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Register a new userFactorization of Hard Processes in QCD
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We summarize the standard factorization theorems for hard processes in QCD, and describe their proofs.
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This is a short review of some hard two-photon processes: $ a) ,,gammagammato {overline P}_1 P_2,,, {overline P}_1 P_2= {pi^+pi^-, K^+ K^-, K_S K_S, pi^opi^o, pi^oeta},, b) ,,gammagammato V_1 V_2,,, V_1 V_2={rho^orho^o, phiphi, omegaphi, omegaomega }, c) ,,gammagammato {rm baryon-antibaryon}, d) ,,gamma^*gammato P^o,,, P^o={pi^o, eta, eta^prime, eta_c}$. The available experimental data are presented. A number of theoretical approaches to calculation of these processes is described, both those based mainly on QCD and more phenomenological (the handbag model, the diquark model, etc). Some theoretical questions tightly connected with this subject are discussed, in particular: the applications of various types of QCD sum rules, the endpoint behavior of the leading twist meson wave functions, etc.
We discuss some problems concerning the application of perturbative QCD to high energy processes. In particular for hard processes, we analyze higher order and higher twist corrections. It is argued that these effects are of great importance for understanding the behaviour of pion electromagnetic form factor at moderately large momentum transfers. For soft processes, we show that summing the contributions of the lowest twist operators leads to a Regge-like amplitude.
A short review of leading term QCD predictions vs those of the handbag model for large angle cross sections gammagamma --> P_2 P_1 (P is the pseudoscalar meson pi^{pm,o}, K^{pm,o}, eta), and for gammagamma --> V_2 V_1 (V is the neutral vector meson rho^o, omega, phi), in comparison with Belle Collaboration measuments
We describe diffractive deeply inelastic scattering in terms of diffractive parton distributions. We investigate these distributions in a hamiltonian formulation that emphasizes the spacetime picture of diffraction scattering. For hadronic systems with small transverse size, diffraction occurs predominantly at short distances and the diffractive parton distributions can be studied by perturbative methods. For realistic, large-size systems we discuss the possibility that diffractive parton distributions are controlled essentially by semihard physics at a scale of nonperturbative origin of the order of a GeV. We find that this possibility accounts for two important qualitative aspects of the diffractive data from HERA: the flat behavior in beta and the delay in the fall-off with Q^2.
We review the origin, and salient features, of the breaking of the conventional linear $k_{perp}$-factorization for an in-nucleus hard pQCD processes. A realization of the nonlinear $k_{perp}$-factorization which emerges instead is shown to depend on color properties of the underlying pQCD subprocesses. We discuss the emerging universality classes and extend nonlinear $k_{perp}$-factorization to AGK unitarity rules for the excitation of the target nucleus.
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