Three-jet events produced in e + e ~ annihilations are used to provide comparisons between quark and gluon jets. Differences between quark-induced and gluon-induced jets are observed. Quark jets tend to have a more tightly collimated structure than gluon jets, which is reflected in the concentration of a larger fraction of the jet's energy near the jet axis. PACS numbers: 13.87.Fh, 12.38.Qk, 13.65.+i Quantum chromodynamics (QCD) has been proposed as a theory for the strong interactions. 1 In this theory, the strong force is mediated by the exchange of massless vector gluons between quarks, the fundamental constituents of strongly interacting particles. The coupling of quarks to gluons is expected to decrease with increasing momentum transfer so that in high-g 2 processes calculations based on perturbation theory are valid. However, in applications of QCD to experimental situations, effects of the transition from the unobservable quarks and gluons to the physically observable hadrons (the socalled hadronization process) are unavoidable. This is a complex sequence of low-g 2 processes for which the techniques of perturbative QCD are not applicable and phenomenological models must be used. Uncertainties in these models present a major obstacle to the detailed testing of QCD. It is thus important to obtain experimental insight into the hadronization process using reactions where the primary parton dynamics are well understood. High-energy e + e ~ annihilations, because of their basic simplicity, are well suited for these purposes. e + e~ annihilation into three jets of hadrons, which is most simply interpreted as consisting of two quark jets and one gluon jet, provides an opportunity for direct 1772
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