In this thesis, I study the formation of structure within the current standard cosmological model using two numerical methods: N-body simulations and semi-analytic models of galaxy formation. In Chapter 1 & 2, I will explain the motivations and objectives of the analysis presented in this thesis, and give a brief review of the relevant background. Chapter 3 is focused on the discreteness effects in $N$-body simulation: Hot/Warm Dark Matter (H/WDM) $N$-body simulations in which the initial uniform particle load is a cubic lattice, exhibit artefacts related to this lattice. In particular, the filaments which form in these simulations break up into regularly spaced clumps which reflect the initial grid pattern. Using numerical simulations, I demonstrate that a similar artefact is present even when the initial uniform particle load is not a lattice, but rather a glass with no preferred directions and no long-range coherence. My study shows that such regular fragmentation occurs also in simulations of the collapse of idealized, uniform filaments, but not in simulations of the collapse of infinite uniform sheets. In H/WDM simulations, all self-bound non-linear structures with masses much smaller than the free streaming mass appear to originate through spurious fragmentation of filaments. These artificial fragments form below a characteristic mass which scales as $M_p^{1/3}k^{-2}_{peak}$. This has the unfortunate consequence that the effective mass resolution of such simulations improves only as the cube root of the number of particles employed. In Chapter 4, I combine $N$-body simulations of structure growth with physical modelling of galaxy evolution to investigate whether the shift in cosmological parameters between the 1-year and 3-year results from the Wilkinson Microwave Anisotropy Probe (WMAP) affects predictions for the galaxy population. Structure formation is significantly delayed in the WMAP3 cosmology, because the initial matter fluctuation amplitude is lower on the relevant scales. The decrease in dark matter clustering strength is, however, almost entirely offset by an increase in halo bias, so predictions for galaxy clustering are barely altered. In both cosmologies, several combinations of physical parameters can reproduce observed, low-redshift galaxy properties; the star formation, supernova feedback, and AGN feedback efficiencies can be played off against each other to give similar results for a variety of combinations. Models which fit observed luminosity functions predict projected 2-point correlation functions which scatter by about 10-20 per cent on large scale and by larger factors on small scale, depending both on cosmology and on details of galaxy formation. Measurements of the pairwise velocity distribution prefer the WMAP1 cosmology, but careful treatment of the systematics is needed. Given current modelling uncertainties, it is not easy to distinguish the WMAP1 and WMAP3 cosmologies on the basis of low-redshift galaxy properties. Model predictions diverge more dramatically at high redshift. Better observational data at z>2 will better constrain galaxy formation and perhaps also cosmological parameters. In Chapter 5, I study whether the apparent universality of halo properties in hierarchical clustering cosmologies is a consequence of their growth through mergers. N-body simulations of Cold Dark Matter (CDM) have shown that, in this hierarchical structure formation model, dark matter halo properties, such as the density profile, the phase-space density profile, the distribution of axial ratio, the distribution of spin parameter, and the distribution of internal specific angular momentum follow `universal' laws or distributions. Here I study the properties of the first generation of haloes in a Hot Dark Matter (HDM) dominated universe, as an example of halo formation through monolithic collapse. I find all these universalities to be present in this case also. Halo density profiles are very well fit by the Navarro et al (1997) profile over two orders of magnitude in mass. The concentration parameter depends on mass as $c \propto M^{0.2}$, reversing the dependence found in a hierarchical CDM universe.However, the concentration-formation time relation is similar in the two cases: earlier forming haloes tend to be more concentrated than their later forming counterparts. Halo formation histories are also characterized by two phases in the HDM case: an early phase of rapid accretion followed by slower growth. Furthermore, there is no significant difference between the HDM and CDM cases concerning the statistics of other halo properties: the phase-space density profile; the velocity anisotropy profile; the distribution of shape parameters; the distribution of spin parameter, and the distribution of internal specific angular momentum are all similar in the two cases. Only substructure content differs dramatically. These results indicate that mergers do not play a pivotal role in establishing the universalities, thus contradicting models which explain them as consequences of mergers.

