QCD and the string model

Infinitisimal variations of the path-dependent phase factor P exp(i∫_σA_μdz_μ) in gauge theory are studied. They are shown to satisfy differential equations which are equivalent to those for a quantized string if the gauge fields meet certain constraints.     

Gluelump spectrum in the QCD string model

The spectrum of gluons in the adjoint source field is computed analytically using the QCD string Hamiltonian, containing only one parameter — string tension, fixed by meson and glueball spectrum. Spin splitting is shown to be small. A good agreement is observed with specially generated gluelump states measured on the lattice. The important role of gluelumps defining the behaviour of field-strength correlators is stressed and correspondence with earlier computations of the latter is established.     

On-shell methods in perturbative QCD

We review on-shell methods for computing multi-parton scattering amplitudes in perturbative QCD, utilizing their unitarity and factorization properties. We focus on aspects which are useful for the construction of one-loop amplitudes needed for phenomenological studies at the Large Hadron Collider.     

Reggeon interactions in perturbative QCD

We study the pairwise interaction of reggeized gluons and quarks in the Regge limit of perturbative QCD. The interactions are represented as integral kernels in the transverse momentum space and as operators in the impact parameter space. We observe conformal symmetry and holomorphic factorization in all cases.     

Heavy flavors in perturbative QCD

I review the techniques used in perturbative QCD to compute open heavy flavor and quarkonium cross sections. I discuss recent theoretical and experimental results, emphasizing the case of bottom production at colliders.Arrival of the final proofs: 29 April 2005     

Secondary reggeons in perturbative QCD

Using the Kirschner-Lipatov equation as a basis, we study high energy elastic quark-quark amplitudes, which correspond to secondary reggeons and pion exchange in the double-logarithmic approximation of perturbative QCD. Received: 19 March 2002 / Revised version: 14 May 2002 / Published online: 26 July 2002     

Generating heavy quarkonia in a perturbative QCD cascade

We present an implementation of heavy quarkonium production within a perturbative QCD cascade based on the Color Dipole Cascade model. We consider the processes most relevant in the context of the ψ′ surplus at the Tevatron; g → ψ′ and c → ψ′ in the color-singlet model and g → ψ′ through the color-octet mechanism. Our implementation is, however, easily extendible to other quarkonia and other production mechanisms. Where comparison is possible we find good agreement with analytical calculations. We present some suggestions for measurements at the Tevatron that would be sensitive to the shape of the fragmentation functions. Our calculations indicate that such measurements could be used to test the color-octet mechanism solution to the ψ′ surplus at the Tevatron.     

Hybrid adiabatic potentials in the QCD string model

The short-and intermediate-distance behavior of the hybrid adiabatic potentials is calculated in the framework of the QCD string model. The calculations are performed with the inclusion of the Coulomb force. The spin-dependent force and the so-called string correction term are treated as a perturbation at the leading-potential-type regime. Reasonably good agreement with lattice measurements takes place for adiabatic curves excited with magnetic components of field strength correlators.     

Symmetric reggeon interaction in perturbative QCD

Integral kernels describing the pair interaction of reggeized gluons and quarks are reconstructed in terms of conformal symmetric 4-point functions in the transverse plane of impact parameters.Received: 2 March 2005, Published online: 4 May 2005     

Large-angle compton scattering in perturbative QCD

The processes γp→γp and γn→γn are studied in perturbative QCD for large scattering angles. The unpolarized cross sections, as well as the cross sections for polarized beams and targets, are given for various choices of the baryon wavefunction. The predictions for the unpolarized cross section of γp→γp are compared with the available data. Possible future experimental verifications of the theory are discussed.     

Pion Compton scattering in perturbative QCD

Pion Campton scattering is studied in perturbative QCD for real and space-like initial photons. Different methods for the convolution of the hard amplitude with the pion wave-functions, which have in the past led to conflicting results, are compared.     

Angular energy flow in perturbative QCD

We discuss a general way in which QCD can be checked in lepton-induced reactions — even with low statistics data. This is the “angular energy flow”, or the average energy fraction outside a cone of opening angle 2σ about the principal jet axis. in the final state of deep inelastic reactions. We illustrate this method by a perturbative calculation of the angular energy flow in $e^{+}{e}^{?}\to q\overline{q}g\to$ jets. This perturbative approach can be extended to other deep inelastic reactions wit h gluon jets. At high enough energy, it should test QCD beyond lowest order in α_s.     

The pole mass in perturbative QCD

We review quark mass definitions in QCD, defending the identification of pole with constituent masses on the one hand and of invariant mass flavour ratios with current mass flavour ratios on the other. We prove that the pole mass is gauge-parameter independent and infrared finite at the two-loop level. We compute the two-loop mass anomalous dimension in the MS renormalization scheme. The renormalization group allows one to connect constituent with current mass ratios. For heavy flavours we predict the invariant masses.     

Renormalization group improved perturbative QCD

The results of perturbative QCD calculations are reformulated as renormalization-scheme independent predictions; in so doing, we obtain a renormalization group improvement of perturbation theory. As an application, we show that asymptotic freedom alone does not give the correct quantitative relation between pseudoscalar charmonium decay and the scaling violations in deep inelastic scattering.     

The odderon intercept in perturbative QCD

We construct, in the framework of QCD, the conformally invariant functional whose maximal value gives theJ-plane location of the leading singularity of thet-channel partial waves in LLA for diagrams withn reggeized gluons. In the case of the odderon the wave function in the impact-parameter space depends on only one anharmonic ratio and the corresponding functional is significantly simplified. By using a variational method with conformal techniques we show that the odderonJ-plane singularity in the LLA approximation of QCD lies above 1.