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G. Marchesini

Publications and source records attributed to G. Marchesini.

At least 19 recordsLinked to original sources
arXiv2009

Monte Carlo and large angle gluon radiation

We discuss the problem of incorporating recoil effects into the probabilistic QCD evolution scheme based on the picture of colour dipoles as done in recent Monte Carlo programs. Such a scheme correctly describes subleading soft contributions to multiplicity distributions. However we find that a simple receipt for incorporating recoil effects into the dipole multiplication picture conflicts the collinear factorization and does not lead to the correct DGLAP equation.

hep-ph↗
arXiv2007

Twist 3 of the sl(2) sector of N=4 SYM and reciprocity respecting evolution

We consider the bosonic sl(2) sector of the maximally supersymmetric N=4 SYM model and show that anomalous dimension of the twist-3 single-trace composite operators built of scalar fields, recently calculated up to the four-loop order, can be generated by a compact reciprocity respecting evolution kernel.

hep-th↗
arXiv2006

N=4 SUSY Yang--Mills: three loops made simple(r)

We construct universal parton evolution equation that produces space- and time-like anomalous dimensions for the maximally super-symmetric N=4 Yang--Mills field theory model, and find that its kernel satisfies the Gribov--Lipatov reciprocity relation in three loops. Given a simple structure of the evolution kernel, this should help to generate the major part of multi-loop contributions to QCD anomalous dimensions, due to classical soft gluon radiation effects.

hep-th↗
arXiv2006

Revisiting parton evolution and the large-x limit

This remark is part of an ongoing project to simplify the structure of the multi-loop anomalous dimensions for parton distributions and fragmentation functions. It answers the call for a "structural explanation" of a "very suggestive" relation found by Moch, Vermaseren and Vogt in the context of the x->1 behaviour of three-loop DIS anomalous dimensions. It also highlights further structure that remains to be fully explained.

hep-ph↗
arXiv2005

Hadron collisions and the fifth form factor

Logarithmically enhanced effects due to radiation of soft gluons at large angles in $2\to 2$ QCD scattering processes are treated in terms of the "fifth form factor" that accompanies the four collinear singular Sudakov form factors attached to incoming and outgoing hard partons. Unexpected symmetry under exchange of internal and external variables of the problem is pointed out for the anomalous dimension that governs soft gluon effects in hard gluon--gluon scattering.

hep-ph↗
arXiv2005

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.

hep-ph↗
arXiv2005

Structure functions for large $x$ and renormalization of Wilson loops

We discuss the relation between partonic distributions near the phase space boundary and Wilson loop expectation values calculated along paths partially lying on the light-cone. Due to additional light-cone singularities, multiplicative renormalizability for these expectation values is lost. Nevertheless we establish the renormalization group equation for the light like Wilson loops and show that it is equivalent to the evolution equation for the physical distributions. By performing a two-loop calculation we verify these properties and show that the universal form of the splitting function for large x originates from the cusp anomalous dimension of Wilson loops.

hep-ph↗
arXiv2004

Exact solution of BFKL equation in jet-physics

It has been recently found that the heavy quark-antiquark QQbar pair multiplicity, in certain phase space region (QQbar at short distance, soft and with small velocity), satisfies an evolution equation formally similar to the BFKL equation for the high energy scattering amplitude. We find the exact solution of the QQbar-equation and discuss the differences with the BFKL scattering amplitude.

hep-ph↗
arXiv2003

BFKL Dynamics in Jet Evolution

We calculate $e^+e^- \to Q\bar{Q}(k) + anything in a certain momentum, k, region for heavy quark-antiquark (Q\bar{Q})production. In our chosen region we find that the number of heavy quark pairs produced is determined by BFKL dynamics and the energy dependence of the number of pairs is given by $αp-1$ the hard pomeron intercept.

hep-ph↗
arXiv2003

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.

hep-ph↗
arXiv2002

HERWIG 6.5: an event generator for Hadron Emission Reactions With Interfering Gluons (including supersymmetric processes)

HERWIG is a general-purpose Monte Carlo event generator, which includes the simulation of hard lepton-lepton, lepton-hadron and hadron-hadron scattering and soft hadron-hadron collisions in one package. It uses the parton-shower approach for initial- and final-state QCD radiation, including colour coherence effects and azimuthal correlations both within and between jets. This article updates the description of HERWIG published in 1992, emphasising the new features incorporated since then. These include, in particular, the matching of first-order matrix elements with parton showers, a more correct treatment of heavy quark decays, and a wide range of new processes, including many predicted by the Minimal Supersymmetric Standard Model, with the option of R-parity violation. At the same time we offer a brief review of the physics underlying HERWIG together with details of the input and control parameters and the output data, to provide a self-contained guide for prospective users of the program. This version of the manual (version 3) is updated to HERWIG version 6.5, which is expected to be the last major release of Fortran HERWIG. Future developments will be implemented in a new C++ event generator, HERWIG++.

hep-ph↗
arXiv2002

Away-from-jet energy flow

We consider interjet observables in hard QCD processes given by the energy flow E_out in a region away from all hard jets. Here the QCD radiation is depleted (E_out << Q), and therefore these observables provide ideal means to study non-perturbative effects. We derive an evolution equation (in the large N_c limit) which resums, for large Q/E_out, all leading terms arising from large angle soft emission (double logarithms are absent). We discuss the analytical features of the result and identify universal and geometry-dependent contributions. Our analysis confirms features found using numerical methods by Dasgupta and Salam.

hep-ph↗
arXiv2002

Azimuthal correlation in DIS

We introduce a new angular correlation in DIS process and study its differential distribution in the region in which the observable is small. We perform a perturbative resummation at single logarithmic accuracy and estimate leading non-perturbative power corrections.

hep-ph↗
arXiv2002

Azimuthal correlation in DIS

We introduce the azimuthal correlation for the deep inelastic scattering process. We present the QCD prediction to the level of next-to-leading log resummation, matching to the fixed order prediction. We also estimate the leading non-perturbative power correction. The observable is compared with the energy-energy correlation in e+e- annihilation, on which it is modelled. The effects of the resummation and of the leading power correction are both quite large. It would therefore be particularly instructive to study this observable experimentally.

hep-ph↗
arXiv2002

Out-of-plane QCD radiation in DIS with high p_t jets

We present a QCD analysis of the cumulative out-of-event-plane momentum distribution in DIS process with emission of high p_t jets. We derive the all-order resummed result to next-to-leading accuracy and estimate the leading power correction. We aim at the same level of accuracy which, in e+e- annihilation, seems to be sufficient for making predictions. As is typical of multi-jet observables, the distribution depends on the geometry of the event and the underlying colour structure. This result should provide a powerful method to study QCD dynamics, in particular to constrain the parton distribution functions, to measure the running coupling and to search for genuine non-perturbative effects.

hep-ph↗
arXiv2002

HERWIG 6.4 Release Note

A new release of the Monte Carlo program HERWIG (version 6.4) is now available. The main new features are: spin correlations between the production and decay of heavy fermions, i.e. top quarks, tau leptons and SUSY particles; polarization effects in SUSY production processes in lepton-lepton collisions; an interface to TAUOLA for tau decays; MSSM Higgs processes in lepton-lepton collisions.

hep-ph↗
arXiv2001

Hard QCD: some results

I recall some points on the present status of QCD results in the short distance regions and illustrate the case of event shape in QCD radiation

hep-ph↗