1s

Jul 29, 2008

Since the awareness of entanglement was raised by Einstein, Podolski, Rosen and Schrödinger in the beginning of the last century, it took almost 55 years until entanglement entered the laboratories as a new resource. Meanwhile, entangled states of various quantum systems have been investigated. Sofar, their biggest variety was observed in photonic qubit systems. Thereby, the setups of today's experiments on multi-photon entanglement can all be structured in the following way: They consist of a photon source, a linear optics network by which the photons are processed and the conditional detection of the photons at the output of the network. In this thesis, two new linear optics networks are introduced and their application for several quantum information tasks is presented. The workhorse of multi-photon quantum information, spontaneous parametric down conversion, is used in different configurations to provide the input states for the networks. The first network is a new design of a controlled phase gate which is particularly interesting for applications in multi-photon experiments as it constitutes an improvement of former realizations with respect to stability and reliability. This is explicitly demonstrated by employing the gate in four-photon experiments. In this context, a teleportation and entanglement swapping protocol is performed in which all four Bell states are distinguished by means of the phase gate. A similar type of measurement applied to the subsystem parts of two copies of a quantum state, allows further the direct estimation of the state's entanglement in terms of its concurrence. Finally, starting from two Bell states, the controlled phase gate is applied for the observation of a four photon cluster state. The analysis of the results focuses on measurement based quantum computation, the main usage of cluster states. The second network, fed with the second order emission of non-collinear type II spontaneous parametric down conversion, constitutes a tunable source of a whole family of states. Up to now the observation of one particular state required one individually tailored setup. With the network introduced here many different states can be obtained within the same arrangement by tuning a single, easily accessible experimental parameter. These states exhibit many useful properties and play a central role in several applications of quantum information. Here, they are used for the solution of a four-player quantum Minority game. It is shown that, by employing four-qubit entanglement, the quantum version of the game clearly outperforms its classical counterpart. Experimental data obtained with both networks are utilized to demonstrate a new method for the experimental discrimination of different multi-partite entangled states. Although theoretical classifications of four-qubit entangled states exist, sofar there was no experimental tool to easily assign an observed state to the one or the other class. The new tool presented here is based on operators which are formed by the correlations between local measurement settings that are typical for the respective quantum state.

1s

Jul 23, 2008

This thesis contributes to the field of transport through quantum dots. These devices allow for a controlled study of quantum transport and fundamental physical effects, like the Kondo effect. In this thesis we will focus on dots that are well described by generalized Anderson impurity models, where the discrete levels of the quantum dot are tunnel-coupled to fermionic reservoirs. The model parameters, like level energy and width, can be tuned in experiments. Therefore these systems constitute a valuable arena for testing experiment against theory and vice versa. In order to describe these strongly correlated systems, we employ the numerical renormalization group method. This allows us to address both longstanding questions concerning experimental results and new physical phenomena in these fundamental models. This thesis consists of three major projects. The first and most extensive one is concerned with the phase of the transmission amplitude through a quantum dot. Measurements of many-electron quantum dots with small level spacing reveal universal phase behaviour, a result not fully understood for almost 10 years. Recent experiments have seen that, contrarily, for dots with only a few electrons, i.e. large level spacing, the phase depends on the mesoscopic dot parameters. Analyzing a multi-level Anderson model, we show that the generic feature of the two regimes can be reproduced in the regime of overlapping levels or well separated levels, respectively. Thereby the universal character follows from Fano-type antiresonances of the renormalized single-particle levels. Moderate temperature supports the universal character. In the mesoscopic regime, we also investigate the effect of Kondo correlations on the transmission phase. In a second project we analyze a quantum dot coupled to a superconducting reservoir. In contrast to previous belief, the energy resolution of our method is not restricted by the energy scale of the superconducting gap, leading to new insights into the method. The high resolution allows us to resolve sharp peaks in the spectral function that emerge for a certain regime of parameters. A third project deals with a quantum dot coupled to two independent channels, a system known to exhibit non-Fermi liquid behaviour. We investigate the existence of the non-Fermi liquid regime when driving the system out of the Kondo regime by emptying the dot. We find that the extent of the non-Fermi liquid regime strongly depends on the mechanisms that couple impurity and reservoirs but prevent mixing of the latter.

