Search for theΘ+pentaquark in the reactionsγp→K¯0K+nandγp→K¯0K0p

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Abstract

The exclusive reactions $\ensuremath{\gamma}p\ensuremath{\rightarrow}{\overline{K}}^{0}{K}^{+}n$ and $\ensuremath{\gamma}p\ensuremath{\rightarrow}{\overline{K}}^{0}{K}^{0}p$ have been studied in the photon energy range 1.6--3.8 GeV, searching for evidence of the exotic baryon ${\ensuremath{\Theta}}^{+}(1540)$ in the decays ${\ensuremath{\Theta}}^{+}\ensuremath{\rightarrow}n{K}^{+}$ and ${\ensuremath{\Theta}}^{+}\ensuremath{\rightarrow}p{K}^{0}$. Data were collected with the CLAS detector at the Thomas Jefferson National Accelerator Facility. The integrated luminosity was about $70\text{ }\text{ }{\mathrm{pb}}^{\ensuremath{-}1}$. The reactions have been isolated by detecting the ${K}^{+}$ and proton directly, the neutral kaon via its decay to ${K}_{S}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}$ and the neutron or neutral kaon via the missing mass technique. The mass and width of known hyperons such as ${\ensuremath{\Sigma}}^{+}$, ${\ensuremath{\Sigma}}^{\ensuremath{-}}$ and $\ensuremath{\Lambda}(1116)$ were used as a check of the mass determination accuracy and experimental resolution. Approximately 100 000 ${\ensuremath{\Lambda}}^{*}(1520)$'s and 150 000 $\ensuremath{\phi}$'s were observed in the ${\overline{K}}^{0}{K}^{+}n$ and ${\overline{K}}^{0}{K}^{0}p$ final state, respectively. No evidence for the ${\ensuremath{\Theta}}^{+}$ pentaquark was found in the $n{K}^{+}$ or $p{K}_{S}$ invariant mass spectra. Upper limits were set on the production cross section of the reaction $\ensuremath{\gamma}p\ensuremath{\rightarrow}{\overline{K}}^{0}{\ensuremath{\Theta}}^{+}$ as functions of center-of-mass angle, $n{K}^{+}$ and $p{K}_{S}$ masses. Combining the results of the two reactions, the 95% C.L. upper limit on the total cross section for a resonance peaked at 1540 MeV was found to be 0.7 nb. Within most of the available theoretical models, this corresponds to an upper limit on the ${\ensuremath{\Theta}}^{+}$ width, ${\ensuremath{\Gamma}}_{{\ensuremath{\Theta}}^{+}}$, ranging between 0.01 and 7 MeV.

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