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The apparent discrepancy between the Rosenbluth and the polarization transfer method for the ratio of the electric to magnetic proton form factors can be explained by a two-photon exchange correction which does not destroy the linearity of the Rosenbluth plot. Though intrinsically small, of the order of a few percent of the cross section, this correction is kinematically enhanced in the Rosenbluth method while it is small for the polarization transfer method, at least in the range of Q 2 where it has been used until now.

This paper presents the reactions which can be performed at COMPASS to study the Generalized Parton Distributions (GPDs). The high energy muon beam at CERN allows to measure Hard Exclusive Meson Production or Deeply Virtual Compton Scattering (DVCS) in the Bjorken regime in a large range of Q 2 and x Bj (1.5 ≤ Q 2≤ 7.5 GeV2 and 0.03 ≤ x Bj ≤ 0.25). Exploratory measurements dedicated to ρ ° or π° production can be investigated with the present setup. DVCS measurement require an upgrade of the COMPASS setup

In this paper, we present the recent studies our group has conducted in order to show the feasibility of a Deeply Virtual Compton Scattering experiment using COMPASS and its high energy muon beam at CERN. The measurement of the cross section and the beam charge asymmetry in the kinematical domain: 0.03

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].

We present a theoretical interpretation of the ( 3 He, t) reaction on nuclei in the Δ resonance region. Our model is consistent with the constraints from pion and photon excitation channels. A sizeable part of the observed downward shift of the resonance is shown to be ascribable to a hitherto undetected nuclear mode at high excitation, the pionic branch. Interpretation of seemingly adverse results is discussed.

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., Comment: 4 pages, 3 figures

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.

We measured the absolute cross sections for photon electro-production off the proton, ep to ep gamma , with the high resolution spectrometers at MAMI at momentum transfer q=600 MeV/c and photon polarization epsilon =0.62. We covered the momentum range for the outgoing real photon q'=33/111 MeV/c. From the extracted virtual Compton scattering amplitude we deduce values for two structure functions related to the generalized polarizabilities of the proton.

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.

Calculations are presented for real Compton scattering on the nucleon in a PQCD formalism by starting from different quark distribution amplitudes of the nucleon. In a second step we present the formalism to extract this distribution amplitude in a model independent way from hard exclusive Compton scattering data.

We present a detailed analysis of the low transfer data for the (ΔT = 1, 0+ ↔ 1+) transition in the A = 12 system. From the experimental observables obtained in electron scattering, in β-decay and in μ− capture, we deduce the experimental form factors of the weak and electromagnetic nuclear currents. The theoretical values of these form factors are estimated with several standard 1p-shell nuclear models. The mesonic exchange currents are introduced in the form of one-pion and one ρ-meson exchange currents. We discuss the problem of radial overlap corrections and the problem of core polarization. The main effect of exchange currents is a large enhancement of the axial charge density with respect to impulse approximation. This is a manifestation of the almost exact chiral invariance of strong interactions. Our analysis shows that this enhancement is required by the experimental value of the corresponding form factor, and this conclusion does not rely upon a particular nuclear model. Finally we point out that all the observables of the transition can be fairly reproduced if we only retain the soft-pion exchange currents, which are determined without ambiguity by chiral symmetry.

It is shown that core polarization by the nuclear pion field has opposite effects on the M1 form factors of 12C(15.11) MeV) and 13C(g.s). New data on 13C are found to agree with this prediction and a commin interpretationof the experiments is shown to be possible for the nuclei in terms of critical opalescence. Discrimination from alternative explanations of the observed anomalies should await further experiments, especially photopion reasctions.

We show that any deviation from the closed-core hypothesis for 16O(0+, g.s.) may strongly affect the theoretical ratio of the transition probabilities for the two inverse processes: 16 O(0 + , g.s. ) → 16 N(O − , 120 keV ) muon-capture) and 16 N(0 − , 120 keV ) → 16 O(0 + , g.s. ) (beta-decay). As a consequence, the possible evidence for exchange currents in this transition must be reexamined. Preliminary calculations do not exclude a still large meson exchange effect.

It is shown that a particularly large mesonic exchange effect is to be expected for the two inverse processes muon-capture and β -decay concerning the transitions between the 16 O(O + ) ground state and the 16 N(O O− ) exited state. It arises from a strong correction to the time part of the axial weak current. It is found that presently available experimental data can be qualitatively consistent with this picture.

A calculation is made of exchange current corrections for partial capture rates of muons in12C,16O,32S and40Ca nuclei. One-pion exchange is taken into account. Adler-Dothan's results are used to obtain the weak pion production amplitude. The nuclear states are described in the Tamm-Dancoff approximation. It is found that the corrections are very erratic and state dependent. A large effect is found for the 0+→0− transitions due to the size of the time part of the axial exchange current.

The implications of the constraints which affect the static bag solutions are investigated in the case of radial excitations. It is found that the Roper resonances cannot be static excited states of the bag.