The Sudbury Neutrino Observatory (SNO), a heavy water Cherenkov experiment, was designed to detect solar Boron-8 neutrinos via their elastic scattering interactions on electrons, or charged current and neutral current (NC) interactions on deuterium. In the third phase of SNO, an array of Helium-3 proportional counters was deployed to detect neutrons produced in NC interactions. A simulation of the current pulses and energy spectra of the main kinds of ionization events inside these Neutral Current Detectors (NCDs) was developed. To achieve this, electron drift times in NCDs were evaluated with a Monte Carlo method, and constrained by using wire alpha activity inside the counters. The pulse calculation algorithm applies to any ionization event, and takes into account processes such as straggling, electron diffusion, and propagation through the NCD hardware. A space charge model was developed to fully explain the energy spectra of neutron and alpha events. Comparisons with data allowed the various classes of alpha backgrounds to be identified, and gave evidence for the spatial non-uniformity of Uranium-238 and Thorium-232 chain nuclei in the counter walls. The simulation was applied to determine the fractional contents of the main types of alpha backgrounds in each NCD string. The number of neutron capture events in the array was extracted via a statistical separation, using Monte Carlo generated alpha background pulse shape parameter distributions and minimal energy information. The inferred total Boron-8 solar neutrino flux is: φ_NC< = 5.74 ± 0.77 (stat) ± 0.39 (sys) x 10⁶ cm^-2s^-1 in agreement with Standard Solar predictions and previous SNO results.

Uncertainties on the parton distribution functions (PDFs), in particular those of the valence quarks, can be constrained at LHC energies using the charge asymmetry in the production of W^&plusmn; bosons. This thesis presents a measurement of the electron channel, lepton charge asymmetry using 497 pb^-1 of data recorded with the ATLAS detector in 2011. The measurement is included in PDF fits using the machinery of HERAPDF and is found to have some constraining power beyond that of existing W charge asymmetry measurements.

This thesis studies the cosmology of ultra-light scalar fields with masses in the range 10⁻³³ eV ? m ? 10⁻¹⁸ eV and their effects on cosmology. The existence of such fields is motivated by the theoretical framework of the "String Axiverse". All types of string theory contain multiple axion fields associated with antisymmetric tensor fields compactified on closed cycles in the compact space. Since the masses of these fields scale exponentially with the volume of the cycle, it is possible for them to be naturally light. We study the effects of these fields as a component of the dark matter and show analytically and numerically that they cause a suppression of structure formation on cosmological scales set by the inverse mass. We show that it will be possible with future galaxy redshift and weak lensing surveys to detect an ultra- light field comprising of order a percent of the total dark matter. If such a field is allowed to couple to the geometry that provided its mass via a phenomenological scalar potential for the axion and modulus, then the expansion of the universe can be altered significantly. In particular, we find that it is possible to have multiple epochs of accelerated expansion over a large region of parameter space, and to have a flat universe with a big crunch in the distant future. Finally, we address the issue of isocurvature perturbations in axion cosmologies, and demonstrate that in the ultra-light case the power spectrum is effected. This may have implications for the conclusions made about fine tuning in the axiverse in relation to a potential detection of tensor modes in the CMB that are different to the case of a standard axion.

This thesis presents the measurement of the inclusive jet cross section using data collected in 2010 by the ATLAS detector, with a particular focus on the reconstruction and calibration techniques used for jets in this measurement and on the estimate of the systematic uncertainty on their energy scale. The inclusive jet cross section measurement is used as input to fits of parton distribution functions. Although the PDF analysis in this thesis is preliminary and its main purpose is to serve as a proof of principle for future studies, improvements in the knowledge of the gluon density are observed thanks to the inclusion of ATLAS data.

Physics measurements at the LHC rely on the imulation of the proton-proton interaction to estimate detector performance and physics ackgrounds. Therefore, a good understanding of ll the processes involved is crucial to the final precision of any measurement. Monte Carlo event generators try to provide a realistic description of the proton-proton collision by combining the theoretical models describing different stages of the interaction process. Within this framework, the baryon number transport and the hadronisation mechanism are currently described by phenomenological models that need input from experimental data. This thesis investigates these subjects in the unique kinematic region covered by the LHCb detector. The production cross section ratios of identified hadrons (protons, kaons and pions) have been measured as a function of pseudorapidity and transverse momentum both at $sqrt{s} = 0.9$ TeV and $sqrt{s} = 7$ TeV collisions using 320 $mub^{-1}$ and 1.8 $nb^{-1}$ of data respectively. The measurements are then compared to the predictions from several tunings of the PYTHIA Monte Carlo generator. In general, current models do not give a satisfactory description of the hadronisation and tend to underestimate the transport of the baryon number into the final state. Critical to this analysis is the particle identification provided by the LHCb Ring Imaging Cherenkov (RICH) system. To ensure high performance, a proper calibration of the RICH photon detectors response is necessary. A system for the calibration of the magnetic distortion induced by the LHCb dipole magnet onto the photon detectors is described. The system currently provides calibration parameters used in the LHCb event reconstruction software and introduces a critical improvement to the overall particle identification performance.

