Evidence for the positive-strangeness pentaquark $\hat{I}{˜}^{+}$ in photoproduction with the SAPHIR detector at ELSA

Abstract

The positive--strangeness baryon resonance $\hat{I}{˜}^{+}$ is observed in photoproduction of the $\mathrm{n}{\mathrm{K}}^{+}{\mathrm{K}}_{\mathrm{s}}^0$ final state with the SAPHIR detector at the Bonn ELectron Stretcher Accelerator ELSA. It is seen as a peak in the $\mathrm{n}{\mathrm{K}}^{+}$ invariant mass distribution with a $4.8\ddot{I}ƒ$ confidence level. We find a mass ${\mathrm{M}}_{\hat{\mathrm{I}}{˜}^{+}}=1540\pm 4\pm 2$ MeV and an upper limit of the width $\hat{\mathrm{I}}{“}_{\hat{\mathrm{I}}{˜}^{+}}<25$ MeV at 90% c.l. From the absence of a signal in the $\mathrm{p}{\mathrm{K}}^{+}$ invariant mass distribution in ${\hat{\mathrm{I}}}^3\mathrm{p}\to \mathrm{p}{\mathrm{K}}^{+}{\mathrm{K}}^{-}$ at the expected strength we conclude that the $\hat{I}{˜}^{+}$ must be isoscalar.

9 pages, 4 figures