Pentaquark Theta^+(1540) production in gamma N --> K Anti-K N

Abstract

The recent developments in the search of exotic pentaquark hadrons are briefly reviewed. We then focus on investigating how the exotic pentaquark $Θ(1540)$ baryon production can be identified in the $γN \to K \bar{K} N$ reactions, focusing on the influence of the background (non-$Θ$ 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 $Λ(1116)$, $Λ(1405)$, $Λ(1520)$, $Σ(1193)$, $Σ(1385)$, and $Δ(1232)$ states. The total cross section data of $γp \to K^+K^- p$ is then used to fix the form factors which regularize the background amplitudes so that the signal of $Θ(1540)$ in $γn \to K^+K^-n$ and $γp \to K^0\bar{K}^0p$ cross sections can be predicted. We find that the predicted $K^+K^-$ and $K^+ n$ invariant mass distributions of the $γn \to K^+ K^- n$ reaction can qualitatively reproduce the shapes of the JLab data. However, the predicted $Θ(1540)$ 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 $Θ$ is more likely to be detected, while it will be difficult to identify an odd-parity $Θ$, even if it exists, from the background continuum, if its coupling constants are small as in the present quark model predictions.