Mechanics of Quark Exchange in High-Energy Hadron Reactions at Forward Angles

The 2-quark back-angle scattering mechanism is shown to reproduce main features of high-energy hh flavor-exchange reactions. Prospects for reduction of the reaction matrix element to a form of convolution of hadron wave functions with the hard scattering kernel are discussed. Wave function models suitable for convolution with the kernel, which is singular at small x, and the emerging form-factor types, are discussed. Reactions of one unit of flavor exchange between two hadrons scattering through small angles at high energy can occur, to LO in Mandelstam s, due to exchange of a single pair of quarks, which enter into a hard collision and completely interchange their large longitudinal momenta (scatter to 180 in c.m.s.). With no rapidity gap between the hadron remnants and their new comoving quarks created, they should with high probability recombine into a new pair of hadrons. In the binary reaction channel the differential cross-section, indeed, exhibits a forward peak of typical p⊥ ∼ 300 MeV width. The peak decreases with the energy rather slowly, as s – approximately as does the Born-level 2-quark back angle scattering differential cross-section (∝ s); the rest of the energy dependence must be attributed to reggeization effects. The Born diagram of the head-on relativistic quark collision is closely similar to that of real photon scattering on a quark, owing to the similarity between hard gluon and hard quark propagators on the light cone. Yet, high energy hadron reactions are much easier to measure then hard forward real Compton scattering is, given the coupling constant factor α s in the cross-section instead of α 2 EM , and no radiative or diffractive background involved. On the theory side, it brings complications, since eikonal (gauge link, Wilson line) phases in hadron remnant interactions emerge, which spoil factorization in terms of GPDs. However, that may actually happen in photon-hadron interactions as well, and the A = 0 gauge condition does not help [1]. As those links are to be dealt with anyway, it would be better to constrain them using cross-checks from different processes. Forward Compton scattering is, also, not the best playground for studying hadron spin effects, since quark SSA for it cancel in the sum of two LO diagrams. As for 2-quark scattering in flavor exchange, there is only one LO diagram, so transverse polarization may be open, provided soft contributions supply necessary phase shifts. Generally, soft effects can stem from eikonal links, and also from initial and final state wave functions sandwiching the complex hard scattering kernel. Both eikonal and initial/final state contributions are proportional to the interaction strength, but still, it is not excluded that some of them may happen to be more significant. To make a sensible decision, it is instructive first to develop qualitative understanding of the dynamics.