Photoproduction of the scalar mesons f0(500) and f0(980) off the nucleon

We investigate photoproduction of scalar mesons off the nucleon, using the effective Lagrangians and the Regge approach. We first study f(0)(980) photoproduction, replacing the Feynman propagator with Regge ones for rho-meson exchange in the t channel. The model parameters are fixed by reproducing the experimental data on the differential cross section. We then apply the same method to f(0)(500) or sigma photoproduction with the same parameters for the Regge propagator. Since the threshold energy of the f(0)(500) production is rather low, N* resonances, which can decay into a nucleon and two pions in the isoscalar and scalar states, can come into play in the s channel. To examine the effects of the N* resonances, we set up two models to look into the respective contribution of the N* resonances. We find that, in particular, N(1440) and N(1680) play important roles in f(0)(500) photoproduction. We discuss the physical implications of the present results.