1s

Jul 18, 2008

This thesis is concerned with two aspects of intersecting D6-brane models: We treat some of the effects that are induced by E2-instantons and we consider one-loop corrections to the gauge kinetic functions in the low-energy effective field theory. We begin in chapter 1 with a general introduction covering the question why to consider string theory as a candidate theory of nature. After that, in chapter 2, we go over some topics related to field theory instantons and finally discuss the instanton generated superpotential in supersymmetric QCD (SQCD), the so-called Affleck–Dine–Seiberg (ADS) superpotential. In chapter 3 we then proceed to reproduce the ADS superpotential in an intersecting D6-brane realization of SQCD as the effect of a single E2-instanton. In order to do so, we first explicate the necessary formalism for calculating the E2-instanton contribution to the superpotential, then construct a (local) intersecting D6-brane realization of SQCD, and finally show that the formalism indeed leads to the Affleck–Dine–Seiberg superpotential. In chapter 4 we consider one-loop corrections to the gauge kinetic functions. These one-loop corrections are related quite intricately to gauge threshold corrections which we compute for intersecting D6-brane models on the Z_2xZ_2 toroidal orbifold. An analysis of the formulas for calculating the E2-instanton contribution to the superpotential raises the question whether this contribution is a holomorphic quantity. In chapter 5 we take up this question showing that indeed it is holomorphic. In addition, we discuss under which circumstances one can expect E2-instanton corrections to the gauge kinetic functions. There follow a number of appendices, some elaborating on points touched upon in the main text, some collecting necessary formulas.

1s

Jul 15, 2008

The Large Hadron Collider, starting in 2008, will be a "top factory" as top-antitop pairs will be produced with a cross section of about 830 pb at an instantaneous luminosity of 10^33 cm^-2 s^-1 during the first year. With about 30% probability top pairs decay semileptonically into a final state with four jets, lepton (electron or muon) and respective neutrino. For another 5% of the top pair events a dileptonic decay is expected. Here the final state signature is composed of two jets, two leptons and two neutrinos. In this thesis the precision for a top pair cross section measurement at the ATLAS experiment in the semileptonic and dileptonic channels with cut based analyses, applicable to the first data, was estimated. The analysis of the semileptonic decay focused especially on the study of background from QCD events either with leptons from semileptonic hadron decays or from hadrons falsely identified as electrons by the calorimeter. For the first 10 fb^-1 and assuming a fake electron probability of 10^-3 a precision for the cross section times the branching ratio of +- 0.5(stat) +- 30.4(syst) +- 24.0(lumi) pb has been estimated, corresponding to a relative precision of 16% for the theoretically predicted cross section times branching ratio of about 240 pb. The analysis in the dileptonic channel achieves a precision of +- 0.2(stat) +- 2.5(syst) +- 2.6(lumi) pb which translates into a relative error of 10% for the cross section times branching ratio of around 38 pb. The errors for both the semileptonic and the dileptonic channel are expected to improve as progress is made on the luminosity determination and the knowledge of the backgrounds from comparisons with measured data. A measurement of the cross-section ratio between the dileptonic and semileptonic channel is sensitive to scenarios of new phenomena with competitive top quark decay modes such as decays involving a charged Higgs boson. It has been estimated that such a ratio should be measurable with a relative precision of +- 0.7%(stat) +- 7.7%(sys) +- 3.1%(lumi) during the first year of ATLAS data-taking. Even though the systematic errors partially cancel in such a ratio the total uncertainty is still around 8% as the background estimates rely on theoretical predictions. This should also improve as soon as the models can be tested against measured data.

