Current propagator: Duality between vector mesons and perturbative QCD

Local duality between vector mesons and perturbative QCD in the current propagator supplies expressions for their leptonic widths in terms of the quark masses, the masses of the lowest vector mesons and their first excitations and the QCD parameter Λ. The experimental data are consistent with this physical principle if ?′(1260 MeV) exists.     

Large perturbative corrections to the Drell-Yan process in QCD

The total cross section $\text{d}\text{σ}\text{d}\text{Q}{}^2$ for the production of a muon pair of invariant mass Q²via the Drell-Yan mechanism and the Feynman x_F differential cross section $\text{d}{}^2\text{σ}\text{d}\text{Q}{}^2\text{d}\text{x}{}_{\mathrm{<mtext>F</mtext>}}$ are calculated in QCD retaining all terms up to order α_s(Q². The calculations are performed using dimensional regularisation of the intermediary infrared and collinear singularities, but we present our results in a form independent of such details. The corrections to both these cross sections coming from radiative corrections to the lowest-order $\text{q}\text{q}$ annihilation diagram are found to be large at present values of Q² and S when the cross section is expressed in terms of parton densities derived from leptonproduction, for all Drell-Yan processes of practical interest. Numerical calculations are presented which show, for any reasonable parametrisation of the parton densities, that the neglect of higher-order terms in α_s(Q²) is not justifiable. The quark-gluon diagrams on the other hand give small corrections in this order and are only important for PP scattering.     

Finite corrections to quark fragmentation functions in perturbative QCD

The finite corrections of order α _s in perturbative QCD to the cross-sections for semi-inclusive hadron production from deep inelastic leptonhadron scattering and electron-positron annihilation are calculated. We define the effective quark fragmentation functions viae ⁺ e ^? → hadron +X including the finite terms in order to estimate these corrections for the reactions lepton + hadron → lepton + hadron + anything. Contrary to the leading term the next-to-leading order term does not factorize into parts depending on the target and the fragment, respectively. For the processese +p →e + π^± +X andv +p →μ ^? + π^± +X the finite corrections of order α _s turn out to be at most 20% in the range of momenta covered by present experiments.     

Beyond leading order QCD perturbative corrections to the pion form factor

The order α_s²(Q²) corrections to the pion form factor, F_π(Q²), are calculated using perturbative quantum chromodynamics and dimensional regularization. In the $\text{MS}$ renormalization scheme these corrections are large. This means that reliable perturbative predictions cannot be made until momentum transfers, Q, of about 300–400 GeV are reached or unless one can sum the large perturbative terms to all orders. Attempts to reorganize the perturbation series so that the first term gives a better approximation of the complete sum indicate that at Q = 10 GeV the pion form factor may be about a factor of two larger than the leading order result.     

QCD power corrections to deep-inelastic scattering

Q^?2 corrections due to next-to-leading twist operators are calculated for the first and the second moments of deep-in-elastic structure functions.     

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.     

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.     

Chiral symmetry breaking and perturbative QCD

Exact results are given for the propagators of axial and vector divergences at the two-loop level of dimensionally regularized quantum chromodynamics. We prove that any combination of propagators that vanishes at zero momentum by virtue of Ward identifies is free of subtractions and satisfies a superconvergence relation. Sum rules for other combinations will, however, involve unknown non-perturbative constants.     

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     

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     

LargeN behaviour of perturbative QCD coupling

A correspondence between perturbative QCD in an axial gauge and dual topological unitarization (DTU) scheme is expressed by linking the QCD scale Λ toN, the number of flavors or colors supposed to be large through a relation which involves a typical hadronic scale.     

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.