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

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.

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

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

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/

A new look for good old parton dynamics

A short review is given of the idea and of the present status of recently proposed evolution equations that respect the Gribov-Lipatov reciprocity between space-like and time-like parton dynamics in all orders.Comment: Talk presented at the XXXIX International Symposium on Multiparticle Dynamics, Gomel, Belorussia, September 2009; 5 page

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

Power corrections to event shapes and factorization

We study power corrections to the differential thrust, heavy mass and related event shape distributions in $e^+e^-$-annihilation, whose values, $e$, are proportional to jet masses in the two-jet limit, $e\to 0$. The factorization properties of these differential distributions imply that they may be written as convolutions of nonperturbative "shape" functions, describing the emission of soft quanta by the jets, and resummed perturbative cross sections. The infrared shape functions are different for different event shapes, and depend on a factorization scale, but are independent of the center-of-mass energy $Q$. They organize all power corrections of the form $1/(eQ)^n$, for arbitrary $n$, and carry information on a class of universal matrix elements of the energy-momentum tensor in QCD, directly related to the energy-energy correlations.Comment: 15 pages, LaTeX style, 1 figure embedded with epsf.st

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