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

Abstract

We investigate the photoproduction of the ${\Theta }^{+}(1540)$ on a nucleon ($\gamma n\to K^{-}{\Theta }^{+}$, $\gamma p\to {\bar{K}}^0{\Theta }^{+}$) and the pion-induced ${\Theta }^{+}$ production reaction on the proton (${\pi }^{-}p\to K^{-}{\Theta }^{+}$). 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 $\mu$b but sensitive to the $K^{\ast }N\Theta$ coupling whose value is not yet known. We also study the production cross section assuming that the ${\Theta }^{+}$ has negative parity. The cross sections are then found to be much suppressed compared to the case where ${\Theta }^{+}$ has positive parity. Hence, the interpretation of the ${\Theta }^{+}$ 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.

Comments: 17 pages, REVTeX 4, 10 figures, revised version, to appear in Phys. Rev. D, typos corrected