Top-Quark Physics at the LHC

The top quark is the heaviest of all known elementary particles. It was discovered in 1995 by the CDF and D0 experiments at the Tevatron. With the start of the LHC in 2009, an unprecedented wealth of measurements of the top quark's production mechanisms and properties have been performed by the ATLAS and CMS collaborations, most of these resulting in smaller uncertainties than those achieved previously. At the same time, huge progress was made on the theoretical side yielding significantly improved predictions up to next-to-next-to-leading order in perturbative QCD. Due to the vast amount of events containing top quarks, a variety of new measurements became feasible and opened a new window to precisions tests of the Standard Model and to contributions of new physics. In this review, originally written for a recent book on the results of LHC Run 1, top-quark measurements obtained so far from the LHC Run 1 are summarised and put in context with the current understanding of the Standard Model.Comment: 35 pages, 25 figures. To appear in "The Large Hadron Collider -- Harvest of Run 1", Thomas Sch\"orner-Sadenius (ed.), Springer, 2015 (532 pages, 253 figures; ISBN 978-3-319-15000-0; eBook ISBN 978-3-319-15001-7, for more details, see http://www.springer.com/de/book/9783319150000

High resolution x-ray diffraction (HRXRD) analysis of molecular beam epitaxy (MBE) grown cubic and hexagonal GaN films on MgF2 and MgO

The start of the Hubble Space Telescope in 1990 was a milestone for the observation and research of the universe. Since the Hubble Space Telescope will reach its end of a lifetime in the late 2020s, alternatives have to be developed. A good alternative could be a UV multi-channel-plate (MCP) detector with a solar-blind photocathode. Therefore, GaN is a suitable material. In this thesis, GaN films for two different concepts of the UV MCP detector were grown by molecular beam epitaxy and afterwards analyzed mainly by high resolution x-ray diffraction. The first detector concept was the window detector concept. Therefore, GaN films were grown on MgF2 (1 0 0) and (1 1 0) single crystal substrates under different growth conditions. The film composition depended strongly on the substrate orientation. On MgF2 (1 0 0), GaN films consisted of up to seven orientations of cubic (c-GaN) and hexagonal GaN (h-GaN). This formation of several orientations could be avoided using a low-temperature GaN buffer layer between substrate and main film. The buffer layer led to a formation of a nearly complete h-GaN (0 0 0 1) film. On MgF2 (1 1 0), GaN films were mainly c-GaN {1 0 0} with inclusions of h-GaN (0 0 0 1) and c-GaN {1 1 1} using a growth temperature of 500 °C. The ratio between c-GaN {1 0 0} and h-GaN (0 0 0 1) switched at a growth temperature of 700 °C resulting in a mainly h-GaN film, so both c-GaN and h-GaN films could be grown on MgF2 (1 1 0). The second concept was the open detector concept. GaN films were grown on MgO {1 0 0} and {1 1 0} single crystal substrates. The GaN films were mainly or completely cubic under nearly all growth conditions due to the cubic structure of MgO and the suitable lattice mismatch of 7.2 %. These films were all fully-relaxed leading to a formation of dislocations and h-GaN (1 0 -1 1) on MgO {1 0 0}. Inclusions of h-GaN could be suppressed on MgO {1 1 0} using growth temperatures of 500 °C and 600 °C. At 700 °C, inclusions of h-GaN (1 0 -1 3) were found increasing with a decreased Ga/N ratio. The lattice constants and FWHM of c-GaN were determined and were in good agreement with values on other substrate materials. All in all, the crystal structure of GaN was mainly influenced by the substrate and its orientation, while an influence of the growth temperature was also found. The lowest influence had the Ga/N ratio. The results were a good basis for the realization of both detector concepts and showed further the high potential of MgO as a substrate material for c-GaN films.Im Rahmen der Dissertation wurde das Wachstum von Galliumnitrid (GaN) Schichten mittels Molekularstrahlepitaxie (MBE) auf jeweils zwei verschiedenen Orientierungen von MgF2 und MgO Einkristallsubstraten untersucht, um Wachstumsbedingungen für zwei unterschiedliche Detektorkonzepte zu evaluieren. Die Dünnschichten wurden danach hauptsächlich mit verschiedenen Methoden der hochauflösenden Röntgendiffraktion (HRXRD), wie z.B. reziproken Gitterkarten und 20- w-scans, analysiert und durch einige externe Transmissionselektronenmikroskopie Messungen ergänzt und validiert. Die wichtigsten Ergebnisse auf MgF2 wiesen große Unterschiede zwischen den beiden untersuchten Substratorientierungen auf. GaN Schichten auf MgF2 (1 0 0) bestehen aus bis zu sieben verschiedenen Orientierungen von kubischem (c-GaN) und hexagonalem GaN (h-GaN). Dies konnte durch eine dünne GaN Zwischenschicht, die bei einer niedrigen Temperatur zunächst auf das Substrat gewachsen wurde, vermieden werden, so dass die Hauptschicht fast ausschließlich aus h-GaN (0 0 0 1) bestand. Auf MgF2 (1 1 0) wurden bei einer Temperatur von 500° C überwiegend c-GaN (1 0 0) Schichten mit Einschlüssen von h-GaN (0 0 0 1) gewachsen. Eine Erhöhung der Wachstumstemperatur drehte das Verhältnis aus c-GaN und h-GaN um, so dass sich sowohl c-GaN als auch h-GaN Schichten auf MgF2 (1 1 0) wachsen ließen. Auf dem zweiten untersuchten Substratmaterial, MgO {1 0 0} und {1 1 0}, wurde bei nahezu allen untersuchten Wachstumsbedingungen kubische GaN Schichten aufgrund der kubischen Kristallstruktur von MgO und einer ausreichend kleinen Gitterfehlanpassung von 7.2 % gewachsen. Die Schichten waren alle voll relaxiert, wodurch es aufgrund der Gitterfehlanpassung zur Ausbildung von Versetzungen kam. Diese führten zu einer Bildung von h-GaN (1 0 -1 1) auf MgO {1 0 0}. h-GaN Einschlüsse ließen sich auf MgO {1 1 0} für Wachstumstemperaturen von 500 °C und 600 °C komplett vermeiden. Die Gitterkonstanten und Halbwertsbreiten wurden bestimmt und waren im Bereich der GaN Gitterkonstanten auf anderen Substratmaterialien. Insgesamt konnte festgestellt werden, dass das Substrat durch seine Gitterstruktur und Orientierung den größten Einfluss auf die Kristallstruktur von GaN besitzt. Die Wachstumstemperatur hat den nächst höheren Einfluss, wohingegen das Ga/N Verhältnis den niedrigsten Einfluss in dieser Dissertation hatte. Die Ergebnisse stellen eine gute Basis für eine weitere Realisierung der beiden untersuchten Detektorkonzepte dar. Darüber hinaus zeigte sich das große Potenzial von MgO als Substratmaterial für das Wachstum von c-GaN Schichten

