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

TL;DR

This paper reports the experimental observation of a positive-strangeness pentaquark $ heta^+$ in photoproduction experiments, providing evidence for its mass, narrow width, and isospin properties, using the SAPHIR detector at ELSA.

Contribution

First experimental evidence of the $ heta^+$ pentaquark in photoproduction with precise measurements of its mass and width, confirming its isoscalar nature.

Findings

Observed as a peak in the nK+ invariant mass distribution at 1540 MeV

Width of the $ heta^+$ is less than 25 MeV at 90% confidence level

Concluded $ heta^+$ is isoscalar based on absence in pK+ channel

Abstract

The positive--strangeness baryon resonance $\Theta^+$ 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 $4.8\sigma$ confidence level. We find a mass $\rm M_{\Theta^+} = 1540\pm 4\pm 2$ MeV and an upper limit of the width $\rm \Gamma_{\Theta^+} < 25$ MeV at 90% c.l. From the absence of a signal in the $\rm pK^+$ invariant mass distribution in $\rm\gamma p\to pK^+K^-$ at the expected strength we conclude that the $\Theta^+$ must be isoscalar.