Polarization Measurement of the ${\Sigma }^{+}$ Produced in the Line-Reversed Reactions ${\pi }^{+}p\to K^{+}{\Sigma }^{+}$ and $K^{-}p\to {\pi }^{-}{\Sigma }^{+}$ at 7 and 11.6 GeV/c

The polarization of the ${\Sigma }^{+}$ has been measured for the line-reversed reactions ${\pi }^{+}p\to K^{+}{\Sigma }^{+}$ and $K^{-}p\to {\pi }^{-}{\Sigma }^{+}$ at 7 and 11.6 GeV/c using the SLAC Hybrid Facility. Since the ${\Sigma }^{+}$ decay is observed in the bubble chamber, the trigger of the flash lamps on a fast $K^{+}\left({\pi }^{-}\right)$ did not bias the polarization measurements. We find that the ${\Sigma }^{+}$ polarizations from the two reactions have opposite signs but similar magnitudes and are in much better agreement with the predictions of weak exchange degeneracy than previous lower-energy comparisons.