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.

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 .

At the Mainz Microtron MAMI, the first high-resolution pion spectroscopy from decays of strange systems was performed by electron scattering off a 9 Be target in order to study the Λ binding energy of light hypernuclei. Positively charged kaons were detected by a short-orbit spectrometer with a broad momentum acceptance at zero degree forward angles with respect to the beam, efficiently tagging the production of strangeness in the target nucleus. In coincidence, negatively charged decay-pions were detected by two independent high-resolution spectrometers. About 10 3 pionic weak decays of hyperfragments and hyperons were observed. The pion momentum distribution shows a monochromatic peak at p π ≈ 133 MeV/c, corresponding to the unique signature for the two-body decay of hyperhydrogen 4 Λ H → 4 He + π − , stopped inside the target. Its Λ binding energy was determined to be B Λ = 2.12 ± 0.01 (stat.) ± 0.09 (syst.) MeV with respect to the 3 H + Λ mass.