PHOTOPRODUCTION OF VECTOR MESONS ON NUCLEONS: RELATIVE ROLE OF POSSIBLE THRESHOLD MECHANISMS T. BABACAN, H. BABACAN, A. GÖKALP and O.YILMAZ Physics Department, Middle East Technical University, 06531 Ankara, Turkey ABSTRACT: We analyze the photoproduction of co-meson on nucleons in order to understand the relative role of different possible threshold mechanisms. Therefore, we propose a set of models with the different contributions of the exchange mechanisms which are equally good for the differential cross-section of y+p—>p+co: pseudoscalar (n) and scalar exchanges (g ) in t-channel and the s+u nucleon term. We fit our model results to the experimental data about the differential cross- section in the energy ranges 1.4 GeV < E., <1.8 GeV and 1.8 GeV< E., < 2.5 GeV, simultaneously, and obtain some sets for the coupling constants, and £y)NN. All calculations are done at Ey = 1.6 GeV ( for the energy range 1.4 GeV < E., < 1.8 GeV ) , and Ey = 2.2 GeV (for the energy range 1.8 GeV< Ey< 2.5 GeV). We found some sensitivity of the differential cross-section on neutron targets and the beam asymmetry on proton and neutron targets as well, to the variations of the different models which are equivalently good for the differential cross-section on proton target. The existing experimental data [1] about the process of photoproduction of co-meson on proton do not allow now to establish definitely the relative role of different possible mechanisms which are typically considered in near threshold region: pseudoscalar and scalar t-channel exchanges, s+u nucleon terms, nucleonic resonance excitations and dfifractive exchange (Pomeron) etc. Therefore, a set of models can be suggested for y+p—>p+co, with different contributions of the above mentioned mechanisms, which are almost equivalently good for the differential cross- section of y+p—>p+co. For t-channel, we consider the pseudoscalar (n) and scalar exchanges (g), Fig. 1(a). The pseudoscalar exchange amplitude can be obtained from the Lagrangian, where Au is the photon field. Following Ref. [2] , we use the coupling constants as g20)ir, = 3.315 and g2reNN/4 tt = 14.0 The form factors that we have used are as follows: ( 1 ) 2 2 2 2 A 7F M 7F -K ( 2) FnNN 2 where Art = 0.7 and A oyKy= 0.77 in GeV unit. [3] 1028