A Dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Master of Science. Johannesburg, 2016. / Plastic scintillators play a key role in reconstructing the energy and tracks of hadronic particles that impinge the Tile Calorimeter of the ATLAS detector as a result of high energy particle collisions generated by the Large Hadron Collider of CERN. In the detector, plastic scintillators are exposed to harsh radiation environments and are therefore susceptible to radiation damage. The radiation damage effects to the optical properties and structural damage were studied for PVT based commercial scintillators EJ200, EJ208, EJ260 and BC408, as well as PS based UPS923A and scintillators manufactured for the Tile Calorimeter. Samples of dimensions 5x5x0.3 mm were subjected to irradiation using 6 MeV protons to doses of approximately 0.8 MGy, 8 MGy, 25 MGy and 25 MGy using the 6 MV tandem accelerator of iThemba LABS. Results show that damage leads to a reduced light output and loss in transmission character. Structural damage to the polymer base and the formation of free radicals occur for doses ≥ 8 MGy leading to reduced scintillation in the base and re-absorption of scintillation light respectively. Scintillators containing a larger Stokes shift, i.e. EJ260 and EJ208 exhibit the most radiation hardness. EJ208 is recommended as a candidate to be considered for the replacement of Gap scintillators in the TileCal for the 2018 upgrade. / LG2017

A dissertation submitted to the Faulty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Master of Science. April 2017. / Plastic scintillators, situated in the Tile Calorimeter (TileCal) of the ATLAS detector at the Large Hadron Collider (LHC), play an integral part in the detection of diffractive, energetic hadronic particles that result from high energy proton-proton collisions. As these particles traversetheplastic, theresultantdecayofthepolymerbaseemitsluminescentlightwhichacts as a signature of this interaction. However, the deleterious radiation environment in which the plastics are situated ultimately degrade the plastic through the formation of free radicals which initiate chemical reactions and alter the structure of the plastic. Radical formation was studied using electron paramagnetic resonance (EPR) spectroscopy in six plastic scintillator types of either polystyrene (PS) or polyvinyl toluene (PVT) base, and computational EPR studies were conducted on two small monomer structures and two large polymer, plastic-like computer models. Damage was simulated in the computational models by removing an increasing number of hydrogen atoms. Plastic samples, of volume 500 ⇥500 ⇥ 250 µm, were to subjected protons accelerated to 6 MeV using the tandem accelerator at iThemba LABS, Gauteng, to increasing target doses of 0.8 MGy, 2.5 MGy, 8.0 MGy, 25 MGy, 50 MGy, and 80 MGy. The experimental EPR data taken after two weeks of the sample exposure to air indicate the presence of peroxy-type radicals that initiate chemical reactions, discolour the plastic, and decrease the efficiency of the plastic. Furthermore, the data suggests that damaged PS and PVT samples are susceptible to different mechanisms of radiation damage. However, results pertaining to the decrease in the g-factor and the increase in normalised EPR intensity suggest that all plastics behave similarly using an EPR analysis as a function of dose. Thus, the EPR analysis could not identify a specific plastic that would perform better than the existing plastics used in the TileCal. The computational chemical potential results indicate that electron transfer between damaged pristine and damaged models is possible. In the two small damaged models, the computational EPR data indicate the presence of a various stable akyl-like radicals depending on the site from which the hydrogen atoms are removed. In the two large damaged models, these results indicate a number of alkyl-, benzyl-, and cyclohexadienyl-like radicals. / LG2017

A study is presented on the production of vector boson (W and Z0) events in association with recoiling hadronic activity in the form of hadronic jets, using proton-proton collisions at the LHC. The dataset was taken by the ATLAS detector during the 2011 data-taking run, at a centre of mass energy p s = 7 TeV, and corresponds to an integrated luminosity of 4.6 fb 1. A measurement of the cross-section ratio of W+jets Z+jets events is de ned, reducing the large systematic uncertainties inherent in the experimental measurement of such events. Inclusive and di erential cross-sections are presented for W+jets Z+jets , as a function of a range of kinematic variables, and are compared to state-of- the-art theoretical predictions.