1s

Jul 08, 2008

Im Rahmen der vorliegenden Arbeit wurde ein spektral hochauflösendes Lidar (HSRL) aufgebaut und während des Feldexperiments SAMUM im Mai/Juni 2006 und im Januar/Februar 2008 an Bord des Forschungsflugzeugs Falcon betrieben. Die Intensität von Lidar–Signalen wird maßgeblich durch die Rückstreuung und die Extinktion der atmosphärischen Teilchen beeinflusst. Dabei stehen die Rückstreuung und die Extinktion der Aerosole in keinem konstanten Verhältnis zueinander. Die Messgröße eines normalen Rückstreu–Lidars ist insofern mit zwei unbekannten Größen behaftet, weshalb die direkte Messung der Aerosolextinktion mit einem solchen Lidar nicht möglich ist. Im Gegensatz dazu wird bei der Methode des HSRL neben der gesamten atmosphärischen Rückstreuung der Teil der molekularen Rückstreuung gesondert gemessen. Die Messung des molekularen Rückstreusignals wird durch die spektrale Filterung der atmosphärischen Rückstreuung mit einem schmalbandigen optischen Filter ermöglicht. Durch den Vergleich des gemessenen molekularen Signals mit dem zu erwartenden kann die Aerosolextinktion direkt bestimmt werden. Zum Aufbau des Instruments wurde eine Joddampfabsorptionszelle konstruiert und in das Empfangsmodul des bestehenden flugzeuggetragenen Lidars des Deutschen Zentrums für Luft– und Raumfahrt integriert. Außerdem wurde der Lasertransmitter des Lidars mit einem neuartigen Verfahren der opto–akustischen Modulation frequenzstabilisiert. Während SAMUM–1 wurden damit erstmalig die optischen Eigenschaften des reinen Saharastaubaerosols, insbesondere dessen Extinktion, das Verhältnis von Extinktion und Rückstreuung sowie die Depolarisation, in der Nähe seiner Quellgebiete untersucht. Die Messungen des neuen HSRL wurden zur Qualitätssicherung mit Hilfe unabhängiger Instrumente validiert. Die Lidar–Verhältnis–Messungen wurden durch Trajektorienanalysen auf mögliche Abhängigkeiten von unterschiedlichen Quellgebieten untersucht. Die HSRL–Messungen der Aerosol–optischen Dicke wurden mit satellitengestützten Messungen verglichen. Südlich des Hohen Atlas Gebirges wurden Aerosol–optische Dicken von 0,50 bis 0,60 gemessen. Es zeigte sich eine ausgeprägte laterale Variabilität der Aerosol–optischen Dicke, die bei homogenen Schichten allein auf deren unterschiedliche Dicke zurückgeführt werden konnte. Die vertikalen Variationen des Lidar–Verhältnisses zwischen 38 sr und 50 sr wurden durch Trajektorienanalysen auf die Anströmung aus unterschiedlichen Quellgebieten zurückgeführt. Im Depolarisationsverhältnis wurden jedoch keine vertikale Variationen beobachtet, was auf eine einheitliche Teilchenform schließen lässt. Die Aerosoldepolarisation betrug in den Staubaerosolschichten 0,30 ± 0,02. Dies bestärkt die Annahme, dass das Lidar–Verhältnis in erster Linie durch die unterschiedliche chemische Zusammensetzung des Aerosols beeinflusst wird. Aufgrund der hohen natürlichen Variabilität erscheint die Angabe eines mittleren Wertes als nicht sinnvoll. Der Vergleich mit MISR–Messungen der Aerosol–optischen Dicke zeigte größtenteils Übereinstimmung innerhalb der Fehlergrenzen. Direkt über dem Hohen Atlas traten signifikante Abweichungen auf, die durch die sich stark ändernde Topographie erklärt werden können.

1s

Jul 08, 2008

In lebenden Organismen spielen Proteine eine wichtige Rolle bei Stoffwechselvorgängen. Für ihre Funktion ist die dreidimensionale Anordnung der Aminosäurekette von entscheidender Bedeutung. Um die frühen Faltungsprozesse von bestimmten Sekundärstrukturelementen zu analysieren, ist die Verwendung von Modellpeptiden nötig, da hier die Bildung solcher Strukturelemente getrennt beobachtet werden kann. Der Einbau eines optischen Schalters wie Azobenzol in Modellpeptide ermöglicht durch dessen lichtgetriebene cis/trans Isomerisierung eine Auslösung der Faltungsprozesse auf ultrakurzer Zeitskala (< 10 ps). Ein wesentliches Merkmal der Kombination dieses Photoschalters mit der in dieser Arbeit verwendeten Methode der UV-Anreg-Infrarot-Abtast-Spektroskopie ist die Möglichkeit, Zwischenzustände, sogenannte Intermediate, zeitlich einordnen und mittels ihrer Infrarotspektren Aussagen über ihre Struktur treffen zu können. Als Modell für das Sekundärstrukturelement des beta-Faltblatts dient eine beta-Hairpin Struktur. Diese Struktur besteht aus zwei anti-parallelen Aminosäuresträngen, welche durch Wasserstoffbrückenbindungen verbunden sind. Eine Kehre aus vier Aminosäuren schließt die Stränge auf einer Seite ab. Als photoschaltbares beta-Hairpin Modellpeptid wurde im Rahmen dieser Arbeit das AzoTrpZip2 mit der Sequenz H-Ser-Trp-Thr-Trp-Glu-AMPP-Lys-Trp-Thr-Trp-Lys-NH2 eingesetzt, wobei AMPP eine auf Azobenzol basierende pseudo-Aminosäure bezeichnet. Das Peptid AzoTrpZip2 bildet als cis-Isomer zu 45 % im Lösungsmittel Methanol-d4 eine beta-Hairpin Struktur aus. Das cis-Isomer des AMPPs ersetzt dabei zwei Aminos¨auren der Kehre. Das Ensemble an trans-Isomeren des AzoTrpZip2 hingegen besitzt eine deutlich weniger definierte Struktur. Ausgelöst durch die Isomerisierung des Schalters AMPP beginnt die Entfaltung der beta-Hairpin Struktur des AzoTrpZip2 mit einem reißverschlussartig sich fortsetzenden Bruch der schalternahen Wasserstoffbrückenbindungen und der Bildung eines desolvatisierten Zustandes mit einer Zeitkonstante von 4,1 ps. Mit 26 ps entsteht ein weiteres Intermediat, das mit einer Zeitkonstante von 630 ps in einer Klappbewegung mit den Strangmitten als Scharnier in einen Zustand übergeht, der dem Endzustand des trans- Ensemble ähnlich ist. Die Entfaltung ist nach 3 ns also weitgehend abgeschlossen. Auch bei der Faltungsreaktion erfolgt die Isomerisierung des Photoschalters auf der Pikosekundenzeitskala. Somit ist die zentrale Kehre der beta-Hairpin Struktur bereits innerhalb weniger Pikosekunden ausgebildet. Ähnlich wie bei der Entfaltung wird mit einer Zeitkonstante von 4,8 ps ein desolvatisierter Zustand erreicht, der mit einer Zeitkonstante von 64 ps in ein Faltungsintermediat übergeht. Daraus wird mit der Faltungszeitkonstante von 30 µs die beta-Hairpin Struktur gebildet. Die in der Literatur kontrovers diskutierte Frage nach dem geschwindigkeitsbestimmenden Schritt konnte für dieses Modellpeptid geklärt werden: Es ist die korrekte Anordnung der Wasserstoffverbrückung der Stränge und nicht die Ausbildung der Schleife.

