The top quark has been extensively studied since the Large Hadron Collider (LHC) began operating in 2010. The excellent performance of both the LHC and the Compact Muon Solenoid (CMS) detector has enabled complex analyses of many properties of the top quark. In this thesis inclusive and differential top pair (tt) production cross sections have been measured. Inclusive tt cross sections of 145.6 ± 8.2 (stat.) +38.1/28.3 (syst.) pb and 237.4 ± 1.9 (stat.) +20.4/-16.9 (syst.) pb were measured at 7 TeV and 8 TeV center-of-mass collision energies using luminosities of 1 fb⁻¹ and 19.7 fb⁻¹, respectively. These measurements were performed in the semi-leptonic channel by means of a maximum likelihood t of the lepton's pseudorapidity. The work in this thesis focuses specifically on the muon-plus-jets channel. The methods used for measuring the inclusive cross sections were built upon to measure differential cross sections with respect to event level observables. These observables include the missing transverse energy (Emiss T ) as well as some other kinematic distributions involving the jets, lepton and Emiss T in the decay. These results are unfolded to remove detector and selection effects and have uncertainties in the range of 3% to 15%. A low uncertainty is achieved by normalising the differential cross section using the total cross section. This leads to cancellations of some uncertainties. The results were compared with different Monte Carlo generators and with different input parameters. No significant deviations from predictions of the Standard Model were observed. This thesis also contains test beam results on CMS ECAL Endcap Lead Tungstate (PbWO4) crystals. These crystals had been damaged using various doses of proton irradiation. The damage for some crystals is expected to be roughly equivalent to 300 fb-1 of integrated luminosity at √s = 14 TeV. The energy resolution for these crystals was seen to reduce by close to a factor of 20.

In the absorption spectra, photoabsorption cross sections and oscillator strengths were measured. Quantum defects were calculated to identify the assignments of absorption features. The measured quantities were also compared with previous results. In fluorescence excitation spectra, ultraviolet and visible emissions were observed, and fluorescence cross sections were measured in the excitation region between 140 and 170 nm. The ultraviolet emission is due to the transition B¡¦2£G¡÷X2£S and the visible emission is associated with B¡¦2£G¡÷B2£S and E2£U+¡÷A2£U+ transitions. In the presence of foreign gas, the collisional quenching effect plays an important role on the fluorescence intensity of C(0) and D(0).We measure the quenching half pressure P1/2 of D(0) respectively. The potential well depth of van der Walls molecule NO-M can be derived from the measured quenching half pressure. In contrast to quenching, the fluorescence intensity of C(1), B(9), B(10) and B(11) are enhanced as the pressure of foreign gas is increased. The enchancement of the fluorescence intensity is attributed to the collisional removal of predissociation.

In the fluorescence excitation spectra, fluorescence cross sections can be measured by the normalization of absorption cross section of A(0) or D(0) states. In the presence of foreign gases, the self-quenching rate constants, the half-quenching pressures and the quenching rate constants of D(0) and D(1) states are measured and the results are compared with the previous works. In the additional of foreign gases, the enhancement of global fluorescence of the strong predissociation states of C(1) and B(9) is due to the collision-induced transition. In the case of N2, the enhancement of fluorescence of £^band is owing to the resonance transfer. Finally, the well depth of the van der Walls molecules which are formed by the excited state of NO with quenching gas can be derived from the measured quenching cross sections.

