Pentaquark production in reactions

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 γ N → K 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 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 vector meson photoproduction part is calculated from a phenomenological model which describes well the experimental data at low energies. We point out that the neutral tensor meson production cannot be due to π 0 -exchange as done by Dzierba et al. [Phys. Rev. D 69 (2004) 051901] because of C parity. The neutral tensor meson production is estimated by considering the vector meson exchange and found to be too weak to generate any peak at the position near Θ ( 1540 ) . For Θ ( 1540 ) production, we assume that it is an isoscalar and hence can only be produced in γ n → K + K - n and γ p → K 0 K ¯ 0 p reactions, but not in γ p → K + K - p and γ n → K 0 K ¯ 0 n . The total cross section data of γ p → K + K - p is thus used to fix the form factors which regularize the background amplitudes so that the signal of Θ ( 1540 ) in γ n → K + K - n and γ p → K 0 K ¯ 0 p cross sections can be predicted. We find that the predicted K + K - and K + n invariant mass distributions of the γ n → K + K - n reaction can qualitatively reproduce the shapes of the JLab data. However, the predicted Θ ( 1540 ) peak cannot 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.