Search for Theta+ via pi- p --> K- X reaction near production threshold

We have searched for ${\mathrm{\Theta }}^{+}$ via ${\pi }^{-}p\to K^{-}X$ reaction using 1.87 and 1.92 GeV/c ${\pi }^{-}$ beam at the K2 beam line of the KEK 12 GeV Proton Synchrotron. In the missing mass spectrum at beam momentum of 1.92 GeV/$c$, a bump has been found at 1530 MeV/$c^2$ which is consistent with the mass reported by several experiments. The statistical significance of this bump, however, is only 2.5$-2.7\sigma$. Therefore we have derived the upper limit of ${\mathrm{\Theta }}^{+}$ production cross section via ${\pi }^{-}p\to K^{-}{\mathrm{\Theta }}^{+}$ reaction which is 4.3$\mu$b at 90% confidence level assuming that ${\mathrm{\Theta }}^{+}$ is produced isotropically in the center of mass system.