Monday, July 8, 2013

1307.1653 (DIANA Collaboration et al.)

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

DIANA Collaboration, V. V. Barmin, A. E. Asratyan, V. S. Borisov, C. Curceanu, G. V. Davidenko, A. G. Dolgolenko, C. Guaraldo, M. A. Kubantsev, I. F. Larin, V. A. Matveev, V. A. Shebanov, N. N. Shishov, L. I. Sokolov, V. V. Tarasov, G. K. Tumanov, V. S. Verebryusov
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.
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