Quantum Effects in Higgs-Boson Production Processes at Hadron Colliders
The search for Higgs bosons and extensions of the Standard Model of Elementary Particle Physics are main tasks of the Large Hadron Collider (LHC) at CERN which will start operation mid-2007. In this thesis processes which can be used to detect supersymmetric Higgs bosons at the LHC were considered. First a computer program was written which completes the toolbox for automatic calculations of hadronic cross sections. Using this program, the supersymmetric QCD corrections to associated H - W + -production and h 0 -production via vector-boson fusion and in association with heavy quarks were calculated. The corrections partly give significant contributions to the total cross section. Additionally, the possibility to measure the quartic Higgs self-coupling via triple-Higgs production was investigated and found to be challenging.

title: NLO QCD corrections to $W^\pm Z\gamma$ production with leptonic decays
abstract: We present a computation of the O(alpha_s) QCD corrections to W^\pm Z \gamma production at the Large Hadron Collider. The photon is considered as real, and we include full leptonic decays for the W and Z bosons. Based on the structure of the VBFNLO program package, we obtain numerical results through a fully flexible Monte Carlo program, which allows to implement general cuts and distributions of the final-state particles. The NLO QCD corrections are sizable and strongly exceed the theory error obtained by a scale variation of the leading-order result. Also, the shapes of relevant observables are significantly altered.

title: Di-boson and Tri-boson production at the LHC
abstract: The status of di-boson and tri-boson production is shortly review. Using the VBFNLO and the LOOPSIM package, approximated results at NNLO QCD are given for WZ production. Results for waa + jet at NLO QCD are also shown.

title: Physics beyond the standard model: supersymmetry
abstract: This collection of studies on new physics at the LHC constitutes the report of the supersymmetry working group at the Workshop 'Physics at TeV Colliders', Les Houches, France, 2007. They cover the wide spectrum of phenomenology in the LHC era, from alternative models and signatures to the extraction of relevant observables, the study of the MSSM parameter space and finally to the interplay of LHC observations with additional data expected on a similar time scale. The special feature of this collection is that while not each of the studies is explicitly performed together by theoretical and experimental LHC physicists, all of them were inspired by and discussed in this particular environment.

title: Helmholtz Alliance Linear Collider Forum
abstract: Supersymmetric scenarios with a very weakly interacting lightest superpartner (LSP) - like the gravitino or axino - naturally give rise to a long-lived next-to-LSP (NLSP). If the NLSP is a charged slepton it leaves a very distinct signature in a collider experiment. At the ILC/CLIC it will be possible to capture a significant fraction of the produced charged sleptons and observe their decays. These decays potentially reveal the nature of the LSP and thus provide a unique possibility to measure the properties of a very weakly interacting LSP which otherwise is most likely hidden from any other observation, like direct or indirect dark matter searches. We review the proposals that have been made to measure the LSP properties at the ILC/CLIC and compare its potential to the capability of the LHC.

title: Complete one-loop calculation of the T-violating D-parameter in neutron decay in the MSSM
abstract: We investigate the violation of time reversal invariance in the decay of the free neutron in the framework of the minimal supersymmetric standard model (MSSM). The coefficient of the triple product of the neutron spin and the momenta of electron and neutrino, the so-called D parameter, is computed at one-loop order including all diagrams. We find that D is mainly sensitive to the trilinear A coupling in the squark sector and to the phase of the coefficient $\mu$ which mixes the two Higgs superfields. The maximal MSSM contribution using parameters still allowed by experiment is however at $D \approx 10^{-7}$, while QED final state interactions give a value of $D_{\mathrm {fsi}} = - 2.3 \cdot 10^{-5}$. Explicit expressions for all relevant diagrams are given in an appendix.

title: THE LYMAN ALPHA FOREST IN THE SPECTRA OF QUASISTELLAR OBJECTS
abstract: Observations of redshifted Lyman alpha (Ly alpha) forest absorption in the spectra of quasistellar objects (QSOs) provide a highly sensitive probe of the distribution of gaseous matter in the universe. Over the past two decades, optical spectroscopy with large ground-based telescopes, and more recently ultraviolet (UV) spectroscopy from space, have yielded a wealth of information on what appears to be a gaseous, photoionized intergalactic medium (IGM), partly enriched by the products of stellar nucleosynthesis, residing in coherent structures over many hundreds of kiloparsecs. 
Recent progress with cosmological hydro-simulations based or hierarchical structure formation models has led to important insights into the physical structures giving rise to the forest. If these ideas are correct, a truly inter- and protogalactic medium [at high redshift (z similar to 3), the main repository of baryons] collapses under the influence of dark matter gravity into flattened or filamentary structures, which are seen in absorption against background QSOs. With decreasing redshift, galaxies forming in the denser regions may contribute an increasing part of the Ly alpha absorption cross section. Comparisons between large data samples from the new generation of telescopes and artificial Ly alpha forest spectra from cosmological simulations promise to become a useful cosmological tool.

