Estimate of the screening corrections to gluon distributions in the smallx region

The screening corrections to gluon distributions in a proton corresponding to the triple gluonic ladder diagram are estimated. They are found to be relatively small: their values does not exceed 10% of the leading order QCD gluon distribution forx?10^?4 andQ ² ?100 GeV².     

Remarks on beyond leading order

In recent years a two-scale expansion was established to study reactions of the type NN→NNπ within chiral perturbation theory. Then the diagrams of some subclasses that are invariant under the choice of the pion field no longer appear at the same chiral order. In this Letter we show that the proposed expansion still leads to well defined results. We also discuss the appropriate choice of the heavy baryon propagator.     

Direct photon production beyond leading order in QCD

The significance of the quark-quark scattering process (quark+quark→quark+quark+photon) for the production of large-q_T real photons is discussed in the framework of perturbative QCD. To extract the finite contribution of this process to the differential cross section dσ/dy d²q_T (hadron 1+hadron 2→photon+anything) we define the gluon distribution and the quark-to-photon fragmentation function beyond the leading approximation. The calculations are performed consistently in the dimensional regularization scheme. Our numerical estimates show the resulting finite qq→qqγ contribution to be a small (order α_s/2π) correction in comparison with the basic QCD subprocesses.     

Lepton-hadron processes beyond leading order in quantum chromodynamics

We present a summary of QCD formulae describing the effects of scaling violation in leptonhadron processes, with the inclusion of recently derived higher order corrections. Deep inelastic leptoproduction, one-hadron inclusive distributions in leptoproduction and ine ⁺ e ^? annihilation and Drell-Yan processes are discussed in detail. Higher order corrections to parton densities, fragmentation functions and to lepton-parton cross-sections in the above processes are presented in a common factorization scheme, so that a comparative analysis of various processes as well as an independent analysis of each of them is possible. A discussion of the various scheme dependences at next-to-leading level is also included.     

Processes involving fragmentation functions beyond the leading order in QCD

We point out that the electron propagator, calculated from a truncated Johnson-Baker-Willey equation, suffers from the following defects: lack of renormalizability gauge dependence of the electron electromagnetic mass, as well as unacceptable analytic structure.     

Hadronic lepton-pair production beyond the leading order in perturbative QCD

Hadronic lepton-pair production in perturbative QCD is calculated to order $\text{g}{}^{\mathrm{<mtext>2</mtext>}}$ to be usable in combination with the existing calculation of the anomalous dimension of order $\text{g}{}^{\mathrm{<mtext>4</mtext>}}$. The structure function of the subprocess $\text{q}\text{+}\text{q}\text{→ γ +}$ anything in an arbitrary covariant gauge for gluons is found to have gauge-dependent contributions which are exactly cancelled by contributions from operator matrix elements to give the gauge-independent “coefficient function” for lepton-pair production in the minimal subtraction scheme. This provides a consistency check of the perturbative QCD recipe. A comparison is made with other existing calculations.     

Regge behaviour of structure function and gluon distribution at low-x in leading order

We present a method to find the gluon distribution from the proton structure function data at low-x assuming the Regge behaviour of the gluon distribution function at this limit. We use the leading order (LO) Altarelli–Parisi (AP) evolution equation in our analysis and compare our result with those of other authors. We also discuss the limitations of the Taylor expansion method in extracting the gluon distribution from the structure function used by those authors. Received: 2 January 2000 / Revised version: 23 February 2000 / Published online: 6 July 2000     

The shape of polarized gluon distributions

The recent high precision SMC data on polarized μp scatterings have again confirmed that very little of the proton spin is carried by quarks. To unravel the mystery of the proton spin structure, it is quite important to know the behavior of the polarized gluon distribution. By using the positivity condition of distribution functions together with the unpolarized and polarized experimental data, we restrict thex dependence of the polarized gluon distribution.     

Integrability in Yang-Mills theory on the light cone beyond leading order

The one-loop dilatation operator in Yang-Mills theory possesses a hidden integrability symmetry in the sector of maximal-helicity Wilson operators. We calculate two-loop corrections to the dilatation operator and demonstrate that, while integrability is broken for matter in the fundamental representation of the SU(3) gauge group, for the ajoint SU(N(c)) matter it survives the conformal symmetry breaking and persists in supersymmetric N=1, N=2, and N=4 Yang-Mills theories.     

Evolution of parton densities beyond leading order: The non-singlet case

We develop a technique, based explicitly on the factorization properties of mass singularities, which allows one to calculate the evolution of parton densities beyond leading order. We present the results for the evolution of hadronic structure functions as well as for parton fragmentation functions into hadrons. Within our scheme the predictions for a particular process are obtained by convoluting a universal parton density with a “short-distance” cross section specific to the process. As an application, we calculate the QCD predictions for the Q² dependence of deep inelastic lepton-hadron scattering and of one-particle iclusive e⁺ e^? annihilation cross sections. Our results for electroproduction agree with those obtained with the operator product expansion technique. Physical quantities in scattering are related to the corresponding ones in annihilation by analytic continuation, whereas the Gribov-Lipatov relation is strongly violated.     

Measurement of the longitudinal structure function and the smallx gluon density of the proton

At HERA energies the smallx region (x?10^?2) can be explored atQ ² values large enough that leading twist QCD calculations are valid. We show how measurement of the longitudinal structure function,F _L (x,Q ²), can lead to accurate measurement of the gluon structure function at such smallx values. Experimental systematic errors are discussed fully and requirements for the measurement outlined. We conclude that it should be possible to distinguish between the widely varying gluon distributions which are currently allowed.