MREX: Mainz Radius Experiment
Final Report Abstract
The nuclear Equation of State (EoS) relates fundamental properties of nuclear matter such as energy, temperature, pressure, density, and isospin. Despite a difference in size of 18 orders of magnitude, nuclei and neutron stars are governed by the same physics enshrined in the nuclear EoS. A particularly important property of the EoS is the symmetry energy, quantifying the cost of converting symmetric nuclear into pure neutron matter. Its density dependence controls both the radius of neutron stars and the neutron-skin thickness in heavy nuclei, which is defined as the difference between neutron and proton rms radii of the nucleus. A precise determination of these quantities will provide important information on the poorly known symmetry energy of neutronrich systems. Heavy nuclei, such as ²⁰⁸Pb provide a portal to study neutron-rich matter around nuclear saturation density in the laboratory. Here, parity-violating electron scattering (PVES) experiments enable the least model dependent determination of the neutron-skin thickness. From the measured parity-violating asymmetry APV the weak form factor is determined at one single value of momentum transfer. Since the form factor is the Fourier transform of the density distribution, which is characterised by the radius and surface thickness parameters, some assumptions concerning the surface thickness must be made to reliably extract the neutron-skin thickness. Performing a PVES experiment at another kinematics will be more sensitive to the surface thickness and would therefore remove the dominant model dependence to extract the neutronskin thickness. Moreover, the contribution of higher order processes, such as two-photon exchange, in PVES experiments can be comparable in size to the observed parity-violating asymmetry APV. Hence, a precise knowledge of this contribution is mandatory to determine the systematic uncertainties. Dedicated measurements of the beam-normal single spin asymmetry (or the so-called transverse asymmetry) An give experimental access to the absorptive part of the two-photon exchange amplitude, as An arises from the interference of the one-photon and two-photon exchange amplitudes. The observation of a transverse asymmetry compatible with zero in ²⁰⁸Pb by the PREX/CREX collaboration is in striking disagreement with available theoretical predictions. Since false asymmetries can contribute substantially to the systematic uncertainty of the determination of the neutron-skin thickness in PVES experiments, further studies of the transverse asymmetry of ²⁰⁸Pb are of utmost importance. Conducting such high-precision asymmetry measurements at A1@MAMI required the development and implementation of a novel FPGA-based data acquisition system. This experimental program contributes significantly to conducting the most stringent model-independent determination of the neutron-skin thickness, thus providing strong constraints on the density dependence of the symmetry energy.
Publications
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Systematic studies of beam-normal single spin asymmetries at MAMI. Proceedings of 25th International Symposium on Spin Physics — PoS(SPIN2023), 177. Sissa Medialab.
Thiel, Michaela; Esser, Anselm; Hoek, Matthias; Merkel, Harald; Müller, Ulrich; Schlimme, Sören & Sfienti, Concettina
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A novel FPGA-based data acquisition for high-precision asymmetry measurements at low rates in electron scattering. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 1072, 170187.
Esser, A.; Del Vincio, A.; Doria, L.; Hoek, M.; Keil, F.; Kozyrev, N.; Merkel, H.; Mihovilovič, M.; Müller, U.; Pochodzalla, J.; Schlimme, B.S.; Shao, T.; Stengel, S.; Thiel, M. & Sfienti, C.
