ExoticΘ⁺ baryon production induced by a photon and a pion

We investigate the photoproduction of the Θ⁺(1540) on a nucleon (γn→K ^-Θ⁺γp→K̄⁰Θ ⁺) and the pion-induced Θ⁺ production reaction on the proton (π p→K^-Θ⁺). The total cross sections near threshold are estimated by using hadronic models with effective interaction Lagrangians and form factors that preserve the gauge invariance of the electromagnetic current. The photoproduction cross sections are found to be a few hundred nb, with the cross section on the proton being larger than that on the neutron. The pion-induced production cross section is found to be around a few hundred μb but sensitive to the K*NΘ coupling whose value is not yet known. We also study the production cross section assuming that Θ⁺ has negative parity. The cross sections are then found to be very suppressed compared to the case where Θ⁺ has positive parity. Hence, the interpretation of the Θ⁺ as an odd-parity pentaquark state seems to be disfavored from the estimates of the cross section for the photon-proton reaction from the SAPHIR experiment.