Search for the ${\Theta }^{+}$ Pentaquark in the Reaction $\gamma d\to p{K}^{-}{K}^{+}n$

A search for the ${\Theta }^{+}$ in the reaction $\gamma d\to p{K}^{-}{K}^{+}n$ was completed using the CLAS detector at Jefferson Lab. A study of the same reaction, published earlier, reported the observation of a narrow ${\Theta }^{+}$ resonance. The present experiment, with more than 30 times the integrated luminosity of our earlier measurement, does not show any evidence for a narrow pentaquark resonance. The angle-integrated upper limit on ${\Theta }^{+}$ production in the mass range of $1.52–1.56\mathrm{GeV}/c^2$ for the $\gamma d\to p{K}^{-}{\Theta }^{+}$ reaction is 0.3 nb (95% C.L.). This upper limit depends on assumptions made for the mass and angular distribution of ${\Theta }^{+}$ production. Using $\Lambda (1520)$ production as an empirical measure of rescattering in the deuteron, the cross section upper limit for the elementary $\gamma n\to K^{-}{\Theta }^{+}$ reaction is estimated to be a factor of 10 higher, i.e., $\sim 3\mathrm{nb}$ (95% C.L.).

Figure 1

The raw data for the missing mass $MM(p{K}^{-}{K}^{+})$ of the reaction $\gamma d\to p{K}^{-}{K}^{+}X$ showing a clean neutron peak (left). The invariant mass spectra of the detected $p{K}^{-}$ (right) show the $\Lambda (1520)$ peak along with higher mass hyperons. The data are shown separately for the low field (top) and high field (bottom) settings of the CLAS torus magnet.

Figure 2

The invariant masses of the $n{K}^{+}$ system for both the low field (top left) and high field (top right) data sets, after applying all event selection cuts. The cross section per mass bin for the combined data (bottom) is shown along with a polynomial fit.

Figure 3

The 95% confidence limit (C.L.) upper limit cross sections for the $\gamma d\to p{K}^{-}{\Theta }^{+}$ reaction using the combined low and high field data. The top plot is a function of mass, whereas the lower plot shows the maximum upper limit for the given ${\theta }^{*}$ bin, for a mass range between 1.52 and $1.56\mathrm{GeV}/c^2$ (solid line) or just at mass $1.54\mathrm{GeV}/c^2$ (dashed line).

Figure 4

The relative cross section fraction for $\Lambda (1520)$ production as a function of the neutron momentum from our data (points). A cutoff of $0.35\mathrm{GeV}/c$ on the horizontal axis corresponds to a factor of 10 loss in detection of the $\Lambda (1520)$ compared with no cut. Also shown by the solid curve is the fraction calculated using only the Fermi motion of the neutron. The reaction diagrams at the bottom show the symmetry between $t$-channel production for $\Lambda (1520)$ and ${\Theta }^{+}$ reactions.

Figure 5

Comparison of the previously published [8] result (points) with the current result (histogram) normalized (by a factor of $1/5.92$) to get the same total number of counts.