Nuclear Physics B151 (1979) 389-398 © North-Holland Publishing Company HADRON DISTRIBUTIONS IN QUARK JETS P. HOYER and C. PETERSON Nordita, Blegdamsvej 17, DK-2100 Copenhagen ~, Denmark C.-H. LAI and J.L. PETERSEN The Niels Bohr Institute, University of Copenhagen DK-2100 Copenhagen ~,, Denmark Received 29 November 1978 We explore the possibility that the longitudinal hadron distributions in quark jets and in soft hadron collisions are similar. Plausibility arguments for this are given in the parton picture of Feynman. It leads to specific predictions for the behaviour of fragmen- tation functions in the z ~ 1 and z ~ 0 limits in terms of Regge trajectory intercepts. Quantum number effects and correlation lengths are discussed. Our results imply a monopole behaviour for the pion form factor, while that of the proton is consistent with a dipole. 1. Introduction In the near future, data from the e+e - storage rings PETRA and PEP will enable us to study the properties of quark jets with energies around 15 GeV. One of the exciting questions to be answered is whether the hadron distribution in the e+e - jets bears any resemblance to that seen in soft hadronic collisions. As far as the transverse momentum component is concerned, the QCD expectation is that the two distributions will be different, with (PT) increasing linearly with energy for the e÷e- jets. Our prejudices concerning the longitudinal hadron distributions are not as clear. In fact, data on jets in e÷e - and deep inelastic lepton-hadron collisions have been successfully fit [1 ] using an iterative model for quark fragmentation that was originally constructed to describe hadron distributions in low-p T collisions [2]. Such models imply a logarithmic growth of multiplicity, a rapidity plateau and short-range correlations, i.e., features that are familiar from soft hadron collisions. In fact, several specific models have been proposed [3] which link the properties of the hadron distributions in hard and soft non-diffractive collisions. A connection between the hadron distributions in hard and soft jets would have a very natural interpretation in the parton model of Feynman [4]. Consider first the usual picture for jet formation in e+e - annihilations (fig. la). The quark and antiquark originally produced at the photon vertex are slowed down (and trans- 389