2+ configuration of valence neutrons. The idea of of extreme peripheral character of the 7H ground state population in this reaction was initiated by the experimental data and then also confirmed by the developed theoretical model based on the plane-wave Born approximation. The measured 6H missing mass spectrum showed a broad bump at ~4–8 MeV above the 3H+3n decay threshold interpreted as a resonant state at 6.8(5) MeV. The obtained missing mass spectrum was practically free of 6H events below 3.5 MeV. The steep rise of the 6H missing mass spectrum at ~3 MeV allowed us to derive the lower limit for the possible resonant-state energy in 6H to be 4.5(3) MeV, which was suggested as the ground state. The obtained results confirm that the decay mechanism of the 7H ground state is the “true” (or simultaneous) 4n emission. The resonance energy profiles and the momentum distributions of fragments of the sequential 6H->5H(g.s.)+n->3H+3n decay were analyzed by the theoretically updated direct four-body-decay and sequential-emission mechanisms. The measured momentum distributions of the 3H fragments in the 6H rest frame indicated very strong “dineutron-type” correlations in the 5H ground state decay.
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The dissertation deals with the analysis of the orbital motion of particles taking place in the close vicinity of neutron stars. It also deals with the properties of external spacetime, comparing different neutron stars models based on equations of state and systematic analysis of the properties of Keplerian and epicyclic frequencies. The Hartle-Thorne metric, whose main parameters are mass, angular momentum and quadrupole moment of a rotating neutron star, is used to describe the external spacetime of rotating neutron stars. The preferred area of interest is the investigation of the conditions determining the presence of the innermost stable circular orbit and examining various models of quasiperiodic oscillations using the available observed data.
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The dissertation is dedicated to the phenomenon of quasiperiodic oscillations (QPOs). Most of the dissertation is based on the fitting of QPO data. A part of the work is dedicated to analysing the timing properties of flux emitted from a fluid torus oscillating in the vicinity of compact objects.
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