Observation of a narrow baryon resonance with positive strangeness formed inK+Xe collisions

Abstract

The charge-exchange reaction ${K}^{+}\mathrm{Xe}\ensuremath{\rightarrow}{K}^{0}p{\mathrm{Xe}}^{\ensuremath{'}}$ is investigated using the data of the DIANA experiment. The distribution of the $p{K}^{0}$ effective mass shows a prominent 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 significance of the signal reaches 5.5$\ensuremath{\sigma}$. We interpret this observation as strong evidence for formation of a pentaquark baryon with positive strangeness, ${\ensuremath{\Theta}}^{+}(uudd\overline{s})$, in the charge-exchange reaction ${K}^{+}n\ensuremath{\rightarrow}{K}^{0}p$ on a bound neutron. The mass of the ${\ensuremath{\Theta}}^{+}$ baryon is measured as $m({\ensuremath{\Theta}}^{+})=1538\ifmmode\pm\else\textpm\fi{}2$ MeV. Using the ratio between the numbers of resonant and nonresonant charge-exchange events in the peak region, the intrinsic width of this baryon resonance is determined as $\ensuremath{\Gamma}({\ensuremath{\Theta}}^{+})=0.34\ifmmode\pm\else\textpm\fi{}0.10$ MeV.