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 $Q^2=1(\mathrm{GeV}/c{)}^2$ 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 $⟨r_E^2{⟩}^{1/2}=0.879(5{)}_{\mathrm{stat}}(4{)}_{\mathrm{syst}}(2{)}_{\mathrm{model}}(4{)}_{\mathrm{group}}\mathrm{fm}$ and $⟨r_M^2{⟩}^{1/2}=0.777(13{)}_{\mathrm{stat}}(9{)}_{\mathrm{syst}}(5{)}_{\mathrm{model}}(2{)}_{\mathrm{group}}\mathrm{fm}$.

This paper describes a precise measurement of electron scattering off the proton at momentum transfers of $0.003\lesssim Q^2\lesssim 1$ ${\mathrm{GeV}}^2$. The average point-to-point error of the cross sections in this experiment is $\sim 0.37\%$. 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.

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 Letter, 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 is described. Exclusion limits in the mass range of $40\mathrm{MeV}/c^2$ to $300\mathrm{MeV}/c^2$, with a sensitivity in the squared mixing parameter of as little as ${\epsilon }^2=8\times 10^{-7}$ 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.

A new exclusion limit for the electromagnetic production of a light $U(1)$ gauge boson ${\gamma }^{\prime }$ 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 $210\mathrm{MeV}/c^2<m_{e^{+}{e}^{-}}<300\mathrm{MeV}/c^2$. This experiment demonstrates the potential of high current and high resolution fixed target experiments for the search for physics beyond the standard model.

We report on a measurement of the parity-violating asymmetry in the scattering of longitudinally polarized electrons on unpolarized protons at a $Q^2$ of $0.230(\mathrm{G}\mathrm{e}\mathrm{V}/c{)}^2$ and a scattering angle of ${\theta }_e=30°–40°$. Using a large acceptance fast ${\mathrm{P}\mathrm{b}\mathrm{F}}_2$ calorimeter with a solid angle of $\Delta \Omega =0.62\mathrm{s}\mathrm{r}$, the A4 experiment is the first parity violation experiment to count individual scattering events. The measured asymmetry is $A_{\mathrm{p}\mathrm{h}\mathrm{y}\mathrm{s}}=(-5.44\pm {0.54}_{\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}}\pm {0.26}_{\mathrm{s}\mathrm{y}\mathrm{s}})\times {10}^{-6}$. The standard model expectation assuming no strangeness contributions to the vector form factors is $A_0=(-6.30\pm 0.43)\times {10}^{-6}$. The difference is a direct measurement of the strangeness contribution to the vector form factors of the proton. The extracted value is $G_E^s+0.225G_M^s=0.039\pm 0.034$ or $F_1^s+0.130F_2^s=0.032\pm 0.028$.