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Giuseppe Marchesini

Publications and source records attributed to Giuseppe Marchesini.

12 recordsLinked to original sources
arXiv2015

The BMS Equation and c\bar{c} Production; A Comparison of the BMS and BK Equations

We introduce two processes where the BMS equation appears in a context quite different from the original context of non-global jet observables. We note the strong similarities of the BMS equation to the BK and FKPP equations and argue that these, essentially identical equations, can be viewed either in terms of the probability, or amplitude, of something not happening or in terms of the nonlinear terms setting unitarity limits. Mostly analytic solutions are given for (i) the probability that no $c\bar{c}$ pairs be produced in a jet decay and (ii) the probability that no-$c\bar{c}$ pairs be produced in a high energy dipole nucleus scattering. Both these processes obey BMS equations, albeit with very different kernels.

hep-ph↗
arXiv2008

High energy gravitational scattering: a numerical study

The S-matrix in gravitational high energy scattering is computed from the region of large impact parameters b down to the regime where classical gravitational collapse is expected to occur. By solving the equation of an effective action introduced by Amati, Ciafaloni and Veneziano we find that the perturbative expansion around the leading eikonal result diverges at a critical value signalling the onset of a new regime. We then discuss the main features of our explicitly unitary S-matrix down to the Schwarzschild's radius R=2G s^(1/2), where it diverges at a critical value b ~ 2.22 R of the impact parameter. The nature of the singularity is studied with particular attention to the scaling behaviour of various observables at the transition. The numerical approach is validated by reproducing the known exact solution in the axially symmetric case to high accuracy.

hep-th↗
arXiv2007

From QCD Lagrangian to Monte Carlo simulation

I discuss old and recent aspects of QCD jet-emission and describe how hard QCD results are used to construct Monte Carlo programs for generating hadron emission in hard collisions. I focus on the program HERWIG at LHC.

hep-ph↗
arXiv2006

QCD Review

This review is focused on QCD theoretical issues and their phenomenological relevance specially for LHC. It is incomplete and mostly neglects the phenomenology of long distance physics

hep-ph↗
arXiv2006

Relating small Feynman and Bjoken $x$

This is a working progress report on the attempt by Yuri Dokshitzer, Gavin Salam and myself to relate the small-$x$ behaviour of the anomalous dimensions in the time- and space-like cases. This relation is based on a ``reciprocity respecting equation'' we propose

hep-ph↗
arXiv2005

Jet evolution and Monte Carlo

In this lecture I discuss jet-shape distributions and describe how from jet evolution one may design Monte Carlo simulations which are used in the analysis of short distance distributions in $\ee$-annihilation, lepton-hadron and hadron-hadron collisions

hep-ph↗
arXiv2004

Jet-shape observables

Studies of jet-shape observables in hard processes are summarized together with future developments

hep-ph↗
arXiv1994

QCD coherence in the structure function and associated distributions at small $x$

We recall the origin of angular ordering of soft parton emission in the region of small $x$ and show that this coherent structure can be detected in associated distributions. For structure functions at small $x$ and at fixed transverse momentum the angular ordering is masked because of the complete inclusive cancellations of collinear singularities for \xt0. Therefore, in this case the dependence on the hard scale is lost and the angular ordered region becomes equivalent to multi-Regge region in which all transverse momenta are of the same order. In this limit one derives the BFKL equation. In general such a complete cancellation does not hold for the associated distributions at small $x$. The calculation, which requires an analysis without any collinear approximation, is done by extending to small $x$ the soft gluon factorization techniques largely uses in the region of large $x$. Since one finds angular ordering in the both regions of small and large $x$, one can formulate a unified evolution equation for the structure function, a unified coherent branching and jet algorithm which allows the calculation of associated distributions in all $x$ regions. Such a unified formulation valid for all $x$ is presented and compared with usual treatments. In particular we show that the associated distributions at small $x$ are sensitive to coherence. By replacing angular ordering with the multi-Regge region one neglects large singular contributions in the associated distributions.

hep-ph↗
arXiv1993

Lattice Perturbation Theory by Langevin Dynamics

We present an application of the standard Langevin dynamics to the problem of weak coupling perturbative expansions for Lattice QCD. This method can be applied to the computation of the most general observables. In this preliminary work we will concentrate in particular on the computation of the perturbative terms of the $1\times 1$ Wilson loop, up to fourth order. It is shown that a stochastic gauge fixing is a possible solution to the problem of divergent fluctuations which affect higher order coefficients.

hep-lat↗