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D. Drechsel

Publications and source records attributed to D. Drechsel.

90 records · Page 5Linked to original sources
arXiv1997

Polarization Phenomena in Small-Angle Photoproduction of e+e- Pairs and the Gerasimov-Drell-Hearn Sum Rule

Photoproduction of $e^+e^-$ pairs at small angles is investigated as a tool to determine the functions $f_1$ and $f_2$ entering the real-photon forward Compton scattering amplitude. The method is based on an interference of the Bethe-Heitler and the virtual Compton scattering mechanisms, generating an azimuthal asymmetry in the $e^+$ versus $e^-$ yield. The general case of a circularly polarized beam and a longitudinally polarized target allows one to determine both the real and imaginary parts of $f_1$ as well as $f_2$. The imaginary part of $f_2$ requires target polarization only. We calculate cross sections and asymmetries of the reaction $p(γ,e^+e^-)p$, estimate corrections and backgrounds, and propose suitable kinematical regions to perform the experiment. Our investigation shows that photoproduction of $e^+e^-$-pairs off the proton and light nuclei may serve as a rather sensitive test of the validity of the Gerasimov-Drell-Hearn sum rule.

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arXiv1997

Generalized polarizabilities of the nucleon studied in the linear sigma model (II)

In a previous paper virtual Compton scattering off the nucleon has been investigated in the one-loop approximation of the linear sigma model in order to determine the 3 scalar generalized polarizabilities. We have now extended this work and calculated the 7 vector polarizabilities showing up in the spin-dependent amplitude of virtual Compton scattering. The results fulfill 3 model-independent constraints recently derived. Compared to the constituent quark model there exist enormous differences for some of the vector polarizabilities. At vanishing three-momentum of the virtual photon, the analytical results of the sigma model and of chiral perturbation theory can be related. The influence of the $π^{0}$ exchange in the $t$ channel has been discussed in some detail. Besides, the vector polarizabilities determine 2 linear combinations of the third order spin-polarizabilities appearing in real Compton scattering.

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arXiv1997

Contributions of strange quarks to the magnetic moment of the proton

Using the Gerasimov-Drell-Hearn sum rule and experimental total cross sections for photoproduction of $η$, $K$ and $ϕ$ mesons on the proton, we obtain upper bounds for the contribution of strange quarks to the anomalous magnetic moment of the proton, $|κ_{s}| \le 0.20 κ_{p}$. The proposed experiments to measure the spin-dependence of the absorption cross section are expected to lower these bounds considerably. The existing data on $η$ production and phenomenological models for $K$ production agree with a negative sign for $κ_{s}$.

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arXiv1996

A dispersion theoretical approach to the threshold amplitudes of pion photoproduction

We give predictions for the partial wave amplitudes of pion photoproduction near threshold by means of dispersion relations at fixed t. The free parameters of this approach are determined by a fit to experimental data in the energy range 160 MeV $\le E_γ \le$ 420 MeV. The observables near threshold are found to be rather sensitive to the amplitudes in the resonance region, in particular to the $Δ$ (1232) and $N^*$ (1440). We obtain a good agreement with the existing threshold data for both charged and neutral pion production. Our predictions also agree well with the results of chiral perturbation theory, except for neutral pion production off the neutron.

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arXiv1996

Nucleon Strangeness and Unitarity

The strange-quark vector current form factors of the nucleon are analyzed within the framework of dispersion relations. Particular attention is paid to contributions made by $K\bar{K}$ intermediate states to the form factor spectral functions. It is shown that, when the $K\bar{K}\to N\bar{N}$ amplitude is evaluated in the Born approximation, the $K\bar{K}$ contributions are identical to those arising from a one-loop calculation and entail a serious violation of unitarity. The mean square strangeness radius and magnetic moment are evaluated by imposing unitarity bounds on the kaon-nucleon partial wave amplitudes. The impact of including the kaon's form factor in the dispersion integrals is also evaluated.

hep-ph↗
arXiv1996

Generalized polarizabilities and the spin-averaged amplitude in virtual Compton scattering off the nucleon

We discuss the low-energy behavior of the spin-averaged amplitude of virtual Compton scattering (VCS) off a nucleon. Based on gauge invariance, Lorentz invariance and the discrete symmetries, it is shown that to first order in the frequency of the final real photon only two generalized polarizabilities appear. Different low-energy expansion schemes are discussed and put into perspective.

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arXiv1996

Virtual Compton Scattering off the nucleon in the linear sigma model

Virtual Compton scattering off the nucleon has been studied in the one-loop approximation of the linear sigma model. The three generalized polarizabilities of the nucleon have been calculated and compared with the existing theoretical predictions. In particular, we find that only two of the three scalar polarizabilities are independent observables.

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arXiv1996

General introduction to Compton scattering

Real and virtual Compton scattering has been and will be an important tool to study the structure of hadronic systems. We summarize the status of real Compton scattering and give an outlook at the new theoretical and experimental developments in the field of virtual Compton scattering.

