We study the non-forward matrix elements of twist-2 QCD light-ray operators and their representations in terms of skewed and double distributions, considering the pion as well as the nucleon. We point out the importance of explicitly including all twist-2 structures in the double distribution representation, which naturally leads to a "two-component" structure of the skewed distribution, with different contributions in the regions |X| > ξ/2 and |X| < ξ/2. We compute the skewed and double quark distributions in the pion at a low normalization point in the effective chiral theory based on the instanton vacuum. Also, we derive the crossing relations expressing the skewed quark distribution in the pion through the distribution amplitude for two-pion production. Measurement of the latter in two-pion production in γ * γ and γ * N reactions could provide direct information about the skewed as well as the usual quark/antiquark-distribution in the pion.

The isosinglet unpolarized and isovector polarized twist-2 quark distributions of the nucleon at low normalization point are calculated in the large-N c limit. The nucleon is described as a soliton of the effective chiral theory. We derive the expressions for the distribution functions in the large-N c limit starting from their definition as numbers of partons carrying momentum fraction x in the infinite momentum frame. We develop a numerical method for computation of the quark and antiquark distributions as sums over the quark single-particle levels in the pion field of the soliton. The contributions of the discrete bound-state level as well as the Dirac continuum are taken into account. The quark-and antiquark distributions obtained explicitly satisfy all general requirements. Results are in reasonable agreement with parametrizations of the data at low normalization point.

At large N c the nucleon can be viewed as a soliton of the effective chiral lagrangian. This picture of nucleons allows a consistent nonperturbative calculation of the leadingtwist parton distributions at a low normalization point. We derive general formulae for the polarized and unpolarized distributions (singlet and non-singlet) in the chiral quark-soliton model. The consistency of our approach is demonstrated by checking the baryon number, isospin and total momentum sum rules, as well as the Bjorken sum rule. We present numerical estimates of the quark and antiquark distributions and find reasonable agreement with parametrizations of the data at a low normalization point. In particular, we obtain a sizeable fraction of antiquarks, in agreement with the phenomenological analysis.

We demonstrate that a trigger on hard dijet production at small rapidities allows us to establish a quantitative distinction between central and peripheral collisions in p p and pp collisions at Tevatron and LHC energies. Such a trigger strongly reduces the effective impact parameters as compared to minimum bias events. This happens because the transverse spatial distribution of hard partons (xտ10 Ϫ2 ) in the proton is considerably narrower than that of soft partons, whose collisions dominate the total cross section. In the central collisions selected by the trigger, most of the partons with xտ10 Ϫ2 interact with a gluon field whose strength rapidly increases with energy. At LHC ͑and to some extent already at Tevatron͒ energies the strength of this interaction approaches the unitarity ͑''blackbody''͒ limit. This leads to specific modifications of the final state, such as a higher probability of multijet events at small rapidities, a strong increase of the transverse momenta and depletion of the longitudinal momenta at large rapidities, and the appearance of long-range correlations in rapidity between the forward or backward fragmentation regions. The same pattern is expected for events with production of new heavy particles ͑Higgs boson, SUSY͒. Studies of these phenomena would be feasible with the CMS-TOTEM detector setup, and would have considerable impact on the exploration of the physics of strong gluon fields in QCD, as well as the search for new particles at LHC.

This article investigates survival and growth of more than 50,000 Upper Austrian farm households within a maximum likelihood sample selection framework. The results suggest that age, schooling and sex of the farm operator, size of the farm family, and off-farm employment status, as well as initial farm size, significantly influence farm growth and survival. The present study provides support for the notion of a “disappearing middle” in the size distribution. The process of polarization is closely related to the off-farm employment status of farms. Correcting for sample-attrition bias is important when analyzing the performance of part-time farms. Copyright 1999, Oxford University Press.

We calculate the isovector ͑flavor-nonsinglet͒ unpolarized quark and antiquark distributions in the nucleon at a low normalization point in the large-N c limit. The nucleon is described as a soliton of the effective chiral theory. The isovector distribution appears in the next-to-leading order of the 1/N c expansion. Numerical results for the quark and antiquark distributions compare well with the parametrizations of the data at a low normalization point. This large-N c approach gives a flavor asymmetry of the antiquark distribution ͑violation of the Gottfried sum rule͒ in good agreement with the measurements. ͓S0556-2821͑99͒01701-4͔

We study the virtual Compton amplitude in the generalized Bjorken region (q 2 → ∞, t small) in QCD by means of a light-cone expansion of the product of e.m. currents in string operators in coordinate space. Electromagnetic gauge invariance (transversality) is maintained by including in addition to the twist-2 operators "kinematical" twist-3 operators which appear as total derivatives of twist-2 operators. The non-forward matrix elements of the elementary twist-2 operators are parametrized in terms of two-variable spectral functions (double distributions), from which twist-2 and 3 skewed distributions are obtained through reduction formulas. Our approach is equivalent to a Wandzura-Wilczek-type approximation for the twist-3 skewed distributions. The resulting Compton amplitude is manifestly transverse up to terms of order t/q 2 . We find that in this approximation the tensor amplitude for longitudinal polarization of the virtual photon is finite, while the one for transverse polarization contains a divergence already at tree level. However, this divergence has zero projection on the polarization vector of the final photon, so that the physical helicity amplitudes are finite.

The leading-twist wave functions of the pion and the photon at a low normalization point are calculated in the effective low-energy theory derived from the instanton vacuum. The pion wave function is found to be close to the asymptotic one, consistent with the recent CLEO measurements. The photon wave function is non-zero at the endpoints. This different behavior is a consequence of the momentum dependence of the dynamical quark mass suggested by the instanton vacuum. We comment on the relation of meson wave functions and off-forward parton distributions in this model.