1s

Jun 24, 2008

Estimates of the ozone layer future evolution must consider both climate dependencies and interannual variability. These considerations imply analyses of transient-scenario realisations with chemistry-climate models (CCM) under realistic boundary conditions. In this context, investigations of ozone variability usually involve multiple regression analysis (MRA), a statistically efficient albeit complicated tool. However, a careful use of advanced regression approaches may improve the variability assessment considerably. The present study addresses climate dependencies in ozone transport, and adopts an advanced regression approach to both quantify deterministic ozone variability and trace it back to the scenario boundary conditions; the investigations refer to transient output of the CCM E39/C. Recent observations show a cooling of the tropical lower stratosphere, and CCMs suggest a spatial coincidence of the cooling with a stronger upward advection of ozone-poor tropospheric air. This advection increase appears to result from a currently unexplained strengthening of the planetary-wave driven mean meridional transport, arguably relating to the anthropogenic climate signal. The present study explores the strengthening by comparing realisations of two different scenarios. Both share the same boundary conditions including concentrations of ozone-depleting substances (ODS), but differ in their climate forcing via sea surface temperatures (SSTs) and well-mixed greenhouse gas concentrations (GHG). In the summer hemisphere tropics, higher SSTs for the warmer scenario amplify deep convection and hence the convective excitation of internal planetary waves. These waves travel upward through easterly winds while dissipating, but still carry enough of the signal into the lower stratosphere to intensify the mean meridional transport. The transport change in turn strengthens the input rate into the tropical lower stratosphere of ozone-poor tropospheric air, ultimately weakening lower-stratospheric ozone concentrations via higher tropical SSTs. The ozone variability assessment relates to monthly-mean total columns from three independent realisations of a 60-year transient scenario with realistic boundary conditions. It focuses on three latitudinal bands: southern/northern midlatitudes (SH/NH) and tropics. Common ozone MRAs are linear and iterate to account for auto-regression-induced nonlinearity. The present MRA is nonlinear and the first to demonstrate the validity of such iterations with respect to the least-squares surface: it detects only a weak distortion of the surface associated with autocorrelation, at least for the ozone time series examined. Also, the present MRA is among the few to demonstrate sufficient compliance with the regression requirements, particularly with that of independent residuals. Additionally, the new approach of response confidence bands permits a correct attribution of individual anomalies to the scenario boundary conditions. As a consequence, the present MRA is the first to explain the year 1985 SH low-ozone event, here reproduced by E39/C. The MRA further captures, e.g., a similar anomaly for the year 1997, and verifies the total-ozone response to stratospheric-transport modulating boundary conditions: tropical-SST anomalies (ENSO) affect the tropics and NH, but not the SH; or, the quasi-biennial oscillation (QBO) causes a seasonally synchronised ozone response at SH and more weakly at NH, but not in the tropics. While these features have already been reported for E39/C data, the present study establishes a firm statistical framework and discusses the physical background. Other responses refer to the 11-year solar cycle (SSC), to sulfate aerosols, and to ODS concentrations. The present nonlinear regression approach provides ample potential for further development. For instance, nonlinear deterministic regression terms may examine the existence of interactions between the NH ENSO response with long-term changes in the probability for northern polar heterogenous ozone depletion. Last, accounting for moving-average regression parameters may improve the compliance with the inference requirements even further. In conclusion, the E39/C boundary conditions modulate the ozone layer as well as stratospheric mean meridional mass transport on long and short time scales. In this respect, the most important result is the universal significance of tropical SSTs controlling stratospheric transport by governing the deep-convective production of internal and, probably, external planetary waves. An important future research task is whether increasing tropical SSTs can cause ENSO-like changes in wintertime mid- and polar-latitude stratospheric planetary-wave activity; such changes could disturb the northern polar vortex against the effect of radiatively induced stabilisation by higher GHG concentrations. E39/C and other CCMS have certain weaknesses, one of which is an unrealistically consistent QBO-related modulation of the northern polar vortex. Keeping these weaknesses in mind, MRA may represent a helpful tool as it improves the statistical efficiency.