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arXiv1996

The position and the residues of the delta resonance pole in pion photoproduction

We have analyzed the $M_{1+}^{(3/2)}$ and $E_{1+}^{(3/2)}$ multipole amplitudes of pion photoproduction in the framework of fixed-$t$ dispersion relations. Applying the speed plot technique to our results for these multipoles, we have determined the position and the residues of the $Δ$ (1232) resonance pole. The pole is found at total $c.m.$ energy $W = (1211 - 50i)$ MeV on the second Riemann sheet, and the ratio of the electric and magnetic residues is $R_Δ = - 0.035 - 0.046 i$, resulting in an E2/M1 ratio for the "dressed" delta resonance of $- 3.5 \%$.

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arXiv1996

Dispersion-theoretical analysis of the nucleon electromagnetic form factors: Inclusion of time-like data

We update a recent dispersion--theoretical fit to the nucleon electromagnetic form factors by including the existing data in the time--like region. We show that while the time--like data for the proton can be described consistently with the existing world space--like data, this is not the case for the neutron. Another measurement of the process $e^+ e^- \to \bar n n$ is called for. We furthermore sharpen the previous estimate of the separation between the perturbative and the non--perturbative regime, which is characterized by a scale parameter $Λ^2 \simeq 10\,$GeV$^2$.

hep-ph↗
arXiv1996

The rare decay $η\toππγγ$ in chiral perturbation theory

We investigate the rare radiative eta decay modes $η\toπ^+π^-γγ$ and $η\toπ^0π^0γγ$ within the framework of chiral perturbation theory at $O(p^4)$. We present photon spectra and partial decay rates for both processes as well as a Dalitz contour plot for the charged decay.

hep-ph↗
arXiv1995

The Anomalous Decay eta -> pi pi gamma gamma

We investigate the rare radiative eta decay modes eta -> pi+ pi- gamma gamma and eta -> pi0 pi0 gamma gamma within the framework of chiral lagrangians at o(p^4) and present photon spectra for both processes.

hep-ph↗
arXiv1995

The Strangeness Radius and Magnetic Moment of the Nucleon Revisited

We update Jaffe's estimate of the strange isoscalar radius and magnetic moment of the nucleon. We make use of a recent dispersion--theoretical fit to the nucleon electromagnetic form factors and an improved description of symmetry breaking in the vector nonet. We find $μ_s = -0.24 \pm 0.03$~n.m. and $r_s^2 = 0.21 \pm 0.03$~fm$^2$. The strange formfactor $F_2^s (t)$ follows a dipole with a cut--off mass of 1.46~GeV, $F_2^s (t)= μ_s (1-t/2.14 \, {\rm GeV}^2 )^{-2}$. These numbers should be considered as upper limits on the strange vector current matrix--elements in the nucleon.

hep-ph↗
arXiv1995

Parity violating pion electroproduction off the nucleon

Parity violating (PV) contributions due to interference between $γ$ and $Z^0$ exchange are calculated for pion electroproduction off the nucleon. A phenomenological model with effective Lagrangians is used to determine the resulting asymmetry for the energy region between threshold and $Δ(1232)$ resonance. The $Δ$ resonance is treated as a Rarita-Schwinger field with phenomenological $N Δ$ transition currents. The background contributions are given by the usual Born terms using the pseudovector $πN$ Lagrangian. Numerical results for the asymmetry are presented.

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arXiv1995

Photo- and Electroproduction of Eta Mesons

Eta photo- and electroproduction off the nucleon is investigated in an effective lagrangian approach that contains Born terms and both vector meson and nucleon resonance contributions. In particular, we review and develop the formalism for coincidence experiments with polarization degrees of freedom. The different response functions appearing in single and double polarization experiments have been studied. We will present calculations for structure functions and kinematical conditions that are most sensitive to details of the lagrangian, in particular with regard to contributions of nucleon resonances beyond the dominant $S_{11}$(1535) resonance.

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arXiv1995

Dispersion-Theoretical Analysis of the Nucleon Electromagnetic Formfactors

Dispersion relations allow for a coherent description of the nucleon electromagnetic form factors measured over a large range of momentum transfer, $Q^2 \simeq 0 \ldots 35$ GeV$^2$. Including constraints from unitarity and perturbative QCD, we present a novel parametrisation of the absorptive parts of the various isoscalar and isovector nucleon form factors. Using the current world data, we obtain results for the electromagnetic form factors, nucleon radii and meson couplings. We stress the importance of measurements at large momentum transfer to test the predictions of perturbative QCD.

hep-ph↗
arXiv1994

The Drell-Hearn-Gerasimov Sum Rule

The Drell-Hearn-Gerasimov (DHG) sum rule relates the helicity structure of the photoabsorption cross section to the anomalous magnetic moment of the nucleon. It is based on Lorentz and gauge invariance, crossing symmetry, causality and unitarity. A generalized DHG sum rule my be derived for virtual photons. At low momentum transfer this generalized sum rule is saturated by the resonance region, at high momentum transfer it may be expressed by the parton spin distributions measured in deep inelastic scattering. The longitudinal-transverse interference determines the Cottingham sum rule, which is related to the electric and magnetic form factors over the whole range of momentum transfer.

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