The impact of recent precision measurements of DIS structure functions and inclusive jet production at the Fermilab Tevatron on the global QCD analysis of parton distribution functions is studied in detail. Particular emphasis is placed on exploring the range of variation of the gluon distribution G(x,Q) allowed by these new data. The strong coupling of G(x,Q) with ␣ s is fully taken into account. A new generation of CTEQ parton distributions, CTEQ4, is presented. It consists of the three standard sets ͓modified minimal subtraction (MS), deep inelastic scattering ͑DIS͒, and leading order ͑LO͔͒, a series that gives a range of parton distributions with corresponding ␣ s 's, and a set with a low starting value of Q. Previously obtained gluon distributions that are consistent with the high E t jet cross section are also discussed in the context of this new global analysis.

The theoretical framework for describing the production of direct photons in hadronic collisions is reviewed. A detailed comparison between the theoretical predictions and existing data is presented along with a critical evaluation of the various sources of theoretical uncertainty. The information available from direct-photon experiments is contrasted with that learned from jet or single-hadron production. Prospects for new types of measurements in future experiments are also presented.

Dihadron spectra in high-energy heavy-ion collisions are studied within the next-to-leading order perturbative QCD parton model with modified jet fragmentation functions due to jet quenching. High-p(T) back-to-back dihadrons are found to originate mainly from jet pairs produced close and tangential to the surface of the dense matter. However, a substantial fraction also comes from jets produced at the center with finite energy loss. Consequently, high-p(T) dihadron spectra are found to be more sensitive to the initial gluon density than the single hadron spectra that are more dominated by surface emission. A simultaneous chi(2) fit to both the single and dihadron spectra can be achieved within a range of the energy loss parameter E(0)=1.6-2.1 GeV/fm. Because of the flattening of the initial jet production spectra at square root s=5.5 TeV, high p(T) dihadrons are found to be more robust as probes of the dense medium.

We have used a large set of high-Q2 data on deep-inelastic scattering, dimuon mass distributions, and J / * X F distributions to extract estimates of the Q2-dependent quark and gluon distribution functions in nucleons. The Q' dependence of these functions is parametrized in a manner convenient for predicting a large variety of reactions for e2=4-200 (Gev/c)' and for extrapolation to the ultrahigh values of Q2 that will be probed by future accelerators in the TeV energy range.

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