THE REGGEON TRAJECTORY IN EXCLUSIVE AND INCLUSIVE LARGE MOMENTUM-TRANSFER REACTIONS

A fundamental prediction of perturbative QCD is that the Reggeon traje ctories alpha(rho) (t) and alpha(A2) (t) governing charge exchange rea ctions at high energies s much greater than -t must monotonically appr oach zero at large spacelike momentum transfers. The asymptotic predic tion lim-t-->infinity alpha(R)(t) = 0 reflects the fact that a weakly interacting quark-antiquark pair is exchanged in the t-channel. Howeve r, measurements of the inclusive processes pi-p --> pi0X at s congruen t-to 300 GeV2 and 8 GeV2 > -t > 2 GeV2 indicate that the effective rho trajectory becomes negative at large -t. We resolve the apparent cont radiction between the perturbative QCD predictions and the experimenta l data by showing that the hard QCD part of the trajectory is weakly c oupled and that its contribution will be hidden until much higher ener gy. We also show that Reggeon contributions to exclusive and inclusive mesonic exchange hadron reactions can be systematically studied in pe rturbative QCD. In particular, the Reggeon contributions to the reacti on pi-p --> pi0X and pi-p --> pi0n is discussed in detail.