The Electroweak Phase Transition

The electroweak phase transition is investigated by means of the perturbatively calculated high temperature effective potential. An analytic result to order $g^4,\lambda^2$ is presented for the Abelian Higgs model, the SU(2)-Higgs model and the standard model and a complete on-shell renormalization at zero temperature is performed. Higher order corrections are found to increase the strength of the first order phase transition in the non-Abelian model, opposite to the Abelian case. This effect is traced back to the infrared contributions from the typical non-Abelian diagrams. The dependence of several phase transition parameters on the Higgs mass is analysed in detail. A new, gauge invariant, approach based on the composite field $\Phi^\dagger\Phi$ is introduced. This method, which supports the above Landau gauge results numerically, permits a conceptually simpler treatment of the thermodynamics of the phase transition. In particular, it enables a straightforward comparison with lattice data and the application of the Clausius-Clapeyron equation to the electroweak phase transition.Comment: Ph.D. thesis, 70 pages LaTeX, figures not included, complete ps-file or hardcopy available from the Autho

On Dynamical Adjustment Mechanisms for the Cosmological Constant

After recalling why dynamical adjustment mechanisms represent a particularly attractive possibility for solving the cosmological constant problem, we briefly discuss their intrinsic difficulties as summarized in Weinberg's no-go theorem. We then comment on some problems of the recently proposed `self-tuning' mechanism in 4+1 dimensions. Finally, we describe an alternative approach which uses the time-evolution of the universe to achieve a dynamical relaxation of the cosmological constant to zero.Comment: 4 pages LaTeX, talk at PASCOS 2001, Chapel Hill, North Carolina, 10-15 April 200

Non-Perturbative High-Energy QCD

It is the aim of this talk to review our understanding of the high-energy limit of QCD, focussing, in particular, on recent theoretical developments. After a brief introduction, I will recall why the true high-energy limit of QCD scattering processes is genuinly non-perturbative and why it has so far not been possible to apply lattice methods to this type of physics. Given the experimental fact of slowly rising hadronic cross sections, we are thus faced with a fundamental problem comparable to that of confinement but without the promise of the lattice. During the last years, the experimental side of this field has largely been driven by the HERA accelerator, which has, naturally, also influenced recent theoretical work in high-energy QCD. I will therefore devote the second part of the talk to small-x deep inelastic scattering, in particular the physics of diffraction, and attempt to describe its impact on the wider field of non-perturbative high-energy QCD.Comment: 20 pages LaTeX, 10 figures, Plenary talk at the International Europhysics Conference on High Energy Physics (EPS HEP 2001), Budapest, 12-18 July 2001, reference adde

High-Energy Scattering and Diffraction: Theory Summary

New developments in the theory and phenomenology of high-energy scattering and diffraction that were presented and discussed at DIS2000 are reviewed.Comment: 9 pages, LaTeX, including one PostScript figure. Talk given at the 8th International Workshop on Deep-Inelastic Scattering (DIS2000), 25th-30th April 2000, Liverpool, England, to appear in the proceeding

Calculating the diffractive from the inclusive structure function

It is demonstrated that the global properties of the rapidity gap events at HERA can be understood based on electron-gluon scattering and a non-perturbative mechanism of colour neutralization. Using the measured inclusive structure function $F_2$ to determine the parameters of the parton model, the diffractive structure function $F_2^D$ is predicted. The ratio of diffractive and inclusive cross sections, $R_D = \sigma_D/\sigma_{incl}\simeq 1/9$, is determined by the probability of the produced quark-antiquark pair to evolve into a colour singlet state.Comment: talk at Workshop on DIS and QCD, Paris, April 1995, 3 pages LaTeX, uses qcdparis.sty, 2 figures (uuencoded

The Ubiquitous Throat

We attempt to quantify the widely-held belief that large hierarchies induced by strongly-warped geometries are common in the string theory landscape. To this end, we focus on the arguably best-understood subset of vacua -- type IIB Calabi-Yau orientifolds with non-perturbative Kaehler stabilization and a SUSY-breaking uplift (the KKLT setup). Within this framework, vacua with a realistically small cosmological constant are expected to come from Calabi-Yaus with a large number of 3-cycles. For appropriate choices of flux numbers, many of these 3-cycles can, in general, shrink to produce near-conifold geometries. Thus, a simple statistical analysis in the spirit of Denef and Douglas allows us to estimate the expected number and length of Klebanov-Strassler throats in the given set of vacua. We find that throats capable of explaining the electroweak hierarchy are expected to be present in a large fraction of the landscape vacua while shorter throats are essentially unavoidable in a statistical sense.Comment: References added, typos fixed. LaTex, 17 pages, 1 figur

Gauge Invariance and Factorisation in Exclusive Meson Production

The structure of the nonperturbative vector meson vertex function complicates the proof of the factorisation theorem for the reaction $\gamma^*p\to Vp$. It leads to additional contributions but, in a simple model for the vertex function, gauge invariance ensures that they cancel and factorisation is preserved.Comment: 11 pages LaTeX, 4 figures included, uses eps