Parton Models and Deep Hadron - Hadron Scattering

We review the recent applications of various parton models, to the problem of hadron-hadron scattering at large momentum transfers. Models which allow direct parton-parton interactions as well as those that do not are considered. Predictions for the structure of hadronic final states (whether or not transverse jets exist), associated multiplicities, and the s and t dependencies of inclusive and exclusive cross-sections are described for the two types of models and compared with available data. The relation of the large momentum transfer (deep) region to the small momentum transfer (Regge) region is also discussed. During the last couple of years, several attempts have been made to extend the intuitions about partons which have been developed in the study of deep electroproduction to purely hadronic interactions at large momentum transfers. In these applications, the hope has been that in this, the deep hadronic region, some sort of impulse approximation is valid so that only the simplest (in some sense) interactions have time to take place. Today I want to discuss some of the major developments which have taken place along these lines. I will describe, in a general way, the theoretical ideas behind various approaches, and then discuss some of the experimental consequences both for inclusive and exclusive (two-body) hadronic interactions. I will also address myself to the question of providing a more precise definition of the deep region. In physical terms this becomes the problem of determining the kinematic region in which coherent (Regge) effects can be ignored. In parton theories of deep hadron scattering one pictures the incident hadrons as being composed of partonic constituents which mediate the basic interactions between the hadrons. The constituents then recombine to produce the.final state hadrons. It is convenient to classify such theories according to whether or not they allow direct parton-parton interactions between partons belonging to different hadrons. A number of authors have described theories which allow direct parton-parton scattering (type I). I While these models differ in their details, they all share the common belief that some sort of * Work supported in part by the U. S. Atomic Energy Commission. t Present address. (Invited paper presented at the 1973 Meeting of the Division of Particles and Fields of the APS, Berkeley, California, August 13-17, 1973)