This thesis presents a study of neutral-current π⁰ production from <i>ν_µ</i> interactions. The study uses data taken at the Tokai-to-Kamiokande off-axis experiment's near detector (ND280). Monte Carlo studies were used to develop selection criteria to reconstruct event signatures produced from such interactions. This physics analysis focuses on the tracker region of the ND280 subdetector. Other π⁰ analyses using different detector regions are either underway or completed. The physics analysis, after being developed from Monte Carlo studies, was then run on real data. This analysis goes partway towards measuring the neutral-current π⁰-producing neutrino interaction cross-section, by measuring the rate of such interactions in the T2K experiment's near detector, ND280.

This thesis presents measurements of the cross section for the production of a <i>W</i> boson in association with jets and the ratio of the cross sections for the production of a <i>W</i> boson and a <i>Z</i> boson in association with jets. Both measurements are performed in proton-proton collisions at &radic;s = 7 TeV with the ATLAS experiment at the Large Hadron Collider using a dataset corresponding to an integrated luminosity of 4.6 fb^-1. The dataset allows for the exploration of a large kinematic range, including jet production up to a transverse momentum of 1 TeV and multiplicities up to seven jets. Results are presented as a function of jet transverse momenta and rapidities and as a function of event variables such as the scalar sum of the transverse momenta of the jets. Results as a function of dijet angular variables are also presented. The measurements are compared to several state-of-the-art QCD predictions including next-to-leading-order perturbative calculations, resummation calculations and Monte Carlo generators. Finally, the effect of the results on parton distribution functions is explored.

No description available.

Die Suche nach dem Higgs Boson ist ein zentraler Forschungsschwerpunkt der modernen Teilchenphysik. Zum einen ist das Higgs Boson das einzige bislang unentdeckte Teilchen des Standard Modells. Zum anderen gibt seine mögliche Entdeckung eine bessere Einsicht in die Natur neuer Physik. Mit der Inbetriebnahme des Large Hadron Colliders, LHC, am europäischen Kernforschungszentrum CERN bei Genf konnten Proton-Proton Kollisionen mit bislang unerreichten Schwerpunktsenergien von √s = 7 TeV beziehungsweise √s = 8 TeV erzielt werden. In der vorliegenden Arbeit wird die Messung des Wechselwirkungsquerschnitts pp→Z/γ→τ⁺τ⁻ im gemischten leptonischen Endzustand mit Daten des ATLAS Detektors durchgeführt. Der Zerfall der kohärenten Summe aus Photon, γ, und Z Boson in zwei τ -Leptonen ist ein irreduzibler Untergrund von Higgs Boson Zerfällen in zwei τ -Leptonen. Eine genaue Kenntniss des Massenspektrums sowie dessen Normierung ist deswegen für die Higgs Boson Suche essentiell. Die verwendeten Daten entsprechen einer integrierten Luminosität von L = 35,51 pb⁻¹ bei einer Schwerpunktsenergie von √s = 7 TeV. Die Messung erzielt einen Wechselwirkungsquerschnitt von σ×BR(Z→ττ ) = (1041 ± 143 ± 74 ± 35) pb. Die angegebenen Unsicherheiten entsprechen den statistischen, den systematischen und den Unsicherheiten aus Luminositätsmessungen. Das Ergebnis stimmt mit dem theoretisch bestimmten Wert sowie dem Ergebnis anderer Experimente innerhalb seiner Unsicherheiten überein. Im zweiten Teil der Arbeit werden verschiedene Massenrekonstruktionsmethoden von Zerfällen in zwei τ-Leptonen untersucht. Die Analyse konzentriert sich dabei auf Higgs Bosonen, der minimalen supersymmetrischen Erweiterung des Standard Modells; insbesondere auf deren Zerfall in zwei τ-Leptonen und weiter in den gemischten leptonischen Endzustand, Φ→τ⁺τ⁻→eμ+4υ. Die untersuchten Massenrekonstruktionsmethoden sind die sichtbare Masse, die effektive Masse, sowie die früh- und spät-projizierte transversale Masse. Weiterhin werden Abhängigkeiten der Massenverteilungen vom transversalen Impuls der Leptonen des Endzustandes, sowie der fehlenden transversalen Energie festgestellt. Eine Abhängigkeit vom transversalen Impuls des energiereichsten Jets hingegen konnte nicht nachgewiesen werden. Eine mögliche Eliminierung der Variablenabhängigkeiten wird untersucht, zeigt jedoch dass sie zu einer schlechteren Trennung zwischen Signal und Untergrundprozessen sowie einer schlechteren Auflösung unterschiedlicher Higgs Boson Massen führt. Mit der Kalibrierung der berechneten Massen der verschiedenen Rekonstruktionstechniken wird die Arbeit abgeschlossen.