1s

Jun 17, 2008

Attosecond physics has become one of the most thriving field of science over the last decade. Although high-order harmonic generation from gaseous media is widely used as the source of attosecond pulses, a demand for more intense coherent extreme ultraviolet (XUV) and soft x-ray (SXR) radiation sources is growing. The process of high-order harmonic generation from plasma surfaces has attracted a strong interest as a promising candidate to meet this demand. Despite many theoretical predictions of the possibilities to generate intense attosecond pulses, experimental verifications are yet to come. The main theme of this thesis is to characterize the temporal structure of the harmonics generated from plasma surfaces. To achieve this goal, several preparatory experiments are made first. The contrast of the laser pulse is one of the most critical parameters for the harmonic generation process and its improvement is demonstrated by using a plasma mirror. The properties of the generated harmonics are studied thoroughly to find the optimal condition for temporal characterization. These experiments provide the groundwork for the autocorrelation measurements of the pulse train. To characterize the temporal structure of the generated harmonics, the technique of the volume autocorrelation using two-photon ionization of helium is applied. The measured autocorrelation traces reveal attosecond structures within the XUV radiation generated from the plasma surfaces for the first time. The observation of attosecond structures prove the potential of this harmonic generation process as a source of attosecond pulses. The process holds a promise to generate attosecond pulses with unprecedented intensities, which will open up a new regime of attosecond physics.

1s

Jun 04, 2008

As the first case ever studied with high - resolution spectroscopy for odd-N nuclei in the second potential minimum, the fission isomers in 237Pu (t1/2 = 110ns/1.1 μs) were investigated using the 235U(alpha,2n) reaction with a pulsed alpha beam (Ealpha =24 MeV, Deltat=400 ns) from the Cologne Tandem accelerator. A metallic 235U target (3.7 mg/cm2)was used, where the 237Pu reaction products were stopped and fission products were emitted in opposite directions. The rare gamma-rays from the second potential well in delayed coincidence with fission products were measured with the MINIBALL spectrometer. The identified level scheme will be presented and compared to single - particle calculations allowing for the first time an identification of the Nilsson quantum numbers. The identification of Nilsson orbitals will provide an important input for the validation and improvement of theoretical nuclear models and will lead to improved predictions for fission barriers and their extrapolations to neutron-rich heavy elements in the mass region of the r-process path of the astrophysical nucleosynthesis.

