Observation of a narrow baryon resonance with positive strangeness formed in $K^+$Xe collisions

The charge-exchange reaction K + Xe → K 0 pXeis investigated using the data of the DIANA experiment. The distribution of the pK 0 effective mass shows a promi- nent enhancement near 1538 MeV formed by nearly 80 events above the background, whose width is consistent with being entirely due to the experimental resolution. Under the selections based on a simulation of K + Xe collisions, the statistical signif- icance of the signal reaches 5.5�. We interpret this observation as strong evidence for formation of a pentaquark baryon with positive strangeness, � + (uudd¯), in the charge-exchange reaction K + n → K 0 p on a bound neutron. The mass of the � + baryon is measured as m(� + ) = 1538 ± 2 MeV. Using the ratio between the num- bers of resonant and non-resonant charge-exchange events in the peak region, the intrinsic width of this baryon resonance is determined as ( � + ) = 0.34±0.10 MeV.