Hump-backed QCD plateau in hadron spectra

New data on inclusive energy distribution of hadrons from quark jets ine ⁺ e ^? annihilation are shown to agree with the quantitative predictions of perturbative QCD, supporting the hypothesis of local parton-hadron duality.     

Resolving the on- and off-shell ambiguity of perturbative QCD using sum rules

The orderg ^?2 coefficient functions for hard processese ⁺ e ^?→hX andlh→l′ h′ X are calculated in perturbative QCD. On- and off-shell regularization of the mass singularities leads to different results. Sum rules of energy and quantum number conservation are satisfied when using the on-shell regularization and force us to reject the result of the conventional off-shell regularization.     

Heavy quark correlations ine + e − annihilations

In heavy quark jets the quark mass acts as a regulator of collinear singularities, making the quark momentum an infra-red safe variable in perturbative QCD. This allows a direct comparison of measured heavy hadron momentum spectra with perturbative calculations. We exploit the factorisation of heavy quark fragmentation to derive QCD predictions for momentum correlations between heavy hadrons produced ine ⁺ e ^? annihilations. We study the practical feasibility and model sensitivity of our approach using Monte Carlo simulations. Higher order perturbative corrections and contributions from non-perturbative effects are found to be at the level of 10%.     

Simulation of QCD jets: The choice of the cutoff

In view of the simulation ofe ⁺ e ^? annihilation events corresponding to QCD multi-jet final states, we examine the dependence of the results on the value of the cutoff, which delimits the perturbative from the non-perturbative regimes. We do this by studying in some detail related effects for the lowest order perturbative contributions (including single gluon emission). We conclude that event simulation is considerably affected by the choice of the cutoff. Up to energies where only two and three jet configurations are dominant, though, this does not prevent from having quantitative tests of QCD, if suitable kinematical cuts are applied.     

Quark-hadron duality and production of charmonia and doubly charmed baryons in <Emphasis Type="Italic">e</Emphasis><Superscript>+</Superscript><Emphasis Type="Italic">e</Emphasis><Superscript>−</Superscript> anninhilation

The cross sections for the production of charmonia and doubly charmed baryons in e ⁺ e ^? annihilation at an interaction energy of \(\sqrt s \) = 10.6 GeV are predicted under the assumption of quark—hadron duality. The method used is shown to remove some contradictions between experimental data and perturbative QCD.     

Chromodynamics and jet acollinearity in e+e− annihilation: Determining the Quark-Gluon coupling

Acollinearity of the quark and antiquark jets produced in e⁺e^? collisions is an obvious signature of gluon emission. We discuss the computation of acollinearity distributions in perturbative QCD and compare the results with recent data. Teh measurements provide us with a direct determination of the running coupling constant α_S.     

On non-perturbative effects in the jet physics

We consider quark scattering off nonperturbative gluonic fields in the vacuum and its impact on the hadronic final state in thee ⁺ e ^?—annihilation. A conservative estimate gives a nonperturbativep _⊥?1GeV. The modification of the final state survives even at an arbitrary high energy, where the corrections to the total cross-section die away. We also argue that the coupling constant extracted from the perturbative treatment of the jet phenomenology can well be different from that governing the correction toσ _tot.     

Comparison of quasiparticle models in the context of relativistic (e, e+, γ) plasma

The two widely studied thermodynamically consistent quasiparticle models are compared by studying the statistics and thermodynamics of relativistic plasma consists of (e, e ⁺, ??). We use different density dependent dispersion relation for electron and photon with the requirment that at high temperature thermal masses go to that of corresponding perturbative results and do the calculation self-consistently. We further compare our results with previous results.     

Perturbative relations between e+e− annihilation and τ decay observables including resummation effects

By exploiting the analyticity properties of the two-point current-current correlator we obtain numerical predictions for the e⁺e⁻ moments in terms of the τ decay rate. We perform a partial resummation of the pertinent perturbative series expansion by solving the renormalization group equation for Adler's function. Our predictions are renormalization scheme independent but depend on the order of the perturbative β-function expansion. The analysis involves the unknown five-loop coefficient k₃ for which we give some new estimates.     

