QCD Perturbation Theory Research Articles

Corrections to the StermanWeinberg jet formula

The fraction of hadronic final states from e +e − annihilation consisting of two jets is calculated exactly to lowest order in QCD perturbation theory. This improves the Sterman-Weinberg expression increasing its range of applicability and may allow for detailed quantitative tests of QCD at feasible experimental energies.

Spin-dependent interactions in heavy quark systems

Some features of spin-dependent forces in heavy quark-antiquark systems are studied in QCD perturbation theory. The spin-dependent potentials in these systems are shown to depend non-trivially on the heavy quark mass. This dependence arises from processes taking place at distances on the order of the quark Compton wavelength. Using these results, the limitations of schemes which have been introduced recently to compute spin-dependent forces due to instantons are illustrated.

Nonperturbative effects on factorization in high-momentum processes

It is pointed out that the usual demonstrations within QCD perturbation theory of factorization at high-momentum transfers are incomplete when nonperturbative instanton effects are incorporated in a dilute gas approximation. The apparent violation of factorization does not vanish at large Q 2.

QCD predictions for jets in e +e − annihilation: Angular correlations and asymmetries

The topological features of hadron jets in e +e − annihilation may reliably be calculated in QCD perturbation theory. To second order the final state hadrons fall into quark-antiquark-gluon initiated jets. The latter induce angular asymmetries which are extensively studied.

High-energy behavior of jet and lepton-pair production

The cross section for jet and lepton-pair production at large transverse momenta has been studied to all orders in QCD perturbation theory. Appropriate moments of these cross sections are seen to factorize short-distance from long-distance dependence, and scale breaking is governed by the anomalous dimensions of twist-two operators.

Comments on the Sterman-Weinberg jet formula

A simple derivation of the Sterman-Weinberg jet formula (the QCD perturbation theory formula for the 2-jet cross section) is described. Conditions on the jet parameters ϵ and δ for this formula to be applicable are discussed and the corrections to the formula up to O(ϵ 2), O(δ 2) are presented.

Gluoproduction of heavy quark flavors in hadronic scattering

Hadronic and photoproduction of heavy flavors are related to each other. QCD perturbation theory fails to describe the production of heavy flavors in hadronic scattering. The results for charm production are compared with the results of the recent CERN beam dump experiments. The possibility that b-flavored particles live longer than 10 −7 s is ruled out.

Quarkonium decay into heavy quark flavours

It is suggested that the production rate of heavy flavours (charm, ψ (J), etc.) in the decay of heavy qyarkonium states (e.g. ϒ(9.4)) can be estimated using QCD perturbation theory. Especially we discuss the energy distribution of ψ (J) mesons produced in quarkonium decay.

QCD predictions for large- p⊥ hadron production by polarized hadrons

Cross sections are given for the scattering of polarized quarks and antiquarks according to lowest-order QCD perturbation theory. Non-vanishing spin asymmetries lead to observable effects for large- p ⊥ hadron production in collisions of polarized hadrons, which are proposed as a test of the relevance of the QCD picture for large- p ⊥ reactions.

QCD predictions for hadronic final states in e +e − annihilation

We defend the applicability of asymptotically free QCD perturbation theory to the description of multi-jet hadronic final states and investigate its predictions for the process e +e − → hadrons both in the hadron “continuum” and in heavy Q Q resonances. We describe the distribution of the hadron energy flow in terms of variables (thrust, spherocity, acoplanarity) chosen to eliminate the sensitivity of the data and of the QCD predictions both to non-perturbative effects related to confinement, and to infra-red divergences which arise in perturbation theory. We present arguments that such analyses should be considered as genuine tests of perturbative QCD. We propose definitions of two- and three-jet final states which can be used to exploit maximally the perturbative QCD predictions. We show that particularly strong tests of the gluonic degrees of freedom are provided by jet distributions in the hadronic decays of heavy Q Q resonances and by the jet-photon angular correlations in sequential decays through even charge conjugation states. Predictions for the hadron energy distributions near new flavour thresholds are also discussed.

Quark decay functions and heavy hadron production in QCD

The evolution of outgoing quarks and gluons into outgoing hadrons (the analog of parton decay functions) is studied in QCD perturbation theory. The results are characterized by logarithmic scaling violations and large transverse momenta. The various functions describing the production of hadrons containing a single heavy quark are calculated from first principles.

The decay of the upsilon into photons and gluons

The hadronic decays of heavy quark-antiquark 1 3S 1 resonances such as the ψ J or ϒ can provide an ideal laboratory for studying the properties of gluon jets. We discuss the predictions of QCD perturbation theory for gluon-jet structures, hadronic multiplicities, fragmentation distributions, and angular distributions. In particular the channel (Q Q→ γ + X , whose rate, spectrum, and angular distribution are predicted in lowest order, should allow the study of the jet and resonance structure produced via two intermediate gluons.