Le quark top à été découvert en 1995 par les collaboration dzero~et cdf~au TeVatron. De part sa masse très grande de l'ordre de 172.5 GeV, il est difficile à produire et certaines de ces propriétés nécéssitent encore d'être étudiées avec précision. Le Modèle Standard prévoit deux types de production pour le quark top qui sont la production par paires et la production solitaire ("single top"). Si les modes de production par paires sont maintenant bien connus, sa production solitaire l'ai beaucoup moins notamment à cause de faibles taux de production et de forts bruits de fond. Cette thèse est consacrée à la mesure de la section efficace de production d'un quark top en association avec un boson W dans le canal dilepton au près de l'expérience ATLAS placée sur le Large Hadron Collider (LHC) au CERN. Deux analyses sont présentées dans ce document. La première mesure mondiale de ce processus, réalisée à l'aide des premiers 35 pb-1 de données, est tout d'abord exposée. Cette mesure se veut simple et robuste et utilise au maximum les données pour les diverses estimations de bruits de fond venant entacher la mesure. Une seconde analyse utilisant cette fois toutes les données 7 TeV recueillies en 2011(4.7 fb-1) est aussi présentée. Cette dernière mesure, de sensitivité beaucoup plus grande, fait appel à des techniques de discrimination poussées utilisant des Arbres de Decision Boostés ainsi qu'un traitement statistique plus performant que celui utilisé précédemment. La valeur mesurée de la section efficace W+t est ensuite utilisée afin de contraindre les secteurs électrofaibles du quark top via une mesure de l'élement de matrice CKM |V_tb|. / The top quark was discovered in 1995 by the D0 and CDF experiments at the TeVatron. Due to its great mass, close to 172.5 GeV, the top quark is difficult to produce and some of its properties still need to be deeply studied. The Standard Model predicts two production modes which are the top quark pair production and the single top production. The top pair production is now well known unlike the single top production which exhibits lower production rates and higher background contamination. This thesis is devoted to the determination of the cross-section of the associated production of a top quark with a W boson in the dilepton channel using the ATLAS detector at the Large Hadron Collider (LHC). Two analyzes are presented in this document. The first analysis corresponds to the first measurement ever of the W+t process using the 2010 ATLAS dataset (35 pb-1 of data). This measurement is simple and robust and uses as much as possible data driven techniques for the various estimates of background contamination. A second analysis that uses the whole 7 TeV dataset collected during 2011 (4.7 fb-1) is also presented. This last study uses advanced discrimination techniques and refined statistical treatments in order to increase the global sensitivity of the measurement. The measured value of the W+t cross section is then used to constrain the top quark electroweak sector via a measurement of the CKM matrix element | V_tb |.

A turbulence model was developed for computing surface velocity coefficients and discharge under steady, uniform flow conditions for rectangular and compound open-channel cross sections. Reynolds-Average Navier-Stokes (RANS) equations, Reynolds stress equations, and kinetic energy and dissipation equations were applied in the model using the finite-volume method with the SIMPLER algorithm. The models show graphical results of the velocity distributions in the longitudinal bed slope direction, secondary velocities, pressure, turbulence kinetic energy, and kinetic energy dissipation rate across the cross section. Also, the surface velocity coefficients were computed at increments of one-eighth of the base width from the vertical walls to the center of the cross section, and the submergence depth of the floating object from zero to 30 cm, with a 5-cm depth increment. Four different sets of Reynolds stress equations (one set by Boussinesq hypothesis and three sets of algebraic stress model) were used to calculate the results. Only one version of the algebraic stress model was successful in predicting the depression of the maximum streamwise velocity below the water surface. The model was calibrated and verified using laboratory data collected at Utah State University. Calculated discharges from the turbulence model had very good agreement with the laboratory data. The surface velocity coefficients from model results were generally lower than the results from the laboratory data, but higher than the values published by the United States Bureau of Reclamation. Standard cross sections of rectangular and compound cross sections were defined to simulate the model results and model sensitivity to parameter changes. The model results were summarized to show the relationship between surface velocity coefficient and channel characteristics compared with the published values by the USBR. For rectangular cross sections, the coefficients from the model are higher than the published USBR values. But the coefficients from the model and USBR are in very close agreement for the tested compound cross sections. The published coefficients by the USBR are a function of only average water depth. However, the model results show that the coefficients are also related to channel size, surface roughness height, float submergence depth, and lateral location of the float object. These factors should be included in the determination of the surface velocity coefficient to improve the discharge estimations from the application of the float method.

