Search for $\Theta^{+}$ via $\pi^-p \to K^{-}X$ reaction near production threshold

We have searched for $\Theta^{+}$ via $\pi^{-}p \to K^{-}X$ reaction using 1.87 and 1.92GeV/c $\pi^{-}$ beam at the K2 beamline of KEK-PS 12GeV Proton Synchrotron. We irradiated 2.9$\times 10^{9}$ and 3.0$\times 10^{9}$ $\pi^{-}$ beam to the SCIFI and the polyethylene targets respectively at beam momentum 1.87GeV/c. At 1.92GeV/$c$, 7.4$\times 10^{9}$ $\pi^{-}$ beam was irradiated to only polyethylene target. In the missing mass spectrum at beam momentum 1.92GeV/$c$, a bump was found at 1530MeV/$c^{2}$ which is consistent with the mass reported by several experiments. The statistical significance of this bump, however, was only 2.5$-$2.7$\sigma$(preliminary). Therefore we derived the upper limit of $\Theta^{+}$ production cross section via $\pi^{-}p \to K^{-}\Theta^{+}$ reaction and obtained 4.1$\mu$b(preliminary) at 90\% confidence level assuming that $\Theta^{+}$ was produced isotropically in the center of mass system.