Perturbative QCD and the exclusive decay of heavy quarkonium into a photon and two mesons

We consider the radiative decays of the ³S₁ ground state Q$\text{Q}$ into a real photon and two light pseudoscalar or vector mesons. For sufficiently large mass of the heavy quark Q, the decay, e.g. ? → γππ, can be calculated in perturbative QCD, provided that the photon does not carry away a predominant fraction of the available energy. We present differential and integrated Dalitz plot distributions and, for quarkonium states produced with transverse polarization in e⁺e^? annihilation, angular distributions of the orientation of the decay triangle relative to the beam direction.     

From asymptotic freedom to quark confinement

Asymptotic freedom is the perturbative expression of paramagnetic screening by the QCD vacuum. When extended to large external fields the perturbative analysis leads to a catastrophe owing to the appearance of negative eigenvalues of the matrix describing the fluctuations of the gluonic field. This is met by a self-consistently determined magnetic-moment density. There results almost complete screening of the external field and the action is dominated by fluctuations of the magnetic moment. Such behaviour is more typical of very strong coupling e ⪢ 1 rather than that expected for e = O(1), the latter being the effective coupling at the QCD length scale. This kind of situation arises in the response to large Wilson loops, so that the derivation of the area law from strong-coupling lattice gauge theory can be rationalized.     

Two-particle correlations in e+e− annihilation

We present measurements of two-particle angular correlations in hadron jets produced in e⁺e^? annihilation between 7.7 and 31.6 GeV c.m. energy. The data are compared to predictions of high order perturbative QCD calculations.     

The transition from the lattice to the continuum: CPN−1 models at large N

We study in detail the large N solution for the CP^N?1 models on a euclidean lattice and their phase structure in D dimensions. In the two-dimensional case, we evaluate some physical quantities and discuss their behaviour in the continuum limit. In particular, for the internal energy, we found the analytic expression for the perturbative tail which, at large β, obscures the predicted renormalization group behaviour; such a tail, which, as we show, is reproduced by a mean field calculation at N=∞, must be subtracted in order to define a suitable scaling quantity. We also investigated the behaviour of the topological charge and we found no perturbative tail in agreement with previous Lüscher's results. Finally, some aspects of the supersymmetric case are discussed.     

Heavy flavour multiplicities at very high energies

We calculate the total multiplicity of heavy flavours in e⁺e^? annihilation at very high energies in the framework of QCD. The problem is dominated by the soft limit of the perturbative expansion and we develop suitable techniques to resum to all orders the leading double logarithmic terms. The results indicate at very high energy a growth of multiplicity faster than any power of log. We find an interesting asymptotic factorization of energy dependence and flavour mass dependence of multiplicities.     

QCD at lowQ 2—A correspondence relation for moments of structure functions

The precocious validity of QCD predictions in deep inelastic lepton nucleon scattering ande ⁺ e ^? annihilation is interpreted as a signal for an underlying “correspondence principle” relating perturbative and nonperturbative physics on theQ ² average. Correspondence relations for nonsinglet moments of deep inelastic structure functions are formulated, discussed and successfully tested against experiment. The relations provide an independent determination of the QCD ?-parameter from lowQ ² data in perfect agreement with results from largeQ ² analyses.     

Why are perturbative corrections to highP T hadron production in photon-photon collisions very small?

TheO(α _s ) corrections to the single hadron inclusive cross-section at high transverse momentum has recently ben found to be very small. This is in sharp contrast to similar corrections to Drell-Yan cross-sections where the first perturbative terms can more than double the size of naive parton model predictions. In this paper we present an intuitive explanation of these results. By considering gauge symmetries of various sets of Feynman diagrams, we have obtained well defined relationships between photon-photon ande ⁺ e ^? annihilation cross-sections. These have been used to study dominant sectors of the phase-space for the two processes, resulting in a simple interpretation of the perturbative results. A by-product is a much simplified algorithm which has enabled us to extend our previous no-tag calculation to the case of single-tag events. In particular, by keeping one of the incomming photons off-shell by an arbitrary amount, we have exposed photon mass dependence of the cross-section right down to the region of deep inelastic scattering on a photon target.