Creating an acute energy deficit without stimulating compensatory increases in appetite: is there an optimal exercise protocol?

Recent years have witnessed significant interest from both the scientific community and the media regarding the influence of exercise on subsequent appetite and energy intake responses. This review demonstrates a consensus among the majority of scientific investigations that an acute bout of land-based endurance exercise does not stimulate any compensatory increases in appetite and energy intake on the day of exercise. Alternatively, preliminary evidence suggests that low volume, supramaximal exercise may stimulate an increase in appetite perceptions during the subsequent hours. In accordance with the apparent insensitivity of energy intake to exercise in the short term, the daily energy balance response to exercise appears to be primarily determined by the energy cost of exercise. This finding supports the conclusions of recent training studies that the energy expenditure of exercise is the strongest predictor of fat loss during an exercise programme

Anomalous diffusion and the Moses effect in a model of aging

We decompose the anomalous diffusive behavior found in a model of aging into its fundamental constitutive causes. The model process is a sum of increments that are iterates of a chaotic dynamical system, the Pomeau-Manneville map. The increments can have long-time correlations, fat-tailed distributions and be non-stationary. Each of these properties can cause anomalous diffusion through what is known as the Joseph, Noah and Moses effects, respectively. The model can have either sub- or super-diffusive behavior, which we find is generally due to a combination of the three effects. Scaling exponents quantifying each of the three constitutive effects are calculated using analytic methods and confirmed with numerical simulations. They are then related to the scaling of the distribution of the process through a scaling relation. Finally, the importance of the Moses effect in the anomalous diffusion of experimental systems is discussed.Comment: This is an author-created, un-copyedited version of an article accepted for publication in the New Journal of Physics. IOP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscript or any version derived from it. The Version of Record is available online at https://doi.org/10.1088/1367-2630/aaeea

Development of a plasma source ion implantation facility for the modification of materials' surfaces

Bibliography: p. 197-203.In Plasma Source Ion Implantation high energy [10-50 keV] plasma ions are implanted into materials to modify surface properties, achieving surface hardening, increased wear and corrosion resistance. Plasma Source Ion Implantation is alos used for doping semiconductors and could form an essential step in the manufacture of multilayered wafers. This thesis describes the development and construction of the plasma implantation facility at the Materials Research Group of the Naitonal Accelerator Centre; in particular, the development of the Plasma Assisted Materials Modification Laboratory, the analytical tools available at the Materials Research Group and surrounding universities, basic research into the implantation of steels, the x-rays emitted as a side-effect of plasma source ion implantation and the development of an analytical technique of interest to silicon wafer-cutting technologies

