Recoil Polarization and Beam-Recoil Double Polarization Measurement of $\eta$ Electroproduction on the Proton in the Region of the $S_{11}(1535)$ Resonance

The beam-recoil double polarization $P_{x^{\prime }}^h$ and $P_{z^{\prime }}^h$ and the recoil polarization $P_{y^{\prime }}$ were measured for the first time for the $p(\overrightarrow{e},e^{\prime }\overrightarrow{p})\eta$ reaction at a four-momentum transfer of $Q^2=0.1{\mathrm{GeV}}^2/c^2$ and a center of mass production angle of $\theta =120°$ at the Mainz Microtron MAMI-C. With a center of mass energy range of $1500\mathrm{MeV}<W<1550\mathrm{MeV}$ the region of the $S_{11}(1535)$ and $D_{13}(1520)$ resonance was covered. The results are discussed in the framework of a phenomenological isobar model (Eta-MAID). While $P_{x^{\prime }}^h$ and $P_{z^{\prime }}^h$ are in good agreement with the model, $P_{y^{\prime }}$ shows a significant deviation, consistent with existing photoproduction data on the polarized-target asymmetry.

Figure 1

Recoil polarization observables as functions of the c.m. energy $W$ at $\theta =120°$, $Q^2=0.1{\mathrm{GeV}}^2/c^2$, and $ϵ=0.718$ (statistical errors only). The solid line shows the prediction of Eta-MAID [11], the dashed line the same model with the energy dependent phase shift of Ref. [16]. The range in c.m. energy $W$ corresponds to the acceptance of the experiment, the data were corrected to the central point of $W=1525\mathrm{MeV}$ as described in the text.