Speaker
Description
Knowledge of whether the proton’s electromagnetic (EM) structure changes when it is bound inside an atomic nucleus is important for a better understanding of nuclear matter and its behavior. If such change is present it is expected to be relatively small and therefore difficult to experimentally determine[1,2].
A measurement of the polarization transfer in A(⃗e,e′p⃗) reaction is a proven technique for the extraction of EM information about the protons. For free protons (A = 1H) the ratio of the transverse to longitudinal polarization-transfer components is proportional to their electromagnetic form factor (FF) ratio GE/GM[3]. Since a similar dependence on is expected for bound protons, a comparison of results for free and bound proton might unveil useful information on their in-medium modification. However, it is often impossible to experimentally distinguish changes in proton EM structure from other nuclear effects such as final state interactions (FSI) and, therefore, inputs from theory are needed.
Experiments searching for in-medium modification of EM FF ratio were carried out on protons from different nuclei (2H, 4He, 12C, 16O)[4-9]. Although the observed polarization transfer ratios deviated significantly (especially for those with higher Fermi momenta) from those of free proton, these differences could be accounted for with the inclusion of different nuclear effects. Furthermore, despite a wide range of nuclear densities covered by these nuclei, we observed that deviations in the measured polarization ratios from those of free-proton scattering have a similar dependence on virtuality (a measure of proton’s off-shellness).
In our last two experiments, instead of comparing results between different nuclei or against a free-proton scattering, we decided to compare polarization transfer to protons extracted from different shells of a chosen target nucleus (12C[10] and 40Ca). This was motivated by theoretical predictions that local nuclear densities experienced by protons from different shells can differ significantly (e.g., approximately by a factor of two for s and p shell of 12C[11]), which could lead to observable changes in the proton EM FF ratio. We will present these new data and whether the universal behavior of polarization ratios as a function of virtuality, seen from different nuclei, is preserved.
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[10] T. Kolar et al. Physics Letters B 811, 135903 (2020)
[11] G. Ron et al., Phys. Rev. C 87, 028202 (2013).
