We study the pion form factor F ␥␥* (Q 2 ) in the light-cone sum rule approach, accounting for radiative corrections and higher twist effects. Comparing the results to the CLEO experimental data on F ␥␥* (Q 2 ), we extract the pion distribution amplitude of twist 2. The deviation of the distribution amplitude from the asymptotic one is small and is estimated to be a 2 ()ϭ0.12Ϯ0.03 at ϭ2.4 GeV, in the model with one nonasymptotic term. The ansatz with two nonasymptotic terms gives some region of a 2 and a 4 , which is consistent with the asymptotic distribution amplitude, but does not agree with some old models.

We report on the perturbative O(α s ) correction to the light-cone QCD sum rule for the B → π transition form factor f + . The correction to the product f B f + in leading twist approximation is found to be about 30%, that is similar in magnitude to the corresponding O(α s ) correction in the two-point sum rule for f B . The resulting cancellation of large QCD corrections in f + eliminates one important uncertainty in the sum-rule prediction for this form factor.

The f + form factors of the B → π, D → π and D → K transitions are calculated from QCD light-cone sum rules (LCSR) and used to predict the widths and differential distributions of the exclusive semileptonic decays B → πlν l , D → πlν l and D → Klν l , where l = e, µ. The current theoretical uncertainties are estimated. The LCSR results are found to agree with the results of lattice QCD calculations and with experimental data on exclusive semileptonic D decays. Comparison of the LCSR prediction on B → πlν l with the CLEO measurement yields a value of |V ub | in agreement with other determinations.

The B * Bπ and D * Dπ couplings have previously been derived from a QCD lightcone sum rule in leading order. Here, we describe the calculation of the O(α s ) correction to the twist 2 term of this sum rule. The result is used for a first nextto-leading order analysis. We obtain g B * Bπ = 22 ± 7 and g D * Dπ = 10.5 ± 3, where the error indicates the remaining theoretical uncertainty.

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HQET currents with the quantum numbers of the ground-state baryons are discussed. One-loop anomalous dimensions are calculated, and exact one-loop matching to QCD currents is found. Two-point correlators of these currents are calculated taking into account d ≤ 9 terms of the OPE. Sum rules for heavy baryons Λ Q and Σ ( * ) Q are analyzed. Three-point correlators of two baryonic currents and a heavy-heavy velocity changing current are calculated with the same accuracy. The baryonic Isgur-Wise form factors are estimated from the corresponding sum rules.

We make an attempt to discuss in detail the effects originating from the final state interaction in the processes involving production of unstable elementary particles and their subsequent decay. Two complementary scenarios are considered: the single resonance production and the production of two resonances. We argue that part of the corrections due to the final state interaction can be connected with the Coulomb phases of the involved charge particles; the presence of the unstable particle in the problem makes the Coulomb phase ``visible''. It is shown how corrections due to the final state interaction disappear when one proceeds to the total cross-sections. We derive one-loop non-factorizable radiative corrections to the lowest order matrix element of both single and double resonance production. We discuss how the infrared limit of the theories with the unstable particles is modified. In conclusion we briefly discuss our results in the context of the forthcoming experiments on the $W^+W^-$ and the $t\bar t$ production at LEP $2$ and NLC.Comment: 33 pages, latex, 6 figures (added), version accepted for publication in Nuc. Phys. B, substantial revisio

Using non-relativistic effective theories, new next-to-next-to-leading order (NNLO) QCD corrections to the total tt production cross section at the Linear Collider have been calculated recently. In this article the NNLO calculations of several groups are compared and the remaining uncertainties are discussed. The theoretical prospects for an accurate determination of top quark mass parameters are discussed in detail. An outlook on possible future improvements is given.