Observation of a narrow baryon resonance with positive strangeness formed in $K^+$Xe collisions
Abstract
The data on the charge-exchange reaction K^+ Xe --> K^0 p Xe', obtained with the Xenon bubble chamber DIANA, are reanalyzed using increased statistics and modified selections. The distribution of the pK^0 effective mass shows a prominent enhancement near 1538 MeV whose width is consistent with being entirely due to experimental resolution. A corresponding enhancement at m(pK^0) \simeq 1538 MeV and p(K^+)\simeq 490 MeV is observed in the m(pK^0)--p(K^+) scatter plot. Using a simulation of K^+ Xe collisions that includes the development of the intranuclear cascade, we show that the observed enhancement is not a spurious structure created by the selections. The statistical significance of the signal, estimated as \sqrt{2\Delta\lnL}, reaches 6.3 standard deviations. We interpret this observation as strong evidence for formation of a pentaquark baryon with positive strangeness, \Theta^+(uudd\bar{s}), in the charge-exchange reaction K^+ p --> K^0 n on a bound neutron. The mass of the \Theta^+ baryon is measured as m = 1538+-2 MeV. Using the ratio between the numbers of resonant and non-resonant charge-exchange events in the peak region, the intrinsic width of this baryon resonance has been determined as \Gamma(\Theta^+) = 0.36+-0.11 MeV.Keywords
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