arXiv subjects

W. Schweiger

Publications and source records attributed to W. Schweiger.

35 records · Page 2Linked to original sources

arXiv2003

Hard exclusive baryon-antibaryon production in 2-$γ$ collisions

We present a perturbative calculation of baryon pair production in two-photon collisions, $γγ\to B \bar{B}$, in which baryons are treated as quark-diquark systems. Our approach accounts for constituent mass effects in a systematic way. Taking the diquark-model parameters from foregoing studies of other electron- and photon-induced baryonic reactions, our results agree well with the most recent large momentum-transfer data for the $p\bar{p}$, $Λ\barΛ$, and $Σ^0\barΣ^0$ channels.

hep-ph↗

arXiv2003

Vector mesons in a relativistic point-form approach

We apply the point form of relativistic quantum mechanics to develop a Poincare invariant coupled-channel formalism for two-particle systems interacting via one-particle exchange. This approach takes the exchange particle explicitly into account and leads to a generalized eigenvalue equation for the Bakamjian-Thomas type mass operator of the system. The coupling of the exchange particle is derived from quantum field theory. As an illustrative example we consider vector mesons within the chiral constituent quark model in which the hyperfine interaction between the confined quark-antiquark pair is generated by Goldstone-boson exchange. We study the effect of retardation in the Goldstone-boson exchange by comparing with the commonly used instantaneous approximation. As a nice physical feature we find that the problem of a too large $ρ$-$ω$ splitting can nearly be avoided by taking the dynamics of the exchange meson explicitly into account.

nucl-th↗

arXiv2002

Mass Effects in Hard Exclusive Photoproduction of $J/Ψ$ Mesons

We report on an attempt to describe hard exclusive photoproduction of $J/Ψ$ mesons, i.e. the reaction $γp \to J/Ψp$, by means of a modified version of the hard-scattering approach, in which the proton is treated as a quark-diquark rather than a three-quark system. In order to improve the applicability of the model at momentum transfers of only a few GeV we take into account constituent-mass effects in the calculation of the perturbative scattering amplitude. With a standard $J/Ψ$-meson distribution amplitude and diquark-model parameters adopted from preceding investigations of other photon-induced reactions our predictions for differential cross sections overestimate the naive extrapolation of the low-momentum transfer ZEUS data. Our results, however, reveal the importance of taking into account the charm-quark mass.

hep-ph↗

arXiv2001

Analysing powers for the reaction $\vec{\rm n} {\rm p} \to {\rm p} {\rm p} π^{-}$ and for np elastic scattering from 270 to 570 MeV

The analysing power of the reaction ${\rm n}{\rm p} \to {\rm p}{\rm p} π^{-}$ for neutron energies between threshold and 570 MeV has been determined using a transversely polarised neutron beam at PSI. The reaction has been studied in a kinematically complete measurement using a time-of-flight spectrometer with large acceptance. Analysing powers have been determined as a function of the c.m. pion angle in different regions of the proton-proton invariant mass. They are compared to other data from the reactions ${\rm n}{\rm p} \to {\rm p}{\rm p} π^{-}$ and ${{\rm p}{\rm p} \to {\rm p}{\rm p} π^{0}}$. The np elastic scattering analysing power was determined as a by-product of the measurements.

nucl-ex↗

arXiv2001

The interplay of soft and hard contributions in the electromagnetic pion form factor

We consider various relativistic models for the valence Fock-state wave function of the pion. These models are obtained from simple instant-form wave functions by applying a Melosh rotation to the spin part and by imposing physical constraints on the parameters. We discuss how the soft and the hard (perturbative) parts of the electromagnetic form factor are affected by the choice of the model and by the Melosh rotation.

hep-ph↗

arXiv2001

Photoproduction of Heavy Quarkonia

We investigate the reaction gamma+p -> V+p, with V denoting a Phi or a J/Psi meson, within the scope of perturbative QCD, treating the proton as a quark-diquark system. Our predictions extrapolate the existing forward differential cross-section data into the few-GeV momentum-transfer region. In case of the J/Psi reasonable results are only obtained by properly taking into account its mass in the perturbative calculation of the hard-scattering amplitude.

