Associative open charm photoproduction $$\gamma + N \to Y_c + \bar D_c (\bar D_c^ * ), Y_c = \Lambda _c^ + or \Sigma _c $$

Abstract

We analyze polarization effects in associative photoproduction of pseudoscalar (D~_) and vector (D:) charmed mesons in exclusive processes 7 + N --, Y~ + De, Y~ = A +, ~. We calculate the differential cross section and all polarization observables in framework of an effective Lagrangian approach. In case of collinear kinematics it is possible to give model independent predictions for polarization observables in case of D~ production, and the analysis for D: is largely simplified.Photoproduction of charmed particles on nucleons has been studied since more than twenty years, mostly through inclusive reactions, in the photon energy range E~ = 20-70 GeV, i.e. far from the threshold region. The production mechanism usually assumed is the photon-gluon fusion mechanism (PGF), "~ + G -~ c + ~ [1], and, in this frame, Dc and D: production are related to the gluon content of the nucleon: polarized reactions are a tool to measure the spill content of tile nucleon which is carried by the gluons.However, the calculation of the properties of the produced charmed inesons and baryons produced from PGF is not straightforward, as it involves assumptions on the hadronization process and the knowledge of fragmentation functions. Whereas it is possible to reproduce the inclusive cross section, the particle/antiparticle ratio, observed in the experiments, in particular near threshold, cannot be reproduced by PGF.In principle, other mechanisms (of non-perturbative nature) should also be taken into account, such as, for example, tile diffractive production of Dc--Dc or AcDc pairs, through Pomeron exchange. Different hadronic exchanges can also occur [2]. We discuss here the simplest two body reactions -y + N ~ Yc + Dc (Dc), among _---0 which the reaction 3' + P --* A + + Dc has the lowest threshold (Ethr = 8.9 GeV).The differential cross section and the polarization observables for these processes can be calculated in terms of hadronic exchanges in s, t and u channels. The effect, ive Lagrangian approach gives a very convenient frame for this study, since all the observables can be calculated with the help of few parameters, which have a definite physical meaning.