Photon-hadron fragmentation: theoretical situation

Models of hadronic fragmentation and their phenomenological comparison are considered using a selection of new experimental results. Indeed, a convenient theory of hadronic fragmentation - for instance based on Q.C.D. - does not exist: low Psub(perpendicular) fragmentation involves the badly known hadronic long-range forces. Models should clarify the situation in the prospect of an eventual future theory. Four models are mentioned: - quark recombination or fusion, quark fragmentation, off shell quark fragmentation and gluon Bremmsstrahlung. The main experimental observables which are relevant to the discussion of fragmentation are discussed. First, features of particle production, then energy dependence of rapidity distributions, energy dependence of multiplicity distributions and finally, transverse radiation. (U.K.)