hep-ph↗

arXiv2001

Two-Quark Correlations in the Hard Electromagnetic Nucleon Form Factors

The, so called, "hard-scattering approach" represents a suitable framework for the perturbative treatment of exclusive hadronic processes at large energies and (transverse) momentum transfers. In this context, diquarks can serve as a useful phenomenological concept to model non-perturbative effects which are still observable in the kinematic range accessible by present-day experiments. We outline how a description of baryons as quark-diquark systems can be understood as an effective theory in the sense that the pure quark hard-scattering approach is recovered in the limit of asymptotically large momentum transfers. Our arguments are based on a reformulation of the hard-scattering formalism in terms of quark-diquark degrees of freedom. This reformulation provides the exact form of photon- and gluon-diquark vertices and corresponding vertex functions (diquark form factors) in the limit of asymptotically large momentum transfers -- and thus also asymptotic constraints which should be fulfilled by phenomenological quark-diquark models for hard scattering. As an application of this reformulation we present an analysis of the hard electromagnetic nucleon form factors with respect to their quark-diquark content.

hep-ph↗

arXiv2000

Hard exclusive photoproduction of $Φ$ and $J/Ψ$ mesons

We present predictions for differential cross sections for the reaction $γp \to Φp$ and give an outlook to which extent our calculations may be generalized to the photoproduction of $J/Ψ$ mesons. Our results are obtained within perturbative QCD treating the proton as a quark-diquark system.

hep-ph↗

arXiv1999

Hard exclusive photoproduction of $Φ$ mesons

We calculate the differential cross section and single-polarization observables for the reaction $γp \to Φp$ within perturbative QCD, treating the proton as a quark-diquark system. The phenomenological couplings of gauge bosons to (spatially extended) diquarks and the quark-diquark distribution amplitude of the proton are adopted from previous investigations of baryon form factors and two-photon processes. Going beyond leading order, we take into account hadron-mass effects by means of a systematic expansion in the small parameter (hadron mass/ photon energy). With the $Φ$-meson distribution amplitude taken from the literature our predictions for the differential cross section at $| t | \agt 4 \text{GeV}^2$ seem to provide a reasonable extrapolation of the low-t data and are also comparable in magnitude with the results of a two-gluon exchange model in which the gluons are considered as a remnant of the pomeron. For momentum transfers of a few GeV hadron-mass effects appear still to be sizeable.

hep-ph↗

arXiv1999

Hard exclusive photoproduction of $Φ$ and $J/Ψ$ mesons

We consider the reaction $γp \to M p$, with M being either a $Φ$ or a $J/Ψ$ meson, within perturbative QCD treating the proton as a quark-diquark system. The phenomenological couplings of gauge bosons to (spatially extended) diquarks and the quark-diquark distribution amplitude of the proton are adopted from previous investigations of baryon form factors and two-photon processes. Going beyond leading order, we take into account hadron-mass effects by a systematic expansion in the small parameter (hadron mass/ photon energy). With the meson distribution amplitudes taken from the literature our predictions for the differential cross section at $| t | \agt 3 \hbox{GeV}^2$ seem to provide a reasonable extrapolation of the low-$t$ data in case of the $Φ$. They fail, however, completely for the $J/Ψ$. A closer inspection reveals that `` hadron-mass corrections\rq\rq become dominant for t values of only a few GeV in case of the $J/Ψ$. This indicates that one has to go to much larger energies and momentum transfers in order to achieve a reliable description of $J/Ψ$ production within the hard-scattering approach.

hep-ph↗

arXiv1999

Diquarks as effective particles in hard exclusive scattering

In the context of hard hadronic reactions diquarks are a useful phenomenological device to model non-perturbative effects still observable in the kinematic range accessible by present-day experiments. In the following we present diquark-model predictions for $γγ\to p \bar{p}$ and $Λ\barΛ$. We also sketch how the (pure quark) hard-scattering formalism for exclusive reactions involving baryons can be reformulated in terms of quarks and diquarks. As an application of these considerations we analyze the magnetic proton form factor with regard to its quark-diquark content.

