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Search for $Θ^{++}$ Pentaquarks in the Exclusive Reaction $γp\to K^+K^-p$

The reaction $γp \to pK^+K^-$ was studied at Jefferson Lab with photon energies from 1.8 to 3.8 GeV using a tagged photon beam. The goal was to search for a $Θ^{++}$ pentaquark, a narrow doubly charged baryon state having strangeness S=+1 and isospin I=1, in the $pK^+$ invariant mass spectrum. No statistically significant evidence of a $Θ^{++}$ was found. Upper limits on the total and differential production cross section for the reaction $γp \to K^-Θ^{++}$ were obtained in the mass range from 1.5 to 2.0 GeV/c$^2$, with an upper limit of about 0.15 nb, 95% C.L. for a narrow resonance with a mass $M_{Θ^{++}}=1.54$ GeV/c$^2$. This result places a very stringent upper limit on the $Θ^{++}$ width.