This thesis presents the measurement of the production cross section of a W boson with a single charm quark in 7 TeV proton-proton collisions using the ATLAS detector at the Large Hadron Collider. The data analysed correspond to an integrated luminosity of 4.6 fb 1 and were collected during 2011. This is the first time that ATLAS data has been used for this particular measurement. This cross section is of particular interest as a probe of the strange quark density of the proton. Typically, the strange quark density is considered to be suppressed relative to that of the other light-quarks in the proton sea. However, some analyses suggest a more symmetric composition of light-quarks in the proton sea. The results of this study aim to improve the precision of the determination of the strange quark density. The analysis uses events where the W boson decays to a muon and a neutrino. In such events, the charm quark is identified by its semileptonic decay to a soft muon within a hadronic jet. The charge correlation between the W boson and the soft muon is exploited to reduce the backgrounds substantially. The analysis results are combined with those obtained using additional decay channels. The measured cross section provides further constraint for the determination of the strange quark density, advancing the knowledge of the fundamental structure of the proton. The results are compared with predictions of next-to-leading-order QCD calculations obtained using various parton distribution function parameterisations. Additionally, the ratio of the strange-to-down sea-quark distributions is determined to be 0.96+0.26 0.30 at Q2 = 1.9 GeV2. This supports the hypothesis of a symmetric composition of light-quarks in the proton sea. The cross section ratio (W+ + c)= (W + c) is also determined and compared with different predictions for the asymmetry of the strange and anti-strange quark distribution functions.

A measurement of the Drell-Yan differential cross section at low invariant mass is presented in the di-muon channel. A 1.64 pb−1 dataset of √s = 7 TeV proton-proton collision data collected by the ATLAS experiment at the LHC is used. The measurement is made in an invariant mass range of 26 < M < 66 GeV where M is the invariant mass of the muon pair. A review of the relevant theoretical physics and the ATLAS detector is made. The analysis is described with particular attention paid to the determination of the isolation efficiency corrections for the Monte Carlo and the estimate of the multijet background. The fiducial differential cross section is calculated with a statistical uncertainty that varies between 0.8% and 1.2%. The systematic uncertainty is seen to vary between 2.4% and 4.1%. A cross section extrapolated to the full phase space is also presented. This is dominated by theoretical uncertainties from the variation of the factorisation and renormalisation scales. The obtained fiducial differential mass cross section is compared to theoretical predictions at NLO and NNLO in perturbative QCD. It is shown that a move beyond NLO is needed to describe the distribution well due to the restrictions of using a fixed order theoretical prediction. A combination with the electron channel measurement is also briefly discussed as well as comparisons to a di-muon measurement in an extended invariant mass range. This allows similar, but stronger conclusions to be drawn. A discussion is made of a PDF fit that uses the measurement presented here. The fit demonstrates the impact of the measurement on the PDFs and further supports the conclusion that a move to NNLO in pQCD is needed to describe the data.

This dissertation documents a study of very rare B-meson decays at the LHCb experiment, using data taken during the first experiment run of the Large Hadron Collider (LHC) and during the second experiment run until September 2016. The LHCb experiment was designed to test the Standard Model of particle physics and to search for New Physics effects that go beyond the scope of the Standard Model through the decay of b hadrons produced in high energy proton-proton collisions at the LHC. The measurements described in this dissertation are made using data samples of proton-proton collisions with integrated luminosities of 1.0, 2.0 and 1.4fb⁻¹, collected at centre-of-mass energies of 7, 8 and 13 TeV, respectively. All results are consistent with the predictions of the Standard Model.

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Tese de mestrado. Engenharia Electrotécnica e de Computadores (Área de especialização de Informática e Automação). Faculdade de Engenharia. Universidade do Porto. 2006