Evidence for the positive-strangeness pentaquark $Θ^+$ in photoproduction with the SAPHIR detector at ELSA

Abstract

The positive--strangeness baryon resonance $Θ^+$ is observed in photoproduction of the $\rm nK^+K^0_s$ final state with the SAPHIR detector at the Bonn ELectron Stretcher Accelerator ELSA. It is seen as a peak in the $\rm nK^+$ invariant mass distribution with a $4.8σ$ confidence level. We find a mass $\rm M_{Θ^+} = 1540\pm 4\pm 2$ MeV and an upper limit of the width $\rm Γ_{Θ^+} < 25$ MeV at 90% c.l. From the absence of a signal in the $\rm pK^+$ invariant mass distribution in $\rmγp\to pK^+K^-$ at the expected strength we conclude that the $Θ^+$ must be isoscalar.