G. De Meyer

First determination of generalized polarizabilities of the proton by a virtual compton scattering experiment.

Absolute differential cross sections for the reaction (e+p ->e+p+gamma) have been measured at a four-momentum transfer with virtuality Q^2=0.33 GeV^2 and polarization \epsilon = 0.62 in the range 33.6 to 111.5 MeV/c for the momentum of the outgoing photon in the photon-proton center of mass frame. The experiment has been performed with the high resolution spectrometers at the Mainz Microtron MAMI. From the photon angular distributions, two structure functions which are a linear combination of the generalized polarizabilities have been determined for the first time.

Photon-induced proton knockout from 208Pb

Abstract Cross sections have been measured for the reaction 208Pb(γ,p) leading to the low-lying ( 1 2 + , 3 2 + ) and ( 11 2 − , 5 2 + ) doublets and 7 2 + state in 207Tl using a high energy-resolution tagged photon beam of 41 to 57 MeV. The data are compared with results obtained with the reaction 208Pb(e,e′p) in the same recoil-momentum range. The recoil-momentum distributions derived from the (γ,p) data do not coincide with those obtained from the (e,e′p) data. Distorted-wave impulse-approximation (DWIA) calculations of which the input is constrained by the (e,e′p) data underestimate the (γ,p) data by typically one order of magnitude. An estimate of meson-exchange currents (MEC) effects using the Siegert theorem brings the DWIA-calculations close to the data. Random-phase approximation calculations also give a fair account of the data, but in this case MEC-effects are predicted to be less important.

Photon-induced proton knockout from 208Pb and 12C

Abstract Cross sections have been measured for the reactions 208Pb(γ,p) and 12C(γ,p) leading to the low-lying and continuum states in 207Tl and 11B using a high energy-resolution tagged photon beam of 41 to 57 MeV. The data are compared with results obtained with the (e,e′p) reaction in the same recoil-momentum range and with various theoretical calculations. A comparison of the (e,e′p) and (γ,p) data in terms of a reduced cross section versus momentum does not show a systematic overlap of the (e,e′p) and (γ,p) results, i.e. no scaling is observed. Distorted-wave impulse-approximation (DWIA) calculations, of which the input is constrained by the (e,e′p) data, and random-phase approximation calculations (RPA) underestimate the 12C(γ,p) data by typically a factor five depending on angle and missing energy. For 208Pb the difference between the calculations and the data for the low-lying states is much less, but an increasing discrepancy between the data and the calculations is found in the continuum. The inclusion of meson-exchange currents (MEC) brings the calculations for both nuclei on average closer to the data, but MEC effects are considerably less important for 208Pb as compared to 12C. Finally, a remarkable similarity is observed between the continuum response of the (γ,p) and (e,e′p) reactions on 208Pb despite the difference in the electromagnetic couplings involved.

Virtual Compton Scattering and polarizabilities

Abstract Virtual Compton Scattering (VCS) off the proton below pion threshold allows access to generalized polarizabilities of the proton. These observables measure the response of the nucleon to low energy electromagnetic field perturbations. This paper presents a comparison with Real Compton Scattering and also results for the first dedicated VCS experiment performed at the Mainz Microtron MAMI with high resolution spectrometers at Q2 = 0.33 GeV2.

The first virtual Compton scattering experiment at MAMI

Abstract Virtual Compton scattering, i.e. the exclusive reaction γ ∗ p → γ′p′ with γ ∗ denoting a virtual photon, provides new insights on the internal structure of the proton. Below π 0 production threshold, this experiment measures the generalized polarizabilities of the proton as defined by Guichon et al [1], [2] and Drechsel et al [3]. These new electromagnetic observables, functions of Q 2 , enlarge the concept of electric (α0 and magnetic (β) polarizabilities in Real Compton Scattering ( Q 2 =0) [4]. The first VCS experiment [5] of this kind was measured at the three spectrometer facility at the Mainz Microtron MAMI for Q 2 =0.33 GeV 2 and we present in this paper the preliminary results.

Alpha-cluster knockout in the 16O(e,e' alpha)12C reaction.

Abstract The knockout of α-clusters induced by electrons has been used to study four-nucleon correlations in 16O. For the transition to the ground state of 12C the (e,e′α) cross section has been measured for three values of the momentum transfer and four values of the missing momentum. The momentum-transfer dependence gives evidence for a quasi-elastic α-knockout mechanism. The shape of the measured missing-momentum distribution is reasonably well described by shell-model and cluster-model calculations, but the theoretical curves overshoot the data by a factor three to four.

Alpha-cluster knockout in the O-16(e,e 'alpha)C-12 reaction

Abstract The knockout of α-clusters induced by electrons has been used to study four-nucleon correlations in 16O. For the transition to the ground state of 12C the (e,e′α) cross section has been measured for three values of the momentum transfer and four values of the missing momentum. The momentum-transfer dependence gives evidence for a quasi-elastic α-knockout mechanism. The shape of the measured missing-momentum distribution is reasonably well described by shell-model and cluster-model calculations, but the theoretical curves overshoot the data by a factor three to four.

Virtual compton scattering under π0 threshold at Q2=0.33 GeV2. Preliminary results

We have measured the absolute unpolarized cross sections for photon electro-production off the proton ep → epγ with the Three-Spectrometer-Setup at MAMI at a momentum transfer q=600 MeV/c and a virtual photon polarization ɛ=0.62. The momentum q ′ of the outgoing real photon range from 33 to 111 MeV/c. We extracted two combinations of the recently introduced generalized polarizabilities [1,2].

Investigation of the40Ca(γ,N) reaction at 60 MeV

The values of the angle-integrated cross section for the decay to various low-lying states in39K following the40Ca(γ,π) reaction at a mean photon energy of 60 MeV are reported on, together with the values of the reduced Legendre coefficientsai. The40Ca(γ,gp) and40Ca(γ,n) reactions yield the same angular cross-section values and these agree with the corresponding RPA-calculations.

The 6Li(γ,p) reaction at intermediate photon energies

Abstract The 6 Li(γ,p) reaction was measured at average tagged photon energies of 〈 E γ 〉 = 59 and 75 MeV. Protons were detected at 5 different angles between 30° and 150°. Most of the observed strength is apparently due to the three-body breakup channels. In particular the semi-inclusive (γ,p(n)) and (γ,p(t)) channels are discussed.