In this work the scattering of an incident plane wave propagating along a plane perpendicular to the xy-plane is studied. The wave is scattered from an infinitely long cylindrical object of arbitrary cross-section. Due to the arbitrary geometry of the obstacle, a finite differences numerical method is employed to approximate the solution of the scattering problems. The wave equation is expressed in terms of generalized curvilinear coordinates. Boundary conforming grids are generated using elliptic grid generators. Then, a explicit marching in time scheme is implemented over these grids. It is found that as time grows the numerical solution converges to a wave with harmonic time dependence. The amplitude of these waves is analyzed and graphed over generalized grids for different geometries. An important physical measure of the energy scattered, the differential scattering cross section, is also obtained. In particular, the method is applied to a circular cylindrical obstacle. For this case, the analytical solution can also be obtained by traditional spectral techniques. The method is validated by comparing this exact solution with the numerical approximation obtained from the application of it.

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Der inelastische Streuquerschnitt von Neutronen mit 7Li weist keine scharfen Resonanzen und eine relativ niedrige Schwelle von 546 keV auf. Bis zur Aufbruchschwelle von 5291 keV findet die inelastische Streuung ausschließlich durch Emission eines 𝛾 -Quants mit 𝐸𝛾 = 477,6 keV statt. Er ist daher als Referenzquerschnitt geeignet. Lithium ist als 3H-Produzent von technischer Bedeutung für die Entwicklung zukünftiger Fusionsreaktoren oder Salzschmelzenreaktoren. Neuere Messungen des inelastischen Streuquerschnitts weichen signifikant von den bisher evaluierten Daten ab. Daher wurden an der Neutronenflugzeitanlage nELBE am HZDR in einem 170 Stunden dauernden Experiment mittels Flugzeit-Methode an einer 4 mm dicken LiF-Scheibe der Streuquerschnitt von 7Li(n,n’𝛾)7Li bestimmt. Die Flugstrecke der Neutronen beträgt 8,3 m. Es wurden 2 Zoll und 3 Zoll-LaBr3-Szintillatoren sowie HPGe- Miniball-Detektoren zum Nachweis der Gammastrahlung in zwei verschiedenen Datenacquisitionen verwendet. Es wird die Abregung von 7Li★ (477,6 keV) gemessen. Das Experiment profitiert von einem hohen Neutronenfluss (ca. 80 n/s/keV @ 1 MeV) sowie einer genauen Spaltkammer der PTB (H19) zur Neutronenflussbestimmung. Die Energie der Neutronen wird mittels Flugzeit-Methode bestimmt. Eine Geant4-Simulation dient der Korrektur der Transmission der Neutronen zum Target, sowie der Mehrfachstreuung und Selbstabsorption. Die bestimmten Wechselwirkungsquerschnitte der LaBr3- und HPGe-Detektoren sind untereinander konsistent, können die Ergebnisse aus [NBI+16] aber nicht bestätigen, die Abweichungen betragen bis zu 20 %. Die Halbwertszeit der gleichzeitig durch Bremsstrahlung in Luft erzeugten Positroniums wurde auf 116(7) ns bestimmt.:1. Einleitung und Motivation 1 2. Theoretische Vorbetrachtungen 3 2.1. Inelastische Neutronenstreuung 3 2.2. Wechselwirkungsquerschnitt 4 2.3. Bisherige Messungen 5 3. Aufbau und Beschreibung des Experimentes 9 3.1. Photoneutronenquelle 10 3.2. Detektoren und Target 11 3.3. Parallelplattenionisationskammer H19 13 3.4. Datenaufnahme 14 3.5. Triggermechanismen 15 4. Auswertung 19 4.1. Kalibrierung 19 4.1.1. Energiekalibrierung 21 4.1.2. Summing-Korrekturen 21 4.1.3. Effizienz-Kalibrierung 22 4.1.4. Effizienzsimulation 27 4.2. Stabilität der Impulshöhen 33 4.3. Totzeitkorrektur 34 