This paper describes a precise measurement of electron scattering off the proton at momentum transfers of 0.003 Q 2 1 GeV 2 . The average point-to-point error of the cross sections in this experiment is ∼0.37%. These data are used for a coherent new analysis together with all world data of unpolarized and polarized electron scattering from the very smallest to the highest momentum transfers so far measured. The extracted electric and magnetic form factors provide new insight into their exact shape, deviating from the classical dipole form, and of structure on top of this gross shape. The data reaching very low Q 2 values are used for a new determination of the electric and magnetic radii. An empirical determination of the two-photon-exchange correction is presented. The implications of this correction on the radii and the question of a directly visible signal of the pion cloud are addressed.

New precise results of a measurement of the elastic electron-proton scattering cross section performed at the Mainz Microtron MAMI are presented. About 1400 cross sections were measured with negative four-momentum transfers squared up to Q² = 1 (GeV/c)² with statistical errors below 0.2%. The electric and magnetic form factors of the proton were extracted by fits of a large variety of form factor models directly to the cross sections. The form factors show some features at the scale of the pion cloud. The charge and magnetic radii are determined to be ½ = 0.879(5)stat(4)syst(2)model(4)group fm and ½ = 0.777(13)stat(9)syst(5)model(2)group fm.

A massive, but light abelian U (1) gauge boson is a well motivated possible signature of physics beyond the Standard Model of particle physics. In this paper, the search for the signal of such a U (1) gauge boson in electron-positron pair-production at the spectrometer setup of the A1 Collaboration at the Mainz Microtron (MAMI) is described. Exclusion limits in the mass range of 40 MeV/c 2 up to 300 MeV/c 2 with a sensitivity in the mixing parameter of down to 2 = 8 × 10 −7 are presented. A large fraction of the parameter space has been excluded where the discrepancy of the measured anomalous magnetic moment of the muon with theory might be explained by an additional U (1) gauge boson.

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A new exclusion limit for the electromagnetic production of a light U (1) gauge boson γ ′ decaying to e + e − was determined by the A1 Collaboration at the Mainz Microtron. Such light gauge bosons appear in several extensions of the standard model and are also discussed as candidates for the interaction of dark matter with standard model matter. In electron scattering from a heavy nucleus, the existing limits for a narrow state coupling to e + e − were reduced by nearly an order of magnitude in the range of the lepton pair mass of 210 MeV/c 2 < m e + e − < 300 MeV/c 2 . This experiment demonstrates the potential of high current and high resolution fixed target experiments for the search for physics beyond the standard model.

Possible differences between free and bound protons may be observed in the ratio of polarizationtransfer components, P x /P z . We report the measurement of P x /P z , in the 2 H( e, e p)n reaction at low and high missing momenta. Observed increasing deviation of P x /P z from that of a free proton as a function of the virtuality, similar to that observed in 4 He, indicates that the effect in nuclei is due to the virtuality of the knock-out proton and not due to the average nuclear density. The measured differences from calculations assuming free-proton form factors (∼ 10%), may indicate in-medium modifications.

We report the first measurements of the transverse (P x and P y ) and longitudinal (P z ) components of the polarization transfer to a bound proton in the deuteron via the 2 H( e, e' p) reaction, over a wide range of missing momentum. A precise determination of the electron beam polarization reduces the systematic uncertainties on the individual components to a level that enables a detailed comparison to a state-of-the-art calculation of the deuteron using free-proton electromagnetic form factors. We observe very good agreement between the measured and the calculated P x /P z ratios, but deviations of the individual components. Our results cannot be explained by medium modified electromagnetic form factors. They point to an incomplete description of the nuclear reaction mechanism in the calculation.Measurements of the polarization transfer P = (P x , P y , P z ) from a polarized electron to a bound nucleon by the A( e, e' p) reaction and their comparison to those of a free proton were suggested as a powerful tool to observe modifications in the bound proton structure [1]. These require detailed calculations incorporating nuclear effects. However, it still might be conceptually difficult to separate such effects from internal nucleon structure changes.

We report on a new experimental method based on initial-state radiation (ISR) in e-p scattering, in which the radiative tail of the elastic e-p peak contains information on the proton charge form factor (G p E ) at extremely small Q 2 . The ISR technique was validated in a dedicated experiment using the spectrometers of the A1-Collaboration at the Mainz Microtron (MAMI). This provided first measurements of G p E for 0.001 ≤ Q 2 ≤ 0.004 (GeV/c) 2 .