On the Irreducible BRST Quantization of Spin-5/2 Gauge Fields

Spin-5/2 gauge fields are quantized in an irreducible way within both the BRST and BRST-anti-BRST manners. To this end, we transform the reducible generating set into an irreducible one, such that the physical observables corresponding to these two formulations coincide. The gauge-fixing procedure emphasizes on the one hand the differences among our procedure and the results obtained in the literature, and on the other hand the equivalence between our BRST and BRST-anti-BRST approaches.Comment: 12 pages, latex 2.09, no figure

Structural variability and the incoherent addition of scattered intensities in single-particle diffraction

X-ray lasers may allow structural studies on single particles and biomolecules without crystalline periodicity in the samples. We examine here the effect of sample dynamics as a source of structural heterogeneity on the resolution of the reconstructed image of a small protein molecule. Structures from molecular-dynamics simulations of lysozyme were sampled and aligned. These structures were then used to calculate diffraction patterns corresponding to different dynamic states. The patterns were incoherently summed and the resulting data set was phased using the oversampling method. Reconstructed images of hydrated and dehydrated lysozyme gave resolutions of 3.7 Å and 7.6 Å, respectively. These are significantly worse than the root-mean-square deviation of the hydrated ͑2.7 Å for all atoms and 1.45 Å for C-␣ positions͒ or dehydrated ͑3.7 Å for all atoms and 2.5 Å for C-␣ positions͒ structures. The noise introduced by structural dynamics and incoherent addition of dissimilar structures restricts the maximum resolution to be expected from direct image reconstruction of dynamic systems. A way of potentially reducing this effect is by grouping dynamic structures into distinct structural substates and solving them separately

Geometric Scaling in a Symmetric Saturation Model

We illustrate geometric scaling for the photon-proton cross section with a very simple saturation model. We describe the proton structure function F2 at small x in a wide kinematical range with an elementary functional form and a small number of free parameters. We speculate that the symmetry between low and high Q2 recently discovered in the data could be related to a well-known symmetry of the two-gluon- exchange dipole-dipole cross section.Comment: 15 pages, 4 figure

Diffractive Higgs boson production at the Tevatron and LHC

Improved possibilities to find the Higgs boson in diffractive events, having less hadronic activity, depend on whether the cross section is large enough. Based on the soft color interaction models that successfully describe diffractive hard scattering at HERA and the Tevatron, we find that only a few diffractive Higgs events may be produced at the Tevatron, but we predict a substantial rate at the LHC.Comment: 4 pages, 4 figures, uses Revtex

Soft Color Interactions and Diffractive Hard Scattering at the Fermilab Tevatron

An improved understanding of nonperturbative QCD can be obtained by the recently developed soft color interaction models. Their essence is the variation of color string-field topologies, giving a unified description of final states in high energy interactions, e.g., diffractive and nondiffractive events in ep and ppbar. Here we present a detailed study of such models (the soft color interaction model and the generalized area law model) applied to ppbar, considering also the general problem of the underlying event including beam particle remnants. With models tuned to HERA ep data, we find a good description also of Tevatron data on production of W, beauty and jets in diffractive events defined either by leading antiprotons or by one or two rapidity gaps in the forward or backward regions. We also give predictions for diffractive J/psi production where the soft exchange mechanism produces both a gap and a color singlet ccbar state in the same event. This soft color interaction approach is also compared with Pomeron-based models for diffraction, and some possibilities to experimentally discriminate between these different approaches are discussed.Comment: 35 pages, 15 figures, uses REVTeX. Minor changes, version to appear in Phys. Rev.

The colour of gluon interactions: Studies of quantum chromodynamics in soft and hard processes

Quantum Chromodynamics (QCD) is the theory of the strong interaction, one of the fundamental forces of nature. The interactions between quarks are mediated by gluons, which are the colour-charged gauge fields in QCD. Hard processes with a large momentum transfer can be calculated using perturbation theory, while soft processes with a small momentum transfer are poorly understood. In this thesis, various aspects of the gluon interactions are studied based on the interplay between hard and soft processes. Soft gluon exchanges do not affect the dynamics of a hard process, but can rearrange the colour topology, resulting in different final states. The soft colour interaction models employ this idea and give a good description of all diffractive hard scattering data observed in pp collisions (W, Z, dijets, bb, J/ψ). This thesis also presents predictions for diffractive Higgs and γγ production at present and future hadron colliders. Multiple gluon exchanges give rise to saturation effects in hadronic collisions at high energies. Implementing this idea in photon-photon collisions gives new insight into the quantum structure of the photon and its interactions at high energies. When combined with perturbative calculations for single gluon exchange, the obtained results are in good agreement with experimental data from e+e- colliders. Off-shell gluon distributions in the photon give another perspective on the photon structure and have been parameterized for the first time in this thesis. These are useful for calculating cross sections of processes where the effects of transverse momenta are crucial, for example heavy quark production in γp or γγ collisions. Quantization of gauge fields which have a richer gauge structure than the gluons in QCD, is studied using the powerful BRST quantization formalism. Thus, first-stage reducible theories, like topological Yang-Mills and spin-5/2 gauge fields, are successfully quantized in an irreducible way. Understanding gluon interactions and the interplay between soft and hard processes paves the way towards solving the longstanding problem of confinement in QCD