Search for S=+1 exotic baryon in γp→π⁺K^-K⁺n

Abstract

The reaction γ p → π+ K-K+n has been investigated 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±; 7±; 10 MeV/c2 was observed in the (nK+)invariant mass spectrum. The width of the peak is consistent with the CLAS resolution (FWHM=26~MeV/c2 ), and its statistical significance is 7.8 ±; 1.0 σ. A baryon with positive strangeness has necessarily exotic structure and cannot be described in the framework of the naive constituent quark model. The signal is consistent with that predicted by a chiral soliton model for a 5-quark baryon state. Partial Wave Analysis was performed for the three-body mesonic background with the photon energy range of 4.8-5.47 GeV . The analysis demonstrates that the observed signal is unlikely to arise from meson production in the same final state. Interesting features were also observed in the 2-, 1- and 1+ partial wave intensity distributions. Due to the low statistics, the resonant nature of these meson waves is not determined.