Search for ${\Theta }^{+}(1540)$ Pentaquark in High-Statistics Measurement of $\gamma p\to {\overline{K}}^0{K}^{+}n$ at CLAS

The exclusive reaction $\gamma p\to {\overline{K}}^0{K}^{+}n$ was studied in the photon energy range between 1.6 and 3.8 GeV searching for evidence of the exotic baryon ${\Theta }^{+}(1540)\to n{K}^{+}$. The decay to $n{K}^{+}$ requires the assignment of strangeness $S=+1$ to any observed resonance. Data were collected with the CLAS detector at the Thomas Jefferson National Accelerator Facility corresponding to an integrated luminosity of $70{\mathrm{pb}}^{-1}$. No evidence for the ${\Theta }^{+}$ pentaquark was found. Upper limits were set on the production cross section as function of center-of-mass angle and $n{K}^{+}$ mass. The 95% C.L. upper limit on the total cross section for a narrow resonance at 1540 MeV was found to be 0.8 nb.

Figure 1

Top: ${\pi }^{+}{\pi }^{-}$ invariant mass and the ${\overline{K}}^0$ peak. Bottom: missing mass for the reaction $\gamma p\to {\overline{K}}^0{K}^{+}X$ after ${\overline{K}}^0$ selection showing a peak at the neutron mass. The mass positions and widths of the measured peaks are given. For comparison, the arrows indicate the accepted value [36] for the mass position. The shaded area corresponds to the events used in the analysis.

Figure 2

Top: $K^{+}$ missing mass distribution with the ${\Lambda }^{*}(1520)$ peak. Bottom: $n{\pi }^{+}$ (left) and $n{\pi }^{-}$ (right) invariant mass distributions with ${\Sigma }^{+}(1189)$ and ${\Sigma }^{-}(1197)$ peaks. The mass position and width of the measured peaks are indicated. For comparison, the arrows indicate the accepted value [36] for the mass position. The shaded area corresponds to the events used in the analysis.

Figure 3

The $n{K}^{+}$ invariant mass distribution after all cuts. It is smooth and no narrow structures are evident. The arrow shows the position where evidence for the ${\Theta }^{+}$ was found by previous experiments. The inset shows the $n{K}^{+}$ mass distribution with specific cuts to reproduce the SAPHIR analysis [4] as described in the text.

Figure 4

The 95% C.L. upper limit on the total cross section as a function of the ${\Theta }^{+}$ mass (top) and on the differential cross section $d\sigma /d\cos {\theta }_{\mathrm{c}.\mathrm{m}.}^{{\overline{K}}^0}$ (bottom) for the reaction $\gamma p\to {\overline{K}}^0{\Theta }^{+}$ for an assumed ${\Theta }^{+}$ mass of 1540 MeV. The dotted line in the top plot is to guide the eye.