title: Faint Resonantly Scattered Lyman Alpha Emission from the Absorption Troughs of Damped Lyman Alpha Systems at z ~ 3
abstract: We demonstrate that the Lyman alpha emission in the absorption troughs of a large sample of stacked damped Lyman alpha absorption systems (DLAS) presented by Rahmani et al (2010) is consistent with the spectral profiles and luminosities of a recently detected population of faint Lyman alpha emitters at z ~ 3. This result supports the suggestion that the faint emitters are to be identified with the host galaxies of DLAS at these redshifts.

title: Direct integration and non-perturbative effects in matrix models
abstract: We show how direct integration can be used to solve the closed amplitudes of multicut matrix models with polynomial potentials. In the case of the cubic matrix model, we give explicit expressions for the ring of nonholomorphic modular objects that are needed to express all closed matrix model amplitudes. This allows us to integrate the holomorphic anomaly equation up to holomorphic modular terms that we fix by the gap condition up to genus four. There is an onedimensional submanifold of the moduli space in which the spectral curve becomes the Seiberg-Witten curve and the ring reduces to the nonholomorphic modular ring of the group Γ(2). On that submanifold, the gap conditions completely fix the holomorphic ambiguity and the model can be solved explicitly to very high genus. We use these results to make precision tests of the connection between the large order behavior of the 1/N expansion and nonperturbative effects due to instantons. Finally, we argue that a full understanding of the large genus asymptotics in the multicut case requires a new class of nonperturbative sectors in the matrix model.

title: High-redshift Metals. II. Probing Reionization Galaxies with Low-ionization Absorption Lines at Redshift Six
abstract: We present a survey for low-ionization metal absorption line systems toward 17 QSOs at redshifts z_(em) = 5.8-6.4. Nine of our objects were observed at high resolution with either Keck/HIRES or Magellan/MIKE, and the remainder at moderate resolution with Keck/ESI. The survey spans 5.3 5.3 in the number density of highly ionized systems traced by C IV. The low-ionization systems at z ~ 6 span a similar range of velocity widths as lower-redshift sub-DLAs but have significantly weaker lines at a given width. This may imply that the mass-metallicity relation of the host galaxies evolves toward lower metallicities at higher redshifts. These systems lack strong Si IV and C IV, which are common among lower-redshift DLAs and sub-DLAs. This is consistent, however, with a similar decrease in the metallicity of the low- and high-ionization phases, and does not necessarily indicate a lack of nearby, highly ionized gas. The high number density of low-ionization systems at z ~ 6 suggests that we may be detecting galaxies below the current limits of i-dropout and Lyα emission galaxy surveys. These systems may therefore be the first direct probes of the "typical" galaxies responsible for hydrogen reionization.

title: The UV spectrum of HS 1700+6416 - I. Predicting the metal line content of the far UV spectrum
abstract: In preparation of a study of the $\ion{He}{ii}$/$\ion{H}{i}$ ratio towards the bright QSO HS 1700+6416, we predict the metal line content of the far-UV spectral range by modelling 18 metal absorption line systems with redshifts $0.2 Methods. For that purpose, we investigate the spectral energy distribution of the metagalactic ionizing radiation field. Simple photoionization models based on 8 different shapes of the ionizing background are tested for each system. The adopted energy distributions comprise the Haardt & Madau (2001, HM) model of metagalactic UV background as well as typical spectra of AGN and starburst galaxies. The models are evaluated and the favoured one is estimated.  We find that the majority of the considered systems is best reproduced with a HM-like ionizing radiation, where the $\ion{He}{ii}$ break, formally located at 4 Ryd, is shifted to lower energies (~3 Ryd), probably due to the opacity of the higher $\ion{He}{ii}$ Lyman series lines. The remaining systems can be reasonably described with models based on the unmodified HM background or the spectra of AGN or starburst galaxies. This finding supports the idea that the UV background is spatially variable due to both IGM opacity variations and to local sources. In comparison to an unmodified HM background, the resulting ionizing spectrum leads to carbon abundances lower by ~0.5 dex. Furthermore, if the ionizing radiation field as determined from metal line systems was typical for the IGM, the expected $\ion{He}{ii}$/$\ion{H}{i}$ ratio would be 150 to 190.