Experimental Evidence for Penta-Quark System

In the K^- missing mass spectrum for the γ + n → K⁺ + K^- + n reaction for which both the kaons are detected at forward angles, a peak is observed at 1540±10 MeV, a width of lower or equal than 25 MeV and a statistical significance of 4.6σ. The strangeness quantum number (S) of the resonance is +1. It can be explained as an exotic 5-quark state, with constituents ( {uuddbar s} ; ) that subsequently decays into a K⁺ and a neutron.