arXiv subjects
Publications and source records attributed to M. Vanderhaeghen.
We report on a dispersion relation formalism for the virtual Compton scattering (VCS) reaction on the proton, which for the first time allows a dispersive evaluation of 4 generalized polarizabilities. The dispersion formalism provides a new tool to analyze VCS experiments above pion threshold, thus increasing the sensitivity to the generalized polarizabilities of the nucleon.
A dispersion relation formalism for the virtual Compton scattering (VCS) reaction on the proton is presented, which for the first time allows a dispersive evaluation of 4 generalized polarizabilities at a four-momentum transfer $Q^2 \leq$ 0.5 GeV$^2$. The dispersive integrals are calculated using a state-of-the-art pion photo- and electroproduction analysis. The dispersion formalism provides a new tool to analyze VCS experiments above pion threshold, thus increasing the sensitivity to the generalized polarizabilities of the nucleon.
The reaction $γ+p\toγ'+p+π^0$ has been suggested as a means to deduce the $Δ^+$ magnetic moment. The cross section for this process is estimated in both the constituent quark model and an effective Lagrangian procedure. The resulting total cross section is of the order 5-10 nb, which is at the limit of present experimental capabilities.
A simple and elegant model, based on Reggeized t-channel exchanges is succesful in reproducing the main features of all existing data of the reactions ep->eKLambda and ep->eKSigma. In particular, the original way gauge invariance is taken into account is found to be essential to describe the ratio between the Coulomb and the Transverse cross-sections at large Q2 that has been measured recently at JLab.
The QED radiative corrections to virtual Compton scattering (reaction $e p \to e p γ$) are calculated to first order in $α_{em} \equiv e^2 / 4 π$. A detailed study is presented for the one-loop virtual corrections and for the first order soft-photon emission contributions. Furthermore, a full numerical calculation is given for the radiative tail, corresponding with photon emission processes, where the photon energy is not very small compared with the lepton momenta. We compare our results with existing works on elastic electron-proton scattering, and show for the $e p \to e p γ$ reaction how the observables are modified due to these first order QED radiative corrections. We show results for both unpolarized and polarized observables of the virtual Compton scattering in the low energy region (where one is sensitive to the generalized polarizabilities of the nucleon), as well as for the deeply virtual Compton scattering.
Fixed-$t$ subtracted dispersion relations are presented for Compton scattering off the nucleon at energies $E_γ\leq 500$ MeV, as a formalism to extract the nucleon polarizabilities from the data with a minimum of model dependence. The scalar polarizabilities difference $α- β$ and the backward spin polarizability $γ_π$ enter directly as fit parameters in this formalism.
The cross sections and transverse spin asymmetries in the hard exclusive electroproduction of decuplet baryons are calculated in the large N_c limit and found to be comparable to that of octet baryons. Large N_c selection rules for the production amplitudes are derived, leading to new sensitive tests of the spin aspects of the QCD chiral dynamics both in the nonstrange and strange sectors. Importance of such studies for the reliable extraction of the pion form factor from pion electroproduction is explained.
Compton scattering results are used to probe proton structure via measurement of higher order polarizabilities. Values for $α_{E2}^p,β_{E2}^p,α_{Eν}^p,$ $β_{Eν}^p$ determined via dispersion relations are compared to predictions based upon chiral symmetry and from the constituent quark model. Extensions to spin-polarizabilities are also discussed.
We give predictions for the leading order amplitudes for deeply virtual Compton scattering and hard meson electroproduction reactions at large Q^2 in the valence region in terms of skewed quark distributions. We give first estimates for the power corrections to these leading order amplitudes. In particular, we outline examples of experimental opportunities to access the skewed parton distributions at the current high-energy lepton facilities : JLab, HERMES and COMPASS.
We estimate the leading order amplitudes for exclusive photon and meson electroproduction reactions at large Q^2 in the valence region in terms of skewed quark distributions. As experimental investigations can currently only be envisaged at moderate values of Q^2, we estimate power corrections due to the intrinsic transverse momentum of the partons in the meson wavefunction and in the nucleon. To this aim the skewed parton distribution formalism is generalized so as to include the parton intrinsic transverse momentum dependence. Furthermore, for the meson electroproduction reactions, we calculate the soft overlap type contributions and compare with the leading order amplitudes. We give first estimates for these different power corrections in kinematics which are relevant for experiments in the near future.
We present fixed-$t$ subtracted dispersion relations for Compton scattering off the nucleon at energies $E_γ\leq$ 500 MeV, as a formalism to extract the nucleon polarizabilities with a minimum of model dependence. The subtracted dispersion integrals are mainly saturated by $πN$ intermediate states in the $s$-channel $γN \to πN \to γN$ and $ππ$ intermediate states in the $t$-channel $γγ\to ππ\to N \bar N$. For the subprocess $γγ\to ππ$, we construct a unitarized amplitude and find a good description of the available data. We show results for Compton scattering using the subtracted dispersion relations and display the sensitivity on the scalar polarizability difference $α- β$ and the backward spin polarizability $γ_π$, which enter directly as fit parameters in the present formalism.
