Measurement of the Reactions ${\pi }^{+}p\to K^{+}{\Sigma }^{+}$ and ${\pi }^{+}p\to K^{+}{Y}^{*+}$ Near Zero Degrees

We have measured the differential cross section for the reaction ${\pi }^{+}p\to K^{+}{\Sigma }^{+}$ at 3, 5, and 7 GeV/c from 0° to ∼3.5° in the laboratory. The cross section at 5 and 7 GeV/c continues to 0° with an exponential $t$ dependence consistent with data at larger angles. However, the 3-GeV/c data exhibit a change in slope near $\mathrm{}\left|t\right|\mathrm{}\simeq 0.1$ ${(\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c})}^2$. We have also measured the differential cross section for ${\pi }^{+}p\to K^{+}{Y}^{*}\mathrm{}\left(1385\right)\mathrm{}$ at 5 GeV/c in the same angular range. These data indicate a dip at 0° as would be expected if the cross section were dominated by the helicity-flip amplitudes.