# N: 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. 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.
• 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 the end of this year. A first comparison with the recent theoretical prediction by Sonia Bacca and collaborators.
• Neutron skin studies via coherent $$\pi^{0}$$ photoproduction
The analysis of the coherent $$(\gamma,\pi^{0})$$ reaction on four spin Zero nuclei ($$^{116,120,124}$$Sn, $$^{58}$$Ni) measured over the photon energy range from threshold to 350 MeV has been finalized. A comparison with precise theorectical predictions is currently being performed. A new experimental campaign is currently being performed at A2 using $$^{40,48}$$Ca targets. 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 being developed, which enables the inclusion of nuclear densities obtained from the aforementioned DFT calculations and the improvement of the pion-nucleus potential used in the exit channel.
• 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 figure below).
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.

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).

• Muonic atoms
Using ab initio few-body methods, we provided the most thorough calculations of nuclear structure corrections to the Lamb shift in light muonic atoms. We collected all our results in a commissioned review on JPG published in 2018, where we show that we substantially improved the uncertainties with respect to previous estimates based on experimental data. For muonic deuterium we performed an order-by-order chiral effective field theory expansion of the two-photon exchange correction, obtaining a solid assessment of uncertainties.
• Photonuclear reactions
Using coupled-cluster theory with the newly implemented three-particle-three-hole (3p-3h) correlations, we improved our previous calculation of the electric dipole polarizability and of the radii in medium-mass nuclei, such as 48Ca. We find that 3p-3h excitations are non-negligible in the electric dipole polarizability, while they essentially do not affect the radii. Interestingly, the neutron-skin thickness, defined as the difference of the neutron distribution from the proton distribution, is confirmed to be smaller than what predicted with density functional theory. The neutron-skin thickness will be studied in the future with parity violating electron scattering.