1s

May 27, 2008

In this thesis we explore the effects of chemical potentials or charge densities inside a thermal plasma, which is governed by a strongly coupled gauge theory. Since perturbative methods in general fail in this regime, we make use of the AdS/CFT correspondence which originates from string theory. AdS/CFT is a gauge/gravity duality (also called holography), which we utilize here to translate perturbative gravity calculations into results in a gauge theory at strong coupling. As a model theory for Quantum-Chromo-Dynamics (QCD), we investigate N=4 Super-Yang-Mills theory in four space-time dimensions. This theory is coupled to fundamental hypermultiplets of N=2 Super-Yang-Mills theory. In spite of being quite different from QCD this model succeeds in describing many of the phenomena qualitatively, which are present in the strong interaction. Thus, the effects discovered in this thesis may also be taken as predictions for heavy ion collisions at the RHIC collider in Brookhaven or the LHC in Geneva. In particular we successively study the introduction of baryon charge, isospin charge and finally both charges (or chemical potentials) simultaneously. We examine the thermodynamics of the strongly coupled plasma. Phase diagrams are given for the canonical and grandcanonical ensemble. Furthermore, we compute the most important thermodynamical quantities as functions of temperature and charge densities~(or chemical potentials): the free energy, grandcanonical potential, internal energy and entropy. Narrow resonances which we observe in the flavor current spectral functions follow the (holographically found) vector meson mass formula at low temperature. Increasing the temperature the meson masses first decrease in order to turn around at some temperature and then increase as the high-temperature regime is entered. While the narrow resonances at low temperatures can be interpreted as stable mesonic quasi-particles, the resonances in the high-temperature regime are very broad. We discuss these two different temperature-regimes and the physical relevance of the discovered turning point that connects them. Moreover, we find that flavor currents with isospin structure in a plasma at finite isospin density show a triplet splitting of the resonances in the spectral functions. Our analytical calculations confirm this triplet splitting also for the diffusion pole, which is holographically identified with the lowest lying quasinormal frequency. We discuss the non-vanishing quark condensate. Furthermore, the baryon diffusion coefficient depends non-trivially on both: baryon and isospin density. Guided by discontinuities in the condensate and densities, we discover a phase transition resembling the one found in the case of 2-flavor QCD. Finally, we extend our hydrodynamic considerations to the diffusion of charmonium at weak and strong coupling. As expected, the ratio of the diffusion coefficient to the meson mass shift at strong coupling is significantly smaller than the weak coupling result. This result is reminiscent of the result for the viscosity to entropy density ratio, which is significantly smaller at strong coupling compared to its value at weak coupling.

1s

May 20, 2008

This Thesis addresses the topic of galaxy formation and evolution in the universe. In collaboration with D. Croton, G. de Lucia, V. Springel, and S.D.M. White, I made use of the Millennium simulation, a very large N-body simulation of dark-matter evolution in a cosmological volume carried out at the MPA in 2005 by Springel 2005, to explore the predictions made by the most recent generation of semi-analytic models for galaxy formation. These models are incorporating a new mode of feedback from active galactic nuclei (AGN), which have their origins in super-massive black holes accreting mass and turning it into energy. Because of its observational signature in the radio regime this feedback is called "radio mode" and it counteracts the cooling flows of cold gas in undisturbed dark-matter haloes hosting galaxy clusters, which would otherwise show much higher star-formation of their central object than is observed. Previous work by Croton 2006 and De Lucia 2006 has shown that with the new semi-analytic model the population of local galaxies can be reproduced quite accurately. In order to study the evolution of the population out to higher redshifts, the semi-analytic predictions have been compared to a number of observations in various filter bands, in particular to two recent efforts to get a comprehensive multi-wavelength dataset of high redshift galaxies carried out by the DEEP2 (Davis 2001) and COSMOS (Scoville 2006) collaborations. The approach taken was to perform as broad a comparison as possible to gain firm constraints on the assumed physics in our model. Therefore a multitude of observational properties was contrasted with the model predictions such as clustering, luminosity functions, stellar mass functions, number counts per area and redshift to a certain magnitude limit. In order to facilitate the comparison between simulations and recent intermediate and high-redshift surveys, it is very useful to have a number of independent mock observations of the simulated galaxies, which provide good enough statistics to get a handle on cosmic variance. To this end I have devised a computer program that calculates the simulated galaxies lying on the backward light cone of a hypothetical observer out to arbitrarily high redshifts, taking advantage of the periodicity of the simulation box but avoiding replications. The output provides accurately interpolated redshifts, positions, observer frame and rest-frame magnitudes, dust extinction, as well as all the intrinsic galaxy properties like stellar mass and star formation rate. Utilising this tool it is also possible to make predictions for future galaxy surveys, deeper in magnitude and redshift than current ones. Presently the mock catalogues are used by the DEEP2 and COSMOS teams as a comparison sample in general and as a means to assess their selection effects and improve their data reduction in particular. First comparisons of counts in apparent magnitude and redshift gave promising results, showing good agreement in the low and intermediate range. The same holds for the angular clustering analysis except for the faintest magnitudes. Thus we conclude that our current understanding of the processes governing galaxy formation and evolution from the very first objects to the present day population is realistic but still incomplete. In particular the treatment of the interplay between star formation and negative feedback and the various processes influencing satellite galaxies in big galaxy clusters have potential for improvement. In the following I will give a brief outline of the thesis. After setting the stage for any kind of model in Chapter 1 by defining the geometry of the universe and the cosmological parameters that determine it, I will describe our semi-analytical model of galaxy formation in Chapter 2, where it will be also explained how to construct realistic mock observations of the simulated galaxies. First in Chapter 3 it will be verified that a simple model which assumes that galaxies are conserved but evolve in luminosity due to their star formation histories cannot account for the observed evolution of the galaxy population in the universe. This fact can be understood in the context of hierarchical models where massive and luminous galaxies assembled from smaller objects. Chapter 4 proceeds with exploring the predictions from the considerably more sophisticated semi-analytic model based on an N-body simulation of the hierarchical growth of dark matter structures. For this analysis a set of mock light-cones was constructed for direct comparison with the data which shows reasonably good agreement between model and observations at low redshift and for bright apparent magnitudes. These light-cones represent one of the largest samples of realistic mock observations currently available. They can be used for testing data analysis techniques usually applied to real observations on a well defined sample of artificial galaxies to verify how well the derivation of galaxy properties from the data works. In Chapter 5 we will demonstrate how one can measure the evolution of the galaxy merger rate from observing close projected galaxy pairs. Interestingly we find that the calibration needed for the conversion is significantly different from what has typically been assumed in previous studies. Additionally we will demonstrate that galaxy merger rates and dark-matter merger rates show considerably different evolution with redshift. Consequently we conclude that merger rate studies are less suitable as a probe of cosmic structure formation than initially assumed, but nonetheless they can be of great help to understand the formation and evolution of galaxies in a hierarchical universe. Finally these results will be summarised and discussed in Chapter 6 where I will also give a brief outlook on the future of this work, a short glimpse of which is already presented in the Appendix.