4.4. Neutronenflussbestimmung 35 4.4.1. Spaltereignisbestimmung 35 4.4.2. Transmissionskorrektur 38 4.4.3. Mehrfachstreuung 41 4.5. Energieauflösungskorrektur 43 4.6. Bestimmung der Anzahl der 478 keV γ-Quanten 45 4.7. Gammaproduktionswirkungsquerschnitt 7Li (𝑛,𝑛′𝛾) 49 4.8. Anisotropie der Gamma-Emission 53 4.9. Systematische Unsicherheiten 54 5. Ergebnisse und Ausblick 57 A. Anhang 59 A.1. Dopplereffekt 59 A.2. CFD vs. LED Effizienz 63 A.3. Elektronik 64 A.4. Mehrfachstreuung im LiF 66 A.5. Wechselwirkungsquerschnitte für 7Li(n,n’𝛾) 66 A.6. Addendum zur Bestimmung der 478 keV 𝛾-Quanten 68 Literaturverzeichnis 71 / The inelastic neutron cross section of 7Li has no sharp resonances and a fairly low threshold of 546keV.Below the breakup threshold at 5291keV only one 𝛾-ray is emitted at 𝐸𝛾 =477, 6keV. It is therefore suited as a reference cross section. Lithium has technical usage as a 3H-producer in future fusion reactors as well as in molten salt reactors. But there are recent measurements [NBI+16] disagreeing with already evaluated data. To resolve this dissonance, an 170 h Experiment was carried out at the nELBE facility of the HZDR. A 4 mm thick LiF-disk was used as a target, the neutron flux was determined with a 235U parallel plate fission chamber. The flight path for the 7Li(n,n’𝛾)7Li reaction was 8,3 m. As detectors four two-inch LaBr3-detectors as well as three three-inch LaBr3-detectors and two miniball-type HPGe detectors with three 60 % crystals each were used. The measurement of the cross section is solely a measurement of the de-excitation of the first exited 7Li-State at 477,6 keV. The second exited state at 4,63 MeV already decays via particle emission and thus does not contribute any 𝛾-radiation. The experiment benefits from the high neutron flux at nELBE (80 n/s/keV @ 1 MeV) as well as from the precise fission chamber of the PTB (H19) for the neutron flux calibration. A Geant4- Simulation is used to determine correction factors as the transmission from the H19 to the target as well as the multiple scattering correction and the self absorption of the 477,6 keV 𝛾-Rays. The deduced cross section from both detector types are consistent, but they can’t reproduce the data from Nyman et al. PRC93 (2016). The deviations are up to 20 %. The half life of the by means of bremsstrahlung in air produced positroniums in the experiment is 116(7) ns.:1. Einleitung und Motivation 1 2. Theoretische Vorbetrachtungen 3 2.1. Inelastische Neutronenstreuung 3 2.2. Wechselwirkungsquerschnitt 4 2.3. Bisherige Messungen 5 3. Aufbau und Beschreibung des Experimentes 9 3.1. Photoneutronenquelle 10 3.2. Detektoren und Target 11 3.3. Parallelplattenionisationskammer H19 13 3.4. Datenaufnahme 14 3.5. Triggermechanismen 15 4. Auswertung 19 4.1. Kalibrierung 19 4.1.1. Energiekalibrierung 21 4.1.2. Summing-Korrekturen 21 4.1.3. Effizienz-Kalibrierung 22 4.1.4. Effizienzsimulation 27 4.2. Stabilität der Impulshöhen 33 4.3. Totzeitkorrektur 34 4.4. Neutronenflussbestimmung 35 4.4.1. Spaltereignisbestimmung 35 4.4.2. Transmissionskorrektur 38 4.4.3. Mehrfachstreuung 41 4.5. Energieauflösungskorrektur 43 4.6. Bestimmung der Anzahl der 478 keV γ-Quanten 45 4.7. Gammaproduktionswirkungsquerschnitt 7Li (𝑛,𝑛′𝛾) 49 4.8. Anisotropie der Gamma-Emission 53 4.9. Systematische Unsicherheiten 54 5. Ergebnisse und Ausblick 57 A. Anhang 59 A.1. Dopplereffekt 59 A.2. CFD vs. LED Effizienz 63 A.3. Elektronik 64 A.4. Mehrfachstreuung im LiF 66 A.5. Wechselwirkungsquerschnitte für 7Li(n,n’𝛾) 66 A.6. Addendum zur Bestimmung der 478 keV 𝛾-Quanten 68 Literaturverzeichnis 71