arXiv subjects

A. Hebecker

Publications and source records attributed to A. Hebecker.

47 records · Page 3Linked to original sources

arXiv1997

High-$p_\perp$ Jets in Diffractive Electroproduction

The diffractive production of high-$p_{\perp}$ jets in deep-inelastic scattering is studied in the semiclassical approach. The $p_{\perp}$-spectra of $q {\bar q}$ and $q {\bar q} g$ diffractive final states are found to be qualitatively different. For $q {\bar q}$ final states, which are produced by `hard' colour-singlet exchange, the $p_{\perp}$-spectrum is much softer than for $q {\bar q} g$ final states, where the colour neutralization is `soft'. Furthermore, the two different final states can be clearly distinguished by their diffractive mass distributions.

hep-ph↗

arXiv1997

Charm as a Key to Diffractive Processes

The diffractive production of open charm in deep-inelastic scattering is studied in the semiclassical approach which has been proposed recently. In this approach, the leading order process contains a charm quark pair and an additional gluon in the diffractive final state. The $p_{\perp}$-spectrum and the diffractive mass distribution are evaluated and compared with predictions based on perturbative two-gluon exchange calculations for charm quark pair production. It is shown that the $p_{\perp}$-spectrum provides a clear test of the underlying partonic process whereas the diffractive mass distribution reflects the non-perturbative mechanism of colour neutralization.

hep-ph↗

arXiv1997

Diffractive Parton Distributions in the Semiclassical Approach

Recently, a semiclassical approach to diffraction has been proposed, which treats the proton as a classical colour field. The present paper demonstrates that this approach is consistent with the concept of diffractive parton distributions. The diffractive quark and gluon distributions are expressed through integrals of non-Abelian eikonal factors in the fundamental and adjoint representation respectively. As a by-product, previously calculated diffractive cross sections for processes with a final state gluon are rederived in a simpler way.

hep-ph↗

arXiv1996

The Drell-Yan Process and Factorization in Impact Parameter Space

The cross section for Drell-Yan pair production in the limit of small $x$-target is derived in the rest frame of the target hadron. Our calculation is based on the fundamental quantity $σ(ρ)$, the cross section for the scattering of a $q\bar{q}$-pair with fixed transverse separation $ρ$ off a hadronic target. As in deep inelastic scattering the result can be given in terms of integrals of $σ(ρ)$. This is consistent with well known factorization theorems and also relates higher-twist terms in both processes. An analysis of the angular distribution of the produced lepton shows that additional integrals of $σ(ρ)$ can be obtained in the Drell-Yan process, which are not measurable in inclusive deep inelastic scattering.

hep-ph↗

arXiv1995

Semiclassical Approach to Structure Functions at Small x

Inclusive and diffractive structure functions for electron-proton scattering are calculated in a semiclassical approach at large momentum transfer $Q^2$ and small values of the scaling variable $x$. The basic process is the production of a quark-antiquark pair in the colour field of the proton. The structure functions are expressed in terms of Wilson lines along the classical trajectories of quark and antiquark passing through the colour field, and their covariant derivatives. Based on some rather general assumptions on properties of the colour field, inclusive and diffractive structure functions are evaluated in terms of four field dependent constants.

hep-ph↗

arXiv1995

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, $R_D = σ_D/σ_{incl}\simeq 1/9$, is determined by the probability of the produced quark-antiquark pair to evolve into a colour singlet state.

hep-ph↗

arXiv1995

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 $g^4,λ^2$ 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 $Φ^\daggerΦ$ 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.

hep-ph↗

arXiv1995

A PARTON MODEL FOR DIFFRACTIVE PROCESSES IN DEEP INELASTIC SCATTERING

We demonstrate that the global properties of the ``rapidity gap'' events, observed at HERA, can be understood based on electron-gluon scattering as the underlying partonic process. 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, $R_D = σ_D/σ_{incl}\simeq 1/9$, is determined by the probability of the produced quark-antiquark pair to evolve into a colour singlet state. This colour singlet cluster may fragment into hadrons independently of the proton remnant, yielding the observed gap in rapidity.

hep-ph↗

arXiv1995

THERMODYNAMICS OF THE ELECTROWEAK PHASE TRANSITION

We discuss several general aspects of the free energy of the standard model at high temperatures. In particular the Clausius-Clapeyron equation is shown to yield a relation between the latent heat and the jump in the order parameter. The free energy is calculated as function of temperature in resummed perturbation theory to two-loop order. A new resummation procedure is proposed in which the symmetric phase and the Higgs phase are treated differently. A quantitative description of the phase transition is achieved for Higgs masses below $\sim 70$ GeV. The results are found to be in agreement with recent numerical simulations on large lattices. The phase transition provides no evidence for strong non-perturbative effects in the symmetric phase.

hep-ph↗

arXiv1994

Gauge Invariant Treatment of the Electroweak Phase Transition

We evaluate the gauge invariant effective potential for the composite field $σ=2Φ^{\dagger}Φ$ in the SU(2)-Higgs model at finite temperature. Symmetric and broken phases correspond to the domains $σ\leq T^2/3$ and $σ> T^2/3$, respectively. The effective potential increases very steeply at small values of $σ$. Predictions for several observables, derived from the ordinary and the gauge invariant effective potential, are compared. Good agreement is found for the critical temperature and the jump in the order parameter. The results for the latent heat differ significantly for large Higgs masses.

hep-ph↗

arXiv1994

Finite Temperature Effective Potential to Order $g^4,\la^2$ and the Electroweak Phase Transition

The standard model effective potential is calculated at finite temperature to order $g^4,\la^2$ and a complete zero temperature renormalization is performed. In comparison with lower order calculations the strength of the first order phase transition has increased dramatically. This effect can be traced back to infrared contributions from typical non-Abelian diagrams and to the infrared behaviour of the scalar sector close to the critical temperature. Several quantities, e.g. surface tension, latent heat and field expectation value are analyzed for an SU(2)-Higgs model and for the full standard model in detail. An explicit formula enabling further analytic or numerical study is presented. (DESY-94-025)

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