We present a quantitative analysis of the electromagnetic pion form factor in the light-cone sum rule approach, including radiative corrections and higher-twist effects. The comparison to the existing data favors the asymptotic profile of the pion distribution amplitude and allows to estimate the deviation: ( du/u φ π (u))/( du/u φ as π (u)) = 1.1± 0.1 at the scale 1 GeV. Special attention is payed to the precise definition and interplay of soft and hard contributions at intermediate momentum transfer, and to matching of the sum rule to the perturbative QCD prediction. We observe a strong numerical cancellation between the soft (end point) contribution and power suppressed hard contributions of higher twist, so that the total nonperturbative correction to the usual pQCD result turns out to be of order 30% for Q 2 ∼ 1 GeV 2 .

Spin asymmetries of semi-inclusive cross sections for the production of positively and negatively charged hadrons have been measured in deep-inelastic scattering of polarized positrons on polarized hydrogen and He-3 targets, in the kinematic range 0.023 < x < 0.6 and 1 GeV2 < Q(2) < 10 GeV2. Polarized quark distributions are extracted as a function of x for up (u + (u) over bar) and down (d + (d) over bar) flavors. The up quark polarization is positive and the down quark polarization is negative in the measured range. The polarization of the sea is compatible with zero. The first moments of the polarized quark distributions are presented. The isospin non-singlet combination dq, is consistent with the prediction based on the Bjorken sum rule. The moments of the polarized quark distributions are compared to predictions based on SU(3)(f) flavor symmetry and to a prediction from lattice QCD. (C) 1999 Published by Elsevier Science B.V. All rights reserved

We analyze power corrections to flavour singlet deep inelastic scattering structure functions in the framework of the infrared renormalon model. Our calculations, together with previous results for the non-singlet contribution, allow to model the x-dependence of higher twist corrections to F 2 , F L and g 1 in the whole x domain.

We calculate the one-loop effective action of the SU (2) Yang-Mills theory for center-vortex configurations, both in 3d and 4d. We find that in both cases there are minima of the effective action, corresponding to vortices of the transverse size approximately 4/g 2 3 and 1.7/Λ M S , respectively. The values of the effective actions at the minima are negative, suggesting that the Euclidian vacuum may be unstable with respect to creation of vortices.

We show that the characteristic p ⊥ spectrum yields valuable information for the test of models for the production of narrow graviton resonances in the TeV range at LHC. Furthermore, it is demonstrated that in those scenarios the parton showering formalism agrees with the prediction of NLO matrix element calculations.