We calculate in the impulse approximation the kaon spectrum from the (\pi -,K+) reaction on 28Si. The strength V0 of the real part of the single particle potential of the \Sigma hyperons produced in the reaction is treated as a free parameter, and the strength W0 of the imaginary (absorptive) part is determined by the \Sigma N cross sections for the \Sigma \Lambda conversion and also for the elastic \Sigma N scattering (this elastic scattering introduces a strong dependence of W0 on the \Sigma momentum). By fitting to the kaon spectrum measured at KEK, we obtain a repulsive V0\simeq 40--60 MeV. This result is much closer to previous estimates of V0 than the results of other analyses of the KEK experiments.
PACS numbers: 21.80.+a
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