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The ATLAS detector is one of the two largest experiments installed at the Large Hadron Collider at CERN, the European Organization for Nuclear Research. During the first run, the ATLAS detector recorded data at centre of mass energies of 7 TeV and 8 TeV, enabling many precision measurements and new physics searches. One important task in ATLAS is measuring the primary vertex, the interaction point of the hardest proton-proton collision in an event. In this thesis, a study of the primary vertex reconstruction performance in data and simulated events using $t\bar{t}$ and ${Z}$ events is presented. Within the statistics available, the performance in data and simulated events is found to be compatible. Motivated by the limitations of the Standard Model of particle physics, searches for supersymmetric particles are performed with the ATLAS experiment. No signal has been observed so far, and the results are used to set exclusion limits on the masses of the supersymmetric particles. As the exclusion limits are derived from analyses which each target only a single decay mode of a supersymmetric particle, the analyses might have lower sensitivity to more complex decay scenarios. In this thesis the sensitivity of one of the ATLAS searches for direct top squark pair production to models with more complex decay modes is investigated. The study concludes that the sensitivity to models where the top squark can decay via heavier charginos and neutralinos is lower than the sensitivity to models where only decays to the lightest chargino or neutralino are present.

The PANDA experiment is one of the main pillars of the Facility for Antiproton and Ion Research (FAIR), currently under construction in Darmstadt, Germany. PANDA will be a fixed target experiment designed for the study of non-perturbative phenomena of the strong interaction. Strange hyperon production is governed by m(s)  ~ 100 MeV, which corresponds to the confinement domain. Thus, hyperons are suitable probes in this energy region. This work is a simulation study focused on the feasibility of studying the production of Σ0-bar and Λ hyperons in the pbar p -> Σ0-bar Λ reaction with the PANDA detector. A 10^4 events sample simulated at p(beam) = 1.771 GeV/c is used to perform a single-tag (inclusive) and a double-tag (exclusive) event selection. From the former, it is concluded that the single-tag method does not provide with the clean signal required for spin observables extraction. In contrast, exclusive event selection provides with a signal reasonably clean from combinatorial background and completely clean from generic hadronic background events. A signal (Σ0-bar Λ) reconstruction efficiency of ε = 5.3 ± 0.2 % is obtained for exclusive event selection. The corresponding signal to background ratio is S/B(Total) ~ 6 and the significance value is ~ 21. In addition, an exclusive event selection is performed on a 10^4 events sample simulated at p_(beam) = 6 GeV/c. Almost all the generic hadronic background events are removed by the applied selection criteria. At this beam momentum, the obtained signal efficiency is ε = 6.1 ± 0.3%, the signal to total background ratio is S/B(Total) ~ 4 and the significance is ~22. Both efficiencies are smaller compared to a previous simulation study on this channel, but are large enough to enable a study of the exclusive production of the pbar p -> Σ0-bar Λ reaction at PANDA. The difference between the results of this thesis work and the previous work is attributed to the more realistic implementation of the signal production mechanism, as well as the detector and reconstruction algorithms.

Quasi-one-dimensional Rydberg atoms exposed to alternating positive and negative electric field pulses (kicks) are an example of a chaotic atomic system. Chaotic ionization is predicted in this system via a phase space turnstile mechanism, and we have explored this experimentally. Turnstiles form a general transport mechanism for numerous chaotic systems, and this study is the first to explicitly illuminate their relevance to atomic ionization. Two experiments are presented. In the first we show that the ionization of the electron depends not only on the initial electron energy, but also on the phase space position of the electron with respect to the turnstile--that part of the electron packet inside the turnstile ionizes quickly, after one period of the applied field, while that part outside the turnstile ionizes after multiple kicking periods. In the second experiment we show the signature of the turnstile manifests itself in the step-function-like behavior of the ionization fraction as a function of the kick strength. This behavior persists for different values of kicking periods and starting electron energies.

This thesis presents calculations of the magnetic helicity injection due to rotating sunspots and a determination of the characteristics of the rotating sunspots in the active regions with simple magnetic configurations. Four active regions are investigated to study the relationship between rotating sunspots and magnetic helicity. The observations indicate that significantly more helicity is injected during the period of rotation in polarities with strong magnetic field. This may be a result of the emergence of a magnetic flux rope from below the solar surface. Moreover, our preliminary study on a large sample of 90 active regions shows that the level of flaring activity increases with the rate of helicity injection. Finally, a statistical study is carried out to determine the relation between rotating sunspots and the emergence of magnetic flux tubes. Among 82 active regions which exhibit flux emergence, 93% are associated with rotating sunspots. Among 50 active regions without well-defined flux emergence, 60% of sunspots are observed to be rotating, though relatively slowly. In addition, we find that sense of the rotation (i.e., clockwise or counter-clockwise) of the sunspots shows a weak hemispherical tendency.

