Observation of an Exotic Baryon with S=+1 in Photoproduction from the Proton

TL;DR

This paper reports the observation of a narrow, exotic baryon with strangeness +1 at 1555 MeV/c^2 in photoproduction experiments, providing evidence for states beyond the naive quark model, consistent with chiral soliton predictions.

Contribution

First experimental observation of an exotic S=+1 baryon state consistent with the $ heta^+$ predicted by chiral soliton models.

Findings

Observed a narrow baryon at 1555 MeV/c^2 with high statistical significance.

No evidence found for doubly-charged exotic baryon states.

The observed mass aligns with theoretical predictions for the $ heta^+$ baryon.

Abstract

The reaction $\gamma p \to \pi^+K^-K^+n$ was studied at Jefferson Lab using a tagged photon beam with an energy range of 3-5.47 GeV. A narrow baryon state with strangeness S=+1 and mass $M=1555\pm 10$ MeV/c$^2$ was observed in the $nK^+$ invariant mass spectrum. The peak's width is consistent with the CLAS resolution (FWHM=26 MeV/c$^2$), and its statistical significance is 7.8 $\pm$ 1.0 ~$\sigma$. A baryon with positive strangeness has exotic structure and cannot be described in the framework of the naive constituent quark model. The mass of the observed state is consistent with the mass predicted by a chiral soliton model for the $\Theta^+$ baryon. In addition, the $pK^+$ invariant mass distribution was analyzed in the reaction $\gamma p\to K^-K^+p$ with high statistics in search of doubly-charged exotic baryon states. No resonance structures were found in this spectrum.