# N1: Accomplishments of the second funding period

• Elastic charge form factor of the Deuteron
Preliminary cross-section results for all of the settings are now available (see left panel Fig. N1.1. Currently the final cross-checks of all sources of systematic errors are being undertaken, while appropriate fitting procedures are being developed to extract the deuteron radius.
The analysis is based on the PhD work of Yvonne Stöttinger, who will defend her thesis by the end of this year.
• Inclusive electron scattering on $$^{4}$$He at low momentum transfer
The analysis of the 2016 experiment to determine the $$^{4}$$He transition form factor $$F_M(q)$$ to the resonant state $$0^+$$ is being finalized and a publication is expected by Summer 2019. A first comparison with the recent theoretical prediction by Sonia Bacca and collaborators is shown in the right panel of Fig. N1.1.
The analysis is based on the PhD work of Simon Kegel, who will defend his thesis before Summer 2019.
• Neutron skin studies via coherent $$\pi^{0}$$ photoproduction
The coherent $$(\gamma,\pi^{0})$$ reaction has been used to obtain information on the matter form factor of four spin zero nuclei ($$^{116,120,124}$$Sn, $$^{58}$$Ni) over the photon energy range from threshold to 350 MeV during the first funding period. The analysis has been finalized and last checks are being performed in order to precisely quantify the systematic errors. A new experimental campaign is currently being performed at A2 using $$^{40,48}$$Ca targets.
The analysis is based on the PhD work of Maria Ferretti -Bondy who will defend her thesis before the end of 2019.
To improve the nuclear description beyond the simplistic Fermi distributions used in earlier studies, ground state proton and neutron densities have been provided for all tin isotopes using density functional theory (DFT). In addition, a more advanced DWIA code is currently being developed, which will enable the inclusion of the nuclear densities from the aforementioned DFT calculations and the improvement of the pion-nucleus potential used in the exit channel (see left panel Fig. N1.2).
The theoretical work is based on the PhD works of Frederic Colomer (Cotutelle with the Université libre de Bruxelles) and Viacheslav Tsaran.

Figure N1.1 Left: First preliminary fits of the measured cross-section to determine the deuteron form factor. A comparison with the world-data is also shown in the upper plot. Right: Comparison between the results obtained for the $$^{4}$$He transition form factor $$F_M(q)$$ to the resonant state $$0^+$$ and the theoretical prediction of Bacca et al. Two different assumptions for the background and for the unknown shape of the resonance are being investigated in order to precisely quantify the systematic error of the experimental data.

• Transverse asymmetry measurement on medium-heavy nuclei
The experimental program on beam-normal single-spin asymmetries for $$^{12}$$C has been completed and published in Physical Review Letters. The experimental results have been compared to a theoretical calculation that relates the beam-normal single-spin asymmetry to the imaginary part of the two-photon exchange amplitude. The result emphasizes that the $$Q^2$$ behavior of the asymmetry given by the ratio of the Compton to charge form factors cannot be treated independently of the target nucleus (see right panel Fig. N1.2).
A new experimental campaign has been initiated on $$^{208}$$Pb and $$^{90}$$Zr nuclei. For this purpose the target cooling system developed in the first funding period to study the elastic electron scattering on Lithium has been successfully used. With incoming beam intensities as high as 20 $$\mu$$A, no melting or density fluctuations have been observed in the target spot. Extensive studies on how to improve the radiation hardness of vacuum seals of the scattering chamber are being undertaken to increase the total number of continuous days of operation for future experiments.
First preliminary results on the recent $$^{90}$$Zr campaign will be shown at the meeting of the Scientific Council.

Figure N1.2 Left: First preliminary theoretical prediction of the coherent $$\pi^{0}$$ photoproduction cross section on Sn isotopes. Right: Extracted transverse asymmetries versus $$Q^2$$. The width of the given boxes indicates the full width at half maximum of the $$Q^2$$ distribution. The statistical and systematic uncertainties are given by the error bars and the height of the boxes, respectively. The theoretical calculation is shown for comparison. The given bands belong to the uncertainty of the Compton slope parameter of 10 % (light grey) and 20 % (dark grey).