In this paper, after a brief summary of the properties of the Off-Forward Parton Distributions, we give some examples of what could be the experimental opportunities to access them at the current high-energy lepton facilities : JLab (E_e >= 6 GeV), HERMES (E_e = 27 GeV) and COMPASS (E_mu = 200 GeV).
We review the recent developments of virtual Compton scattering. We focus on the kinematical regimes which look the most promising. The threshold regime gives access to the generalized polarizabilities of the proton. The hard scattering regime allows tests of perturbative QCD predictions and of the valence quark wave function. The Bjorken regime is closely related to deep inelastic scattering. It gives access to the off-forward parton distributions which generalize the ordinary parton distributions and may shed a new light on the spin problem. For each regime we discuss the experimental perspectives and the role of the Bethe-Heitler background.
We present a model to calculate cross sections for electroinduced two-nucleon emission from finite nuclei. Short-range correlations in the wave functions and meson-exchange contributions to the photoabsorption process are implemented. Effects of the short-range correlations are studied with the aid of a perturbation expansion method with various choices of the Jastrow correlation function. The model is used to investigate the relative importance of the different reaction mechanisms contributing to the A(e,e$'$pn) and A(e,e$'$pp) process. Representative examples for the target nuclei $^{12}$C and $^{16}$O and for kinematical conditions accessible with contemporary high-duty cycle electron accelerators are presented. A procedure is outlined to calculate the two-nucleon knockout contribution to the semi-exclusive (e,e$'$p) cross section. Using this technique we investigate in how far semi-exclusive (e,e$'$p) reactions can be used to detect high-momentum components in the nuclear spectral function.
The similarities in the experimental indications for multinucleon mechanisms in $(γ,p)$ and $(e,e'p)$ processes are pointed out. For both types of reactions, the substantial role of two-nucleon emission processes for transitions to high excitation energies in the residual nucleus is stressed. A microscopic model for the calculation of the two-body knockout contributions to the inclusive $(γ,N)$ reaction is presented. It is based on an unfactorized formalism for the calculation of electromagnetically induced two-nucleon emission cross sections. The model is shown to yield a reasonable description of the overall behaviour of the $^{12}$C$(γ,p)$ and $^{12}$C$(γ,n)$ data at high excitation energies in the residual nucleus. In the calculations effects from non-resonant and resonant pion exchange currents are included. Photoabsorption on these currents are predicted to produce the major contributions to the exclusive $^{16}$O$(γ,n_0)^{15}$O process at photonenergies above the pion threshold. Double differential cross sections for photon induced $pp$ and $pn$ emission from $^{16}$O are calculated and compared with the data.
A model is presented to describe electromagnetically induced two-nucleon emission processes in a shell-model picture. Distortions in the outgoing nucleon waves are accounted for by performing a partial-wave expansion in a real mean-field potential. The antisymmetry condition for the A-body wavefunctions is shown to be naturally preserved. The model is used to calculate ($γ$,pn) and ($γ$,pp) cross sections off the target nuclei $^{16}$O and $^{12}$C for photon energies ranging from 50 MeV up to the $\bigtriangleup$(1232) isobar threshold. Effects due to the pionic currents and intermediate $\bigtriangleup$ creation are implemented. The impact of the distortions due to the interaction of the outgoing nucleon waves with the (A-2) core is examined. Hadronic form factors are introduced to regularize the $π$NN vertices and the sensitivity of the cross section to the pion cut-off mass is examined. The relative contribution of the ($γ$,pp) and the ($γ$,pn) channel to the total photoabsorption strength is discussed. Further, the photon energy dependence of the ($γ$,pp)/($γ$,pn) ratio is investigated.
Calculations have been performed for the $^{16}$O($γ$,pn) and the $^{16}$O($γ$,pp) reaction in the photon-energy range $E_γ$ = 60-300 MeV. Besides the contribution from the more common photoabsorption on the pionic degrees of freedom, we have investigated the influence of heavier meson exchange ($ρ, σ, ω$) and intermediate $Δ$ creation with $π$ and $ρ$ exchange. Whereas the $π$ meson is found to set the main trends, the $ρ$ meson is found not to be discardable in a theoretical description of the ($γ$,pn) reaction. The incorporation of an energy dependence and a decay width in the $Δ$ propagator is observed to be essential in order to arrive at a more realistic description of ($γ$,NN) reactions at higher photon energies.