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.     

Angular energy flow in leptoproduction and QCD

The perturbative QCD prediction of the angular energy flow in deep inelastic leptoproduction is discussed. A numerical estimate of its magnitude is given in the Breit frame and in the c.m. frame of the virtual photon and proton.     

Energy-energy correlations in e+e− annihilation into hadrons

Measurements of energy-energy correlations in hadronic final states produced in e⁺e^? annihilation at c.m. energies between 7.7 and 31.6 GeV are presented. The data are compared to perturbative QCD predictions. Good qualitative agreement above 20 GeV c.m. energy is found. The importance of non-perturbative effects is discussed, as well as the detailed behaviour of the correlation near 180°.     

QCD and short range nuclear phenomena

We have found essential QCD diagrams for the leading particle production in the high energy processes. The high momentum component of the deuteron (nucleus) wave function is estimated in terms of perturbative QCD. Reasonable agreement of the formulae obtained with the observed cross section of the deep inelastic reaction e + D → e + X at x > 1, and the observed momentum dependence of the high energy reactions h + A → fast backward, p, π + X indicate that the nuclear core and cumulative particle production are actually QCD phenomena. The crucial importance of the deep inelastic 1A scattering at x > 1 for the investigation of the short range nuclear structure is explained.     

Unified description of rapidity gaps and energy flows in DIS final states

We show that the ‘orthogonal’ characteristics of the observed rapidity gaps and large forward energy flows in deep inelastic scattering at HERA, can be described within a single framework. Our Monte Carlo model is based on perturbative QCD matrix elements and parton showers together with Lund string model hadronization, but has in addition a new mechanism for soft colour interactions which modifies the perturbative colour structure and thereby the hadronization. Effects of perturbative multiparton emission are investigated and the non-perturbative treatment of the proton remnant is discussed and comparison to the observed transverse energy flow is made. We investigate the resulting diffractive-like properties of the model; such as rapidity gap events, t- and M _X -distributions and the diffractive structure function in comparison to H1 data.     

Production of low transverse energy clusters in pp collisions at √s=0.2–0.9 TeV and their interpretation in terms of QCD jets

The production of transverse energy clusters in minimum bias proton-antiproton collisions at the CERN SPS Collider is studied with the UA1 detector over a new range of centre of mass energies (√s = 0.2−0.9 TeV). This study is intended to investigate how low in transverse momentum perturbative QCD is able to describe the dynamics of hadron collisions. We observe that clusters with transverse energy in excess of a few GeV exhibit properties in agreement with QCD expectations for parton scattering, supporting their interpretation in terms of jet production. We find that the jet-event rate represents a sizeable fraction of the inelastic rate and is increasing with √s over the measured energy range.     

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.     

DiffractiveJ/Ψ electroproduction in LLA QCD

Cross section of diffractiveJ/ψ production in deep inelastic scattering in the Born and the leading-log approximations of perturbative QCD are calculated.     

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.     

Two-loop QCD corrections to the coefficient functions of the deep inelastic structure functionsF 2 andF L

We present the analytic two-loop perturbative QCD corrections in the leading twist approximation to the coefficient functions of the operator product expansion for the second to tenth moments of the nonsinglet and singlet deep inelastic structure functionsF ₂ andF _L .     

Testing QCD scaling violations in the HERA energy range

High energy electron-proton colliders open new kinematical domains for the investigation of scaling violations in nucleon structure functions. Focussing on HERA as a definite study case we explore the precision which can be expected in determinations of the QCD scale parameter Λ and the running coupling constant α _s (Q ²). Our results provide a quantitative basis for discussions of the possibilities and requirements of QCD tests in inclusive deep inelastic scattering at HERA energies.     

