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

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

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.

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

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

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.

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

On large angle multiple gluon radiation

Jet shape observables which involve measurements restricted to a part of phase space are sensitive to multiplication of soft gluon with large relative angles and give rise to specific single logarithmically enhanced (SL) terms (non-global logs). We consider associated distributions in two variables which combine measurement of a jet shape V in the whole phase space (global) and that of the transverse energy flow away from the jet direction, Eout (non-global). We show that associated distributions factorize into the global distribution in V and a factor that takes into account SL contributions from multi-gluon ``hedgehog'' configurations in all orders. The latter is the same that describes the single-variable Eout distribution, but evaluated at a rescaled energy VQ.Comment: 16 page

Perturbative QCD description of multiparticle correlations in quark and gluon jets

The QCD evolution equations in Modified Leading Log Approximation for the factorial moments of the multiplicity distribution in quark and gluon jets are numerically solved with initial conditions at threshold by fully taking into account the energy conservation law. After applying Local Parton Hadron Duality as hadronization prescription, a consistent quantitative description of available experimental data for factorial cumulants and factorial moments of arbitrary order and for their ratio both in quark and gluon jets and in $e^+e^-$ annihilation is achieved.Comment: LaTeX, 15 pages, 3 figures, typos corrected in the labels and caption of Figure

Cumulants of QCD Multiplicity Distributions in Small Phase Space Bins

It is shown that, as functions of their rank, cumulants of QCD multiplicity distributions in small phase space bins possess the quasi- oscillating behavior similar to that found for them in the total rapidity range. First minimum moves to lower ranks for smaller bins. For the total rapidity range, it moves to higher ranks with energy increase. The running property of the QCD coupling constant is in charge of these effects which can be verified in experiment.Comment: 7 pages, no Figs, preprint IP-ASTP-19; FIAN-TD 94/