We have investigated $\sp3$He spin diffusion in two dilute solutions of $\sp3$He in $\sp4$He, with atomic fraction $x\sp3=1.82\times 10\sp{-3}$ and 6.26 $\times$ 10$\sp{-4}$, spin-polarized by an 8 T field. We do not find evidence that the diffusion coefficient for spins transverse to the average magnetization ($D\sb\perp$) declines or saturates at temperatures down to 0.20$T\sb{\rm F}$, contrary to previous experiment (Gully and Mullin 1984) but in accord with current theory. We have compared our measurements with the latest theory of Jeon and Mullin (1991); our data is mostly in good agreement with their theory if a slightly modified version of a $\sp3$He-$\sp3$He interaction due to Ebner (1967) is used. The congruence between data and theory supports the conclusion that the s-wave approximation to the interaction is not useful for transport calculations even for these rather dilute solutions. There may be an one unresolved discrepancy: our diffusion constant for the lowest concentration at the lowest temperatures is 25% higher than theory predicts. This could be due to a polarization dependance for $D\sb\perp$ or to a modification of the boundary condition by a bound $\sp3$He state, or possibly due to errors in the theoretical calculation.

Collisional excitation and ionization cross sections suitable to highly excited states of the target atom at low and intermediate kinetic energy of the scattering particle have been derived following the semiclassical Impact Parameter approximation. An analytical expression for cross section involving transitions with $\Delta l > 1$ has been obtained when the semiclassical requirement and the Ehrhardt conditions are valid. The analytical cross sections are explicit functions of the initial quantum number of the target atom, the incident velocity of the colliding particle and $\Delta n$ (for bound-bound transitions), allowing a generalization to any transition needed in a spectral modeling code. The cross sections obtained with the Impact Parameter approximation present a close agreement with previous theoretical results for dipole and quadrupole ($\Delta l$ = 2) cross sections of bound-bound transitions. Results of octupolar ($\Delta l$ = 3) cross sections are also given for $\Delta n$ = 1 and $\Delta n$ = 2 and a wide range of incident energies. The results indicate that the cross sections corresponding to transitions with $\Delta l > 1$ are significant up to incident energies about 100 times the transition energy. The Impact Parameter cross sections for bound-free transitions present a very good agreement with experimental values, in particular at low incident energies.

NNBAR is a proposed experiment for the European Spallation Source. Thegoal of the experiment is to observe the transformation n −  ̄n. Currently a cutbased analysis is used to select signal events and discriminate against cosmic raybackground. To further increase the signal efficiency machine learning was used.Most machine learning algorithms resulted in a higher signal efficiency at the costof lowering the background rejection. However using the Linear DiscriminantAnalysis resulted in a new signal efficiency of 94% whilst having a predictedbackground rejection of roughly 100%. These results show that machine learningis a promising tool for increasing the signal efficiency at NNBAR.

Accelerator-driven systems have been proposed for incineration of transuranic elements from spent nuclear fuel. For safe operation of such facilities, a robust method for reactivity monitoring is required. In this thesis, the most important existing reactivity determination methods have been evaluated experimentally in the subcritical YALINA-experiments in Belarus. It is concluded that the existing methods are sufficient for calibration purposes, but not for reactivity monitoring during regular operation of an accelerator-driven system. Conditions for successful utilization of the various methods are presented, based on the experimental experience. / QC 20101115

The Standard Model has been greatly successful in predicting the laws that govern our universe. Yet there are still seemingly missing pieces to the model. Detector development plays a crucial role in advancing our understanding of particle physics and helps answer some of the most pressing questions in the field, such as the nature of dark matter and why a matter-antimatter asymmetry is observed. This thesis has covered the work of developing detectors for two different experiments. The HIBEAM/NNBAR experimental program will be a search after neutron-sterile neutron and neutron-antineutron oscillations housed in the the European Spallation Source (ESS) in Lund, Sweden. The experiment will reach unprecedented sensitivity for free-neutron searches, surpassing the last the oscillation time limit by 3 orders of magnitude. This thesis presents an overview of the experimental goals and the opportunities afforded by the ESS infrastructure. The primary work for this thesis has been the design, simulation and construction of a prototype calorimeter for NNBAR stage of the experiment, which is presented here. The ATLAS experiment is currently undergoing upgrades to meet the requirements of the high-luminosity, high-radiation environment at the HL-LHC. This thesis provides an overview of the LHC and the ATLAS experiment, with special focus on the hadronic Tile calorimeter. TileCal will be upgraded to provide full granularity data at the lowest trigger level of the upgraded ATLAS trigger and data acquisition system. The work presented here focuses on TileCal upgrades to Daughterboard (DB), the interlink board responsible for the management of on- and off-detector data transmission. This work has been primarily focused on the design and fabrication of two printed circuit boards to test the implementation of a commercial, radiation-hardened FPGA in the upgraded DB design. The FPGA will control access to remote control JTAG in the DB Xilinx FPGA interface.