Pentaquark Theta^+(1540) production in gamma N --> K Anti-K N
  The recent developments in the search of exotic pentaquark hadrons are briefly reviewed. We then focus on investigating how the exotic pentaquark $\Theta(1540)$ baryon production can be identified in the $\gamma N \to K \bar{K} N$ reactions, focusing on the influence of the background (non-$\Theta$ production) mechanisms. By imposing the SU(3) symmetry and using various quark model predictions, we are able to fix the coupling constants for evaluating the kaon backgrounds, the $K\bar{K}$ production through the intermediate vector meson and tensor meson photoproduction, and the mechanisms involving intermediate $\Lambda(1116)$, $\Lambda(1405)$, $\Lambda(1520)$, $\Sigma (1193)$, $\Sigma(1385)$, and $\Delta (1232)$ states. The total cross section data of $\gamma p \to K^+K^- p$ is then used to fix the form factors which regularize the background amplitudes so that the signal of $\Theta(1540)$ in $\gamma n \to K^+K^-n$ and $\gamma p \to K^0\bar{K}^0p$ cross sections can be predicted. We find that the predicted $K^+K^-$ and $K^+ n$ invariant mass distributions of the $\gamma n \to K^+ K^- n$ reaction can qualitatively reproduce the shapes of the JLab data. However, the predicted $\Theta(1540)$ peak can not be identified unambiguously with the data. High statistics experiments are needed to resolve the problem. We also find that an even-parity $\Theta$ is more likely to be detected, while it will be difficult to identify an odd-parity $\Theta$, even if it exists, from the background continuum, if its coupling constants are small as in the present quark model predictions. 
Yongseok Oh, K. Nakayama, T.-S. H. Lee