L. Zana

Information on the size and shape of the neutron skin on (208)Pb is extracted from coherent pion photoproduction cross sections measured using the Crystal Ball detector together with the Glasgow tagger at the MAMI electron beam facility. On exploitation of an interpolated fit of a theoretical model to the measured cross sections, the half-height radius and diffuseness of the neutron distribution are found to be c(n)=6.70±0.03(stat.) fm and a(n)=0.55±0.01(stat.)(-0.03)(+0.02)(sys.) fm, respectively, corresponding to a neutron skin thickness Δr(np)=0.15±0.03(stat.)(-0.03)(+0.01)(sys.) fm. The results give the first successful extraction of a neutron skin thickness with an electromagnetic probe and indicate that the skin of (208)Pb has a halo character. The measurement provides valuable new constraints on both the structure of nuclei and the equation of state for neutron-rich matter.

The reactions $\ensuremath{\gamma}p\ensuremath{\rightarrow}\ensuremath{\eta}p$ and $\ensuremath{\gamma}p\ensuremath{\rightarrow}{\ensuremath{\eta}}^{\ensuremath{'}}p$ are measured from their thresholds up to the center-of-mass energy $W=1.96\text{ }\text{ }\mathrm{GeV}$ with the tagged-photon facilities at the Mainz Microtron, MAMI. Differential cross sections are obtained with unprecedented statistical accuracy, providing fine energy binning and full production-angle coverage. A strong cusp is observed in the total cross section for $\ensuremath{\eta}$ photoproduction at the energies in the vicinity of the ${\ensuremath{\eta}}^{\ensuremath{'}}$ threshold, $W=1896\text{ }\text{ }\mathrm{MeV}$ (${E}_{\ensuremath{\gamma}}=1447\text{ }\text{ }\mathrm{MeV}$). Within the framework of a revised $\ensuremath{\eta}\mathrm{MAID}$ isobar model, the cusp, in connection with a steep rise of the ${\ensuremath{\eta}}^{\ensuremath{'}}$ total cross section from its threshold, can only be explained by a strong coupling of the poorly known $N(1895)1/{2}^{\ensuremath{-}}$ state to both $\ensuremath{\eta}p$ and ${\ensuremath{\eta}}^{\ensuremath{'}}p$. Including the new high-accuracy results in the $\ensuremath{\eta}\mathrm{MAID}$ fit to available $\ensuremath{\eta}$ and ${\ensuremath{\eta}}^{\ensuremath{'}}$ photoproduction data allows the determination of the $N(1895)1/{2}^{\ensuremath{-}}$ properties.

We present deeply virtual π^{0} electroproduction cross-section measurements at x_{B}=0.36 and three different Q^{2} values ranging from 1.5 to 2 GeV^{2}, obtained from Jefferson Lab Hall A experiment E07-007. The Rosenbluth technique is used to separate the longitudinal and transverse responses. Results demonstrate that the cross section is dominated by its transverse component and, thus, is far from the asymptotic limit predicted by perturbative quantum chromodynamics. Nonetheless, an indication of a nonzero longitudinal contribution is provided by the measured interference term σ_{LT}. Results are compared with several models based on the leading-twist approach of generalized parton distributions (GPDs). In particular, a fair agreement is obtained with models in which the scattering amplitude includes convolution terms of chiral-odd (transversity) GPDs of the nucleon with the twist-3 pion distribution amplitude. This experiment, together with previous extensive unseparated measurements, provides strong support to the exciting idea that transversity GPDs can be accessed via neutral pion electroproduction in the high-Q^{2} regime.

The Dalitz decays $\ensuremath{\eta}\ensuremath{\rightarrow}{e}^{+}{e}^{\ensuremath{-}}\ensuremath{\gamma}$ and $\ensuremath{\omega}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{0}{e}^{+}{e}^{\ensuremath{-}}$ have been measured in the $\ensuremath{\gamma}p\ensuremath{\rightarrow}\ensuremath{\eta}p$ and $\ensuremath{\gamma}p\ensuremath{\rightarrow}\ensuremath{\omega}p$ reactions, respectively, with the A2 tagged-photon facility at the Mainz Microtron. The value obtained for the slope parameter of the electromagnetic transition form factor of $\ensuremath{\eta}, {\mathrm{\ensuremath{\Lambda}}}_{\ensuremath{\eta}}^{\ensuremath{-}2}=(1.97\ifmmode\pm\else\textpm\fi{}0.{11}_{\mathrm{tot}})\phantom{\rule{4pt}{0ex}}{\mathrm{GeV}}^{\ensuremath{-}2}$, is in good agreement with previous measurements of the $\ensuremath{\eta}\ensuremath{\rightarrow}{e}^{+}{e}^{\ensuremath{-}}\ensuremath{\gamma}$ and $\ensuremath{\eta}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}\ensuremath{\gamma}$ decays. The uncertainty obtained in the value of ${\mathrm{\ensuremath{\Lambda}}}_{\ensuremath{\eta}}^{\ensuremath{-}2}$ is lower than in previous results based on the $\ensuremath{\eta}\ensuremath{\rightarrow}{e}^{+}{e}^{\ensuremath{-}}\ensuremath{\gamma}$ decay. The value obtained for the $\ensuremath{\omega}$ slope parameter, ${\mathrm{\ensuremath{\Lambda}}}_{\ensuremath{\omega}{\ensuremath{\pi}}^{0}}^{\ensuremath{-}2}=(1.99\ifmmode\pm\else\textpm\fi{}0.{21}_{\mathrm{tot}})\phantom{\rule{4pt}{0ex}}{\mathrm{GeV}}^{\ensuremath{-}2}$, is somewhat lower than previous measurements based on $\ensuremath{\omega}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{0}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$, but the results for the $\ensuremath{\omega}$ transition form factor are in better agreement with theoretical calculations, compared to earlier experiments.

We have measured the nuclear transparency of the incoherent diffractive A(e,e′ρ0) process in 12C and 56Fe targets relative to 2H using a 5 GeV electron beam. The nuclear transparency, the ratio of the produced ρ0ʼs on a nucleus relative to deuterium, which is sensitive to ρA interaction, was studied as function of the coherence length (lc), a lifetime of the hadronic fluctuation of the virtual photon, and the four-momentum transfer squared (Q2). While the transparency for both 12C and 56Fe showed no lc dependence, a significant Q2 dependence was measured, which is consistent with calculations that included the color transparency effects.