Review of Deep Inelastic Charged Lepton Scattering

The present situation in deep inelastic lepton-nucleon scattering is reviewed from an experimenter's point of view. After describing, in some detail, the quark-parton model (QPM) predictions the basic ideas of Quantum Chromodynamics (QCD) are illustrated. The perturbative QCD predictions for deep inelastic scattering are summarised discussing also their theoretical uncertainties of today. Past and present experiments on deep inelastic electron and muon scattering are compared regarding their ability to test these predictions. It is shown that all data is in very good agreement with the QPM and fully consistent with perturbative QCD. New results on lepton scattering off nuclei are discussed and confronted to recently invented phenomenological models.     

Scaling violations and perturbative quantum chromodynamics

We try to understand the meaning of the recent data on scaling violations of the moments of the structure function F₃ measured in ν and $\text{ν}$ deep inelastic scattering, and their relevance as a test of QCD. We do this by reducing to the minimum the theoretical machinery and prejudices and stressing the perturbative nature of the problem. We are led to a definition of the perturbation coupling constant σ_S(Q = 2.5 GeV) = 0.61 ± 0.06, in termas of which the corrective terms for all quantities computed so far turn out to be relatively small.     

Pomeron structure function and diffraction dissociation of virtual photons in perturbative QCD

We study scaling properties of the diffraction dissociation of virtual photons in a deep inelastic scattering. We concentrate on the total diffraction dissociation rate, diffraction excitation mass spectrum and the pomeron structure function to the lowest order in perturbative QCD. We calculate the valence structure function and the strangeness and charm content of the pomeron and estimate the ocean structure function using the pomeron factorization property. We find that quarks carry ≈ 10% of pomeron's momentum. Differential cross section of the (virtual) photon-pomeron scattering is found to exhibit features typical of the hadronic two-body reactions, supporting a treatment of the Pomeron as a particle, whereas the flavor dependence of structure functions does not support the particle treatment of the pomeron. Diffraction dissociation of photons is predicted to make ≈ 15% of the total deep inelastic scattering rate at smallx and largeQ ². Detailed predictions for the mass spectrum and angular distribution of jets produced on the valence component of the pomeron are presented.     

Nuclear effects in massive lepton pair production

Precise measurements of nuclear effects in the production of massive lepton pairs from heavy targets by pion and proton beams would extend knowledge of the nuclear dependence of antiquark and quark structure functions of nucleons into important intervals of the fractional longitudinal momentum variable x_Bj not easily accessible in deep inelastic scattering. Isolating nuclear dependence is also essential before definite conclusions can be drawn on the behavior of the pion structure function and on the magnitude and kinematic variation of the perturbative QCD K-factor obtained from heavy target data.     

Measuring the charges of QCD jets

We investigate the effects of perturbative branching upon the accuracy with which one can determine the charge of the underlying QCD quantum from the charge structure of a given hadronic jet. We show explicitly how at asymptoticQ ² we lose all such charge information. We investigate these effects at current PETRA energies using the Monte Carlo program of Fox and Wolfram; and find that a reasonably accurate charge determination is still possible at these energies. We suggest the variation of the jet charge structure with multiplicity, at a given energy, as a sensitive probe of the onset of perturbative branching inside jets.     

Corrections to angular ordering in multiple hadroproduction

One of the key ideas in describing the multiparticle production at high energies in the perturbative QCD is the angular ordering in successive soft-gluon emission. We analyse the angular distribution of particles in a jet, and investigate the corrections to the angular ordering. At a small angle from the jet direction, the angular ordering is exact at the next-to-leading order. At a large angle, the angular ordering overestimates the angular particle density by a factor (1?γln cos²(γ/2)), whereγ is the anomalous dimension of the multiplicity. TheO(γ) correction restores the boost invariance of the cross section.     

Singlet structure function at small x: Unitarization of gluon ladders

The singlet structure function of a deep inelastic process, D(x, q², is studied in the double log approximation of perturbative QCD at small x and large q². It is shown, how the increase of D at small x is screened by multiladder diagrams, so that the unitarity is not violated. The equation that sums such diagrams is written and solved, the screening effects being reduced to a specific boundary condition for the ladder equation.