Abstract
We report measurements of the induced polarization P→ of protons knocked out from 2H and 12C via the A(e,e′p→) reaction. We have studied the dependence of P→ on two kinematic variables: the missing momentum pmiss and the “off-coplanarity” angle ϕpq between the scattering and reaction planes. For the full 360° range in ϕpq, both the normal (Py) and, for the first time, the transverse (Px) components of the induced polarization were measured with respect to the coordinate system associated with the scattering plane. Px vanishes in coplanar kinematics, however in non-coplanar kinematics, it is on the same scale as Py.We find that the dependence on ϕpq is sine-like for Px and cosine-like for Py. For carbon, the magnitude of the induced polarization is especially large when protons are knocked out from the p3/2 shell at very small pmiss. For the deuteron, the induced polarization is near zero at small |pmiss|, and its magnitude increases with |pmiss|. For both nuclei such behavior is reproduced qualitatively by theoretical results, driven largely by the spin-orbit part of the final-state interactions. However, for both nuclei, sizeable discrepancies exist between experiment and theory.
Highlights
Within the shell model a spin-orbit term is required in the mean-field potential of atomic nuclei in order to explain the energy splitting of the single-particle levels for reproducing the magic numbers [1,2,3]
In view of the fact that it is largely insensitive to details of the nucleon electromagnetic form factors, the induced polarization serves as an effective probe of final-state interactions (FSI) effects in quasi-elastic A(e, e p )
Previous measurements of the normal component, Py, of the induced polarization have been performed at MIT-Bates on 2H [13] at low pmiss and 12C with large pmiss in coplanar kinematics [14]
Summary
Within the shell model a spin-orbit term is required in the mean-field potential of atomic nuclei in order to explain the energy splitting of the single-particle levels for reproducing the magic numbers [1,2,3]. We present measurements of the induced polarization of quasielastic protons from 2H and 12C over a wide range in the missing momentum, pmiss. Previous measurements of the normal component, Py, of the induced polarization have been performed at MIT-Bates on 2H [13] at low pmiss and 12C with large pmiss in coplanar kinematics [14]. The induced polarization measurements presented here were performed at the Mainz Microtron (MAMI), during four run periods from 2012-2017 Our measurements for both nuclei cover a large range in missing momentum, pmiss, and the full 360◦ range in the off-coplanarity angle, φpq (See Fig. 1). We present the data for both nuclei and their dependence on pmiss and φpq in Secs.
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