Three essays on environmental and resource economics

Environmental issues in modern day Iowa are a perfect example of the indirect consequences of exploiting natural resources. Over 12,000 years ago, glaciers left the state with rich and fertile soil, perfect for agriculture. Over the course of more than one hundred years, Iowa\u27s landscape has been cleared and drained to gain access to this valuable farmland. While the economic benefits of agriculture are clear, it is important to understand the environmental consequences of this transformation. This dissertation uses economic tools and analysis to investigate three environmental and resource issues related to the complex interplay between Iowa agriculture and the environment. The first chapter examines the relationship between an important adaptive tool, tile drainage, and climate. Tile drainage is largely responsible for transforming Iowa from mostly wetlands into prime farmland. It fundamentally changes the relationship between land, climate, and soil, by draining away excess water, allowing crops to grow. This chapter uses observations from over 800,000 farms across the U.S. to estimate the relationship between farmland value and climate while explicitly incorporating tile drainage. We find fundamental differences in the relationship between tile drained and non-tile drained land, which has not been accounted for in previous research. Using climate projections, we estimate the impact of climate change on farmland and show how these estimates can be biased when tile drained and non-tile drained farms are pooled together. The second chapter looks at the relationship between land change and lake water quality. While most of Iowa\u27s lakes are artificial, many are popular destinations for fishing, boating, swimming, and other recreational activities. But their close proximity to farmland results in high nutrient levels and decreased water quality, which can reduce recreational and ecosystem benefits. This chapter combines fifteen years of water quality measurements with satellite images of land use to estimate the impact of land use change on water quality. These estimates are used to assess the lake water quality impacts of the Renewable Fuel Policy, a government policy which has had a large impact on agriculture and land use in Iowa. The third chapter is concerned with the optimal management of the Iowa deer population through hunting licenses. Although not all species have benefited from the transformation of Iowa\u27s landscape, the deer population has thrived due to a lack of predators and an abundant new food source in crops. While deer hunters enjoy a large population of deer, farmers and drivers face costs due to crop depredation and deer vehicle collisions, creating a complex management problem. This chapter uses the tools of dynamic programming to solve for an optimal policy that balances these opposing interests. Altering the natural landscape turned Iowa into one of the most productive farming regions in the world, but has also created the need to balance intensive farming practices with the impacts on the surrounding environment. The tools of economics provide an appealing framework to propose solutions to these problems. The goal of the following three chapters is to use these tools to shed some light on three such issues Iowa currently faces. The insight and results from this research will hopefully help inform future researchers and policymakers in Iowa and beyond

A Universal Stellar Initial Mass Function? A Critical Look at Variations

Few topics in astronomy initiate such vigorous discussion as whether or not the initial mass function (IMF) of stars is universal, or instead sensitive to the initial conditions of star formation. The distinction is of critical importance: the IMF influences most of the observable properties of stellar populations and galaxies, and detecting variations in the IMF could provide deep insights into the process by which stars form. In this review, we take a critical look at the case for IMF variations, with a view towards whether other explanations are sufficient given the evidence. Studies of the field, local young clusters and associations, and old globular clusters suggest that the vast majority were drawn from a "universal" IMF: a power-law of Salpeter index ($\Gamma=1.35$) above a few solar masses, and a log normal or shallower power-law ($\Gamma \sim 0-0.25$) between a few tenths and a few solar masses (ignoring the effects of unresolved binaries). The shape and universality of the IMF at the stellar-substellar boundary is still under investigation and uncertainties remain large, but most observations are consistent with a IMF that declines ($\Gamma < -0.5$) well below the hydrogen burning limit. Observations of resolved stellar populations and the integrated properties of most galaxies are also consistent with a "universal IMF", suggesting no gross variations in the IMF over much of cosmic time. There are indications of "non-standard" IMFs in specific local and extragalactic environments, which clearly warrant further study. Nonetheless, there is no clear evidence that the IMF varies strongly and systematically as a function of initial conditions after the first few generations of stars.Comment: 49 pages, 5 figures, to appear in Annual Reviews of Astronomy and Astrophysics (2010, volume 48

IN-SYNC. VIII. Primordial Disk Frequencies in NGC 1333, IC 348, and the Orion A Molecular Cloud

In this paper, we address two issues related to primordial disk evolution in three clusters (NGC 1333, IC 348, and Orion A) observed by the INfrared Spectra of Young Nebulous Clusters (IN-SYNC) project. First, in each cluster, averaged over the spread of age, we investigate how disk lifetime is dependent on stellar mass. The general relation in IC 348 and Orion A is that primordial disks around intermediate mass stars (2--5$M_{\odot}$) evolve faster than those around loss mass stars (0.1--1$M_{\odot}$), which is consistent with previous results. However, considering only low mass stars, we do not find a significant dependence of disk frequency on stellar mass. These results can help to better constrain theories on gas giant planet formation timescales. Secondly, in the Orion A molecular cloud, in the mass range of 0.35--0.7$M_{\odot}$, we provide the most robust evidence to date for disk evolution within a single cluster exhibiting modest age spread. By using surface gravity as an age indicator and employing 4.5 $\mu m$ excess as a primordial disk diagnostic, we observe a trend of decreasing disk frequency for older stars. The detection of intra-cluster disk evolution in NGC 1333 and IC 348 is tentative, since the slight decrease of disk frequency for older stars is a less than 1-$\sigma$ effect.Comment: 25 pages, 26 figures; submitted for publication (ApJ