hep-ph↗

arXiv1997

Exclusive photo- and electroproduction of mesons in the GeV region

We consider the reactions gamma p --> M B, with M B being either K+ Lambda, K+ Sigma0, or Pi n, within a diquark model which is based on perturbative QCD. The model parameters and the quark-diquark distribution amplitudes of the baryons are taken from previous investigations of electromagnetic baryon form factors and Compton-scattering off protons. Reasonable agreement with the few existing photoproduction data at large momentum transfer is found for meson distribution amplitudes compatible with the asymptotic one (proportional to x (1-x)). We present also first results for hard electroproduction of the K+ Lambda final state. Our predictions exhibit some characteristic features which could easily be tested in forthcoming electroproduction experiments.

hep-ph↗

arXiv1996

Exclusive photo- and electroproduction of mesons at large momentum transfer

We analyze the reactions gamma+p => M+B, with M B being either K+ Lambda, K+ Sigma0, or pi+ n, within perturbative QCD, allowing for diquarks as quasi-elementary constituents of baryons. The diquark-model parameters and the quark-diquark distribution amplitudes of the baryons are taken from previous investigations of electromagnetic baryon form factors and Compton- scattering off protons. Reasonable agreement with the few existing data at large momentum transfer is achieved if the asymptotic form (c x (1-x)) is chosen for the meson distribution amplitudes.

hep-ph↗

arXiv1996

Exclusive Photoproduction of Large Momentum-Transfer K and K* Mesons

The reactions gamma p -> K+ Lambda and gamma p -> K* Lambda are analyzed within perturbative QCD, allowing for diquarks as quasi-elementary constituents of baryons. The diquark-model parameters and the quark-diquark distribution amplitudes of proton and Lambda are taken from previous investigations of electromagnetic baryon form factors and Compton-scattering off protons. Unpolarized differential cross sections and polarization observables are computed for different choices of the K and K* distribution amplitudes. The asymptotic form of the K distribution amplitude (proportional to x1 x2) is found to provide a satisfactory description of the K photoproduction data.

hep-ph↗

arXiv1995

Exclusive Photon-Induced Hadronic Reactions at Large Momentum Transfers

It is generally assumed that due to factorization of long- and short-distance dynamics perturbative QCD can be applied to exclusive hadronic reactions at large momentum transfers. Within such a perturbative approach diquarks turn out to be a useful phenomenological device to model non-perturbative effects still observable in the kinematic range accessible by present-days experiments. The basic ingredients of the perturbative formalism with diquarks, i.e. Feynman rules for diquarks and quark-diquark wave functions of baryons, are briefly summarized. Applications of the diquark model to the electromagnetic form factors of the proton in the space- as well as time-like region, Compton-scattering off protons, gamma-gamma -> p-pbar, and photoproduction of Kaons are discussed.

hep-ph↗

arXiv1993

Octet-Baryon Form Factors in the Diquark Model

We present an alternative parameterization of the quark-diquark model of baryons which particularly takes care of the most recent proton electric form-factor data from the E136 experiment at SLAC. In addition to electromagnetic form factors of the nucleon, for which good agreement with data is achieved, we discuss the weak axial vector form factor of the nucleon as well as electromagnetic form factors of $Λ$ and $Σ$ hyperons. Technical advance in calculating the pertinent analytic expressions within perturbative quantum chromodynamics is gained by formulating the wave function of the quark-diquark system in a covariant way. Finally, we also comment on the influence of Sudakov corrections within the scope of the diquark model.

hep-ph↗

arXiv1993

On Exclusive Reactions in the Time-Like Region

The electromagnetic form factors of the proton in the time-like region and two-photon annihilations into proton-antiproton are investigated. To calculate these processes at moderately large $s$ we use a variant of the Brodsky-Lepage hard-scattering formalism where diquarks are considered as quasi-elementary constituents of baryons. The proton wave function and the parameters controlling the diquark contributions are determined from fits to space-like data. We also comment on the decay $η_c \to p\bar{p}$.

hep-ph↗