1s

May 20, 2008

Ultraviolet radiation can lead to photo-lesions and mutations in DNA, which can cause several diseases, most notably skin carcinomas. The most abundant UV-induced photo-lesions result from the formation of a cyclobutane ring between adjacent thymine bases (CPD-lesions). Although the formation of these thymine dimers has been reported in the early 1960s, until now neither the time scale of dimer formation nor the reaction mechanism has been resolved. The focus of this work is on the investigation of photophysical and photochemical processes in nucleic acids on the picosecond time scale. Pump-probe spectroscopy allows the investigation of ultrafast, photo-induced processes. The applied spectrometer is based on a central femtosecond laser system. The emitted short light pulses (800 nm, 90 fs) are converted via nonlinear processes into the required spectral regions (ultraviolet pump: ~ 270 nm, mid infrared probe: 3 - 10 µm). In this way the high-structure sensitivity of vibrational spectroscopy can be combined with a time resolution in the sub picosecond regime. For the investigation of thymine dimer formation, two thymine derivatives were chosen: the 18-mer all-thymine single strand (dT)18 and the mononucleotide thymidine-5'-monophosphate. Additional experiments were performed on the all-adenine single strand poly(A) and the mononucleotide adenine-5'-monophosphate, as adenine is the complementary base of thymine in the DNA double helix. While in AMP virtually all excited adenine bases return to the vibrationally excited ground state via a fast, internal conversion (< 1 ps), there is an additional population of long-lived, electronic states in poly(A), with lifetimes in the 100 ps- and ns-regime. The population of these states correlates with the amount of stacked bases in poly(A). This can be explained by the formation of excimer states. The IR absorption of these states could be deduced in this paper for the first time. To resolve the UV-induced formation of CPD-lesions, the characteristic IR absorption of thymine dimers was determined from stationary irradiation experiments on (dT)18. Afterward the formation of thymine dimers in (dT)18 was shown to occur within one picosecond (10^-12 s) by comparing the time resolved measurements on TMP and (dT)18. Long-lived electronic states (100 ps - 1 ns) do not lead to dimer formation. Therefore, the photoreaction can only take place if the conformation of two bases at the moment of UV absorption is already suitable for dimerization. This interpretation can be transferred to the DNA double helix, in which deviations from the ideal helix structure are necessary for dimer formation. In this work a fundamental question of photochemical reactions in DNA is resolved, which is of central importance to the understanding of the frequency of damage and mutation patterns in the genome.

1s

May 16, 2008