First Measurement of Transferred Polarization in the Exclusive $\overrightarrow{e}p\to e^{\prime }{K}^{+}\overrightarrow{\Lambda }$ Reaction

The first measurements of the transferred polarization for the exclusive $\overrightarrow{e}p\to e^{\prime }{K}^{+}\overrightarrow{\Lambda }$ reaction have been performed at Jefferson Laboratory using the CLAS spectrometer. A 2.567 GeV beam was used to measure the hyperon polarization over $Q^2$ from 0.3 to $1.5(\mathrm{G}\mathrm{e}\mathrm{V}/c{)}^2$, $W$ from 1.6 to 2.15 GeV, and over the full $K^{+}$ center-of-mass angular range. Comparison with predictions of hadrodynamic models indicates strong sensitivity to the underlying resonance contributions. A nonrelativistic quark-model interpretation of our data suggests that the $s\overline{s}$ quark pair is produced with spins predominantly antialigned. Implications for the validity of the most widely used quark-pair creation operator are discussed.

Figure 1

Missing-mass spectra (GeV) for the reactions (a) $p(e,e^{\prime }{K}^{+})X$ and (b) $p(e,e^{\prime }{K}^{+}p)X$. (c) The hyperon distribution after cutting on the low-mass peak in (b). CLAS data from 2.567 GeV summing over all $Q^2$ and $W$.

Figure 2

Center-of-mass coordinate system highlighting the definitions of the different spin-quantization axis choices for the final-state $\Lambda$ hyperon used in the polarization analysis.

Figure 3

Transferred $\Lambda$ polarization components $P_{z^{\prime }}^{\prime }$ and $P_{x^{\prime }}^{\prime }$ (left panel) and $P_z^{\prime }$ and $P_x^{\prime }$ (right panel) at 2.567 GeV vs $W$ summed over all $Q^2$ and $d{\Omega }_K^{*}$. Curves correspond to the hadrodynamic models: WJC92 (dotted), BM98 (dashed), BM02 (solid), J02 (dot-dashed), averaged over the experimental bins.

Figure 4

Transferred $\Lambda$ polarization components $P_{z^{\prime }}^{\prime }$ and $P_{x^{\prime }}^{\prime }$ (upper panels) and $P_z^{\prime }$ and $P_x^{\prime }$ (lower panels) at 2.567 GeV vs $\cos {\theta }_K^{*}$ summed over all $Q^2$ and $\Phi$ for three $W$ bins centered at 1.69, 1.84, and 2.03 GeV. The curves are the same as in Fig. 3.