Soft gluons at large angles in hadron collisions

A general discussion is presented of the single logarithmic soft factor that appears in two scale QCD observables in processes involving four partons. We treat it as the ``fifth form factor'', accompanying the four collinear singular Sudakov form factors attached to colliding and outgoing hard partons. The fifth form factor is expressed in terms of the Casimir operators (squared colour charges) of irreducible representations in the crossing $t$- and $u$-channels. As an application we revisit the problem of large angle radiation in $gg\to gg$ and give a relatively simple solution and interpretation of the results. We found an unexpected symmetry of the soft anomalous dimension under exchange of internal and external variables of the problem whose existence calls for explanation.Comment: 29 pages, pictures generated with feynmf graphic packag

Power corrections and the interplay between perturbative and non-perturbative phenomena

We discuss the issue of interplay between perturbative and non-perturbative phenomena for power corrections to e+e- event shapes.Comment: 7 pages, to appear in the proceedings of QCD99 Euroconference, Montpellier, July 199

A Comparison of the Momentum Spectra in Deep Inelastic Scattering with the Modified Leading Logarithmic Approximation

The evolution of the logarithmic scaled energy spectra with energy scale, in the framework of the modified leading logarithmic approximation (MLLA), is investigated. The behaviour of the higher order moments of HERA deep inelastic scattering data are compared with the analytic predictions of MLLA.Comment: 9 pages incl 4 figure

Near-to-planar 3-jet events in and beyond QCD perturbation theory

We present the results of QCD analysis of out-of-event-plane momentum distribution in 3-jet e+e- annihilation events. We consider the all-order resummed perturbative prediction and the leading power suppressed non-perturbative corrections to the mean value and the distribution and explain their non-trivial colour structure. The technique we develop aims at improving the accuracy of the theoretical description of multi-jet ensembles, in particular in hadron-hadron collisions.Comment: 11 pages, 2 figure

Perturbative universality in QCD jet physics

I survey some recent advances in the applications of the analytical perturbative approach to the description of particle distributions in multi-jet processes. New tests of the perturbatively based picture in the (semi) soft region are discussed.Comment: LaTeX, 15 pages, talk at the XXVII International Symposium on Multiparticle Dynamics, Frascati (Rome), Italy, 8-12 September 199

Tests of hard and soft QCD with e+e- Annihilation Data

Experimental tests of QCD predictions for event shape distributions combining contributions from hard and soft processes are discussed. The hard processes are predicted by perturbative QCD calculations. The soft processes cannot be calculated directly using perturbative QCD, they are treated by a power correction model based on the analysis of infrared renormalons. Furthermore, an analysis of the gauge structure of QCD is presented using fits of the colour factors within the same combined QCD predictions.Comment: 6 pages, 7 figures, contribution to Siena 2001, Seventh Topical Seminar on The legacy of LEP and SLC, Siena, Italy, 8 to 11 October 2001, proceedings to appear in Nucl. Phys. B (Proc. Suppl.

Hadronic Antenna Patterns as a Probe of Leptoquark Production at HERA

Hadronic antenna patterns can provide a valuable diagnostic tool for probing the origin of the apparent excess of high $x,Q^2$ events at HERA. We present quantitative predictions for the distributions of soft particles and jets in standard deep inelastic scattering $eq \to eq$ events and in events corresponding to the production of a narrow colour-triplet scalar (`leptoquark') resonance. The corresponding distributions of soft photon radiation, which are sensitive to the leptoquark electric charge, are also presented. The distribution for one particular leptoquark assignment is shown to contain a radiation zero.Comment: 16 pages, LaTeX, incl. 6 postscript figures, uses epsfig.sty. (High resolution pictures may be obtained via email.

Power correction analyses in e+e- annihilation

The current status of theoretical work and experimental analyses on power corrections in QCD for e+e- annihilation will be reviewed. Measurements of the number of active quark flavours n_f and the QCD colour factors C_A and C_F derived from QCD fits to event shape distributions at lower energies and LEP energies will be presented. The fits are based on O(alpha_s^2)+NLLA QCD predictions with power corrections to model hadronisation.Comment: Contribution to High Energy Physics International Conference on Quantum Chromo Dynamics (QCD00), Montpellier, 6 to 12 July, 200

Perturbative QCD analysis of near-to-planar three jet events

We present the all-order resummed thrust minor distribution as a measure of aplanarity in three-jet events.Comment: 5 pages, 2 figures, Talk presented by Andrea Banfi at QCD 00 Euroconference, Montpellier, July 200

Inclusive hadronic distributions in jets and sub-jets with jet axis from color current

The hadronic \kt-spectrum and the gluon to quark average multiplicity ratio $r=N_g/N_q$ inside a high energy sub-jet are determined from a precise definition of the jet axis, including corrections of relative magnitude ${\cal O}(\sqrt{\alpha_s})$ with respect to the Modified Leading Logarithmic Approximation (MLLA), in the limiting spectrum approximation (assuming an infrared cut-off Q_0 =\lqcd). The results for the \kt-spectrum in the limiting spectrum approximation are found to be, after normalization, in impressive agreement with measurements by the CDF collaboration and the ratio has not been measured yet.Comment: Invited lecture at Ringberg Workshop: New Trends in HERA Physics 2008, October 5-10, 2008, Ringberg Castle, Tegernsee-Bavaria, German