Exotic Theta^+ baryon production induced by a photon and a pion

Abstract

We investigate the photoproduction of the $Θ^+(1540)$ on a nucleon ($γn \to K^- Θ^+$, $γp \to \bar{K}^0 Θ^+$) and the pion-induced $Θ^+$ production reaction on the proton ($π^- p \to K^- Θ^+$). The total cross sections near threshold are estimated by using hadronic models with effective interaction Lagrangians and form factors that preserve 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 $μ$b but sensitive to the $K^* N Θ$ coupling whose value is not yet known. We also study the production cross section assuming that the $Θ^+$ has negative parity. The cross sections are then found to be much 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 cross section for the photon-proton reaction from the SAPHIR experiment.