We describe the physics potential of e + e − linear colliders in this report. These machines are planned to operate in the first phase at a center-of-mass energy of 500 GeV, before being scaled up to about 1 TeV. In the second phase of the operation, a final energy of about 2 TeV is expected. The machines will allow us to perform precision tests of the heavy particles in the Standard Model, the top quark and the electroweak bosons. They are ideal facilities for exploring the properties of Higgs particles, in particular in the intermediate mass range. New vector bosons and novel matter particles in extended gauge theories can be searched for and studied thoroughly. The machines provide unique opportunities for the discovery of particles in supersymmetric extensions of the Standard Model, the spectrum of Higgs particles, the supersymmetric partners of the electroweak gauge and Higgs bosons, and of the matter particles. High precision analyses of their properties and interactions will allow for extrapolations to energy scales close to the Planck scale where gravity becomes significant. In alternative scenarios, like compositeness models, novel matter particles and interactions can be discovered and investigated in the energy range above the existing colliders up to the TeV scale. Whatever scenario is realized in Nature, the discovery potential of e + e − linear colliders and the high-precision with which the properties of particles and their interactions can be analysed, define an exciting physics programme complementary to hadron machines.

We present new sets of fragmentation functions for charged pions and kaons, both at leading and next-to-leading order. They are fitted to TPC data taken at energy √ s = 29 GeV and describe excellently a wealth of other e + e − data on charged-hadron production, ranging from √ s = 5.2 GeV way up to LEP 1 energy.They also agree with data on the production of neutral pions and kaons, if one makes the natural assumption that the respective fragmentation functions are related to the charged counterparts by SU(2) symmetry. We also list simple parameterizations of the x and Q 2 dependence of our results, which may be implemented conveniently in applications.

We present new sets of fragmentation functions for D * ± mesons, both at leading and next-to-leading order. They are determined by fitting LEP1 data on inclusive D * ± production in e + e − annihilation. In one of the sets, we take the charm-quark fragmentation function to be of the form proposed by Peterson et al. and thus obtain updated values of the c parameter and the c → D * + branching ratio. The new fragmentation functions lead to an excellent description of other e + e − data with centre-of-mass energies between 10 and 35 GeV. They also nicely agree with recent HERA data on inclusive D * ± photoproduction in ep collisions, which may be considered as a test of the universality of the fragmentation into D * ± mesons.

We study inclusive charged-hadron production in collisions of quasireal photons at next-to-leading order (NLO) in the QCD-improved parton model, using fragmentation functions recently extracted from PEP and LEP1 data of e + e − annihilation. We consistently superimpose the direct (DD), single-resolved (DR), and double-resolved (RR) γγ channels. We consider photon spectra generated by electromagnetic bremsstrahlung and/or beamstrahlung off colliding e + and e − beams as well as those which result from backscattering of laser light off such beams. First, we revisit existing single-tag data taken by TASSO at PETRA and by MARK II at PEP (with e + e − energy √ S ≈ 30 GeV) and confront them with our NLO calculations imposing the respective experimental cuts. We also make comparisons with the neutral-kaon to charged-hadron ratio measured by MARK II. Then, we present NLO predictions for LEP2, a next-generation e + e − linear collider (NLC) in the TESLA design with √ S = 500 GeV, and a Compton collider obtained by converting a 500-GeV NLC. We analyze transverse-momentum and rapidity spectra with regard to the scale dependence, the interplay of the DD, DR, and RR components, the sensitivity to the gluon density inside the resolved photon, and the influence of gluon fragmentation. It turns out that the inclusive measurement of small-p T hadrons at a Compton collider would greatly constrain the gluon density of the photon and the gluon fragmentation function.

We present new sets of nonperturbative fragmentation functions for D* Ϯ mesons, both at leading and next-to-leading order in the MS factorization scheme with five massless quark flavors. They are determined by fitting the latest OPAL and ALEPH data on inclusive D* Ϯ production in e ϩ e Ϫ annihilation. We take the charm-quark fragmentation function to be of the form proposed by Peterson et al. and thus obtain new values of the ⑀ c parameter, which are specific for our choice of factorization scheme. With these fragmentation functions, recent data on inclusive D* Ϯ photoproduction in ep collisions at DESY HERA are reasonably well reproduced.

We present new sets of fragmentation functions for neutral kaons, both at leading and next-to-leading order. They are fitted to data on inclusive K 0 production in e + e − annihilation taken by MARK II at PEP ( √ s = 29 GeV) and by ALEPH at LEP. Our fragmentation functions lead to a good description of other e + e − data on inclusive K 0 production at various energies. They also nicely agree with the K 0 S transverse-momentum spectra measured by H1 at the DESY ep collider HERA, by UA5 at the CERN SppS Collider, and by CDF at the Fermilab Tevatron.