High-resolution search for the ${\mathrm{\Theta }}^{+}$ pentaquark via a pion-induced reaction at J-PARC

The pentaquark ${\mathrm{\Theta }}^{+}$ has been searched for via the ${\pi }^{-}p\to K^{-}X$ reaction with beam momenta of 1.92 and 2.01 GeV/$c$ at J-PARC. A missing mass resolution of 2 MeV (FWHM) was achieved but no sharp peak structure was observed. The upper limits on the production cross section averaged over the scattering angle from 2${}^{\circ }$ to 15${}^{\circ }$ in the laboratory frame were found to be less than 0.28 $\mu$b/sr at the 90% confidence level for both the 1.92- and 2.01-GeV/$c$ data. The systematic uncertainty of the upper limits was controlled within 10%. Constraints on the ${\mathrm{\Theta }}^{+}$ decay width were also evaluated with a theoretical calculation using effective Lagrangian. The present result implies that the width should be less than 0.36 and 1.9 MeV for the spin-parity of $1\mathrm{/}{2}^{+}$ and $1\mathrm{/}{2}^{-}$, respectively.