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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.
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We summarize the standard factorization theorems for hard processes in QCD, and describe their proofs.
The QCD improved parton model is a very successful concept to treat processes in hadronic interactions, whenever large partonic transverse momenta are involved. However, cross sections diverge in the limit p_T -> 0, and the usual treatment is the definition of a lower cutoff p_T_min, such that processes with a smaller p_T -- so-called soft processes -- are simply ignored, which is certainly not correct for example at RHIC energies. A more consistent procedure amounts to introduce a technical parameter Q_0^2, referred to as soft virtuality scale, which is nothing but an artificial borderline between soft and hard physics. We will discuss such a formalism, which coincides with the improved parton model for high p_T processes and with the phenomenological treatment of soft scattering, when only small virtualities are involved. The most important aspect of our approach is that it allows to obtain a smooth transition between soft and hard scattering, and therefore no artificial dependence on a cutoff parameter should appear.
We calculate the probability that the rapidity gaps in diffractive processes survive both eikonal and enhanced rescattering. We present arguments that enhanced rescattering, which violates soft-hard factorization, is not very strong. Accounting for NLO effects, there is no reason to expect that the black disc regime is reached at the LHC. We discuss the predictions for the survival of the rapidity gaps for exclusive Higgs production at the LHC.
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.
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
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