ar X iv : n uc l - ex / 0 21 20 10 v 1 1 1 D ec 2 00 2 1 Three - nucleon mechanisms in photoreactions

The 12 C(γ, ppn) reaction has been measured for Eγ =150-800 MeV in the first study of this reaction in a target heavier than 3 He. The experimental data are compared to a microscopic many body calculation. The model, which predicts that the largest contribution to the reaction arises from final state interactions following an initial pion production process, overestimates the measured cross sections and there are strong indications that the overestimate arises in this two-step process. The selection of suitable kinematic conditions strongly suppresses this two-step contribution leaving cross sections in which up to half the yield is predicted to arise from the absorption of the photon on three interacting nucleons and which agree with the model. The results indicate (γ, 3N) measurements on nuclei may be a valuable tool for obtaining information on the nuclear three-body interaction. The study of (γ, 3N) reactions in nuclei offers a possible new means of learning about the character of three-nucleon forces (3NF), which are thought to play a small but essential role in nuclear structure and reactions[1]. Since the details of this role and the relative importance of the different 3NF mechanisms are not well established it is of interest to examine three-nucleon knockout reactions, which can take place via closely related mechanisms, to see what light they can cast on the three-nucleon force. An early indication of the significance of the 3NF came from binding energy calculations for the few-nucleon systems, which fail to reproduce the measured binding if only the two-nucleon force is used. The present position is reviewed in [2]. The latest calculations underestimate the binding energy of 3 H and 3 He by ∼0.5-0.9 MeV[3,4] and of 4 He by 2.0-4.2 MeV[3,5] but the inclusion of the 3NF largely removes these discrepancies and also reduces the disagreements observed in the description of nucleon-deuteron scattering. Residual discrepancies in the description of polarisation observables in nucleon-deuteron scattering may be attributable to additional 3NF mechanisms not presently included in the calculations. Many 3NF models are based on 2π exchange forces and intermediate ∆ excitation but other Feynman diagrams have been proposed, in particular short range components involving N * ex-citation and the exchange of heavier mesons such as the ρ and ω. Recent insights from chiral perturbation theory are helping to guide the development of 3NF models[1,6]. However experimental input from a range of reactions will be needed to identify …