%============================================================================== 
% % mpi08 Worshop proceedings: Individual contribution % 
%============================================================================== 
\documentclass[10pt]{mpi08} \usepackage{graphicx} 
\usepackage{cite,./mcite} \def\bibtex{{\sc BiBTeX}} 
\setcounter{footnote}{1} 
%============================================================================== 
% Here comes the document. 
%============================================================================== 
\begin{document} 
\title{Introduction to the Monte Carlo Models session} 
\author{Jonathan M. Butterworth$^1$, Torbj\"orn Sj\"ostrand$^2$}
\institute{$^1$Department of Physics and Astronomy, 
University College London\\ 
$^2$Department of Theoretical Physics, Lund University}
\maketitle
%Do we need Abstract? Hopefully not!
%\begin{abstract}
%Monte Carlo modelling and tuning are indispensable activities.
%Much of the progress in recent years would not have happened 
%without them.
%\end{abstract}

%\section{}
\label{sec:32}

There is hardly any area of hadron collider physics where event generators
play such a central a role as they do for the exploration of MPI.
One reason is that MPI, although extending well into the perturbative
region, have their biggest impact close to, or inside, the nonperturbative 
regime. Another is that MPI studies by necessity probe \textit{all} 
the main physics aspects of hadron colliders in an nontrivial admixture, 
including multiple partonic collisions, initial- and final-state 
radiation, beam remnant structure, colour flow issues, the 
impact-parameter picture, and hadronization. 

If the study of MPI has for the first time become fashionable 
within the particle 
physics community, it is in large part owing to the interplay between 
experimental studies and Monte Carlo modelling and tuning in recent 
years. Specifically, the CDF studies, already reviewed by Rick Field, 
have largely relied on the availability of generators that could 
provide a framework for the interpretation of the data. One case 
in point is that a unified description of mimimum-bias and 
underlying-event physics comes about quite naturally in MPI-based 
Monte Carlo implementations. Conversely, the renewed interest in 
improving and tuning models that have lain dormant for many years 
would not have happened without the influx of new data to digest.

The session on Monte Carlo Models collects talks within two areas.
Firstly presentations of several of the main generators, with an 
overview of new ideas and current status. Secondly presentations of
new tunes of these generators, which also introduce new tools that
allow a more systematic approach to the whole tuning effort. But it 
should be emphasized that event generators are central to many other 
studies presented at this meeting, in particular in sessions I and II.

Since it is all too easy to get carried away by the ``Yes, we can''
spirit that exists in the MPI community nowadays, in this introduction 
we would still like to remind the reader that many tough issues remain 
poorly understood and modelled. Thus there is still scope for significant 
improvements in the future, driven both by theoretical insights and
experimental studies. Several such topics made for corridor talk 
during the meeting, but are maybe not so well represented in the 
individual writeups, so here are a few examples:
\begin{itemize}
\item How to model and measure multi-parton density functions, 
that depend on multiple flavour choices and multiple $x$ and $Q^2$ scales? 
\item How does close-packing of partons in the initial state, 
especially at small $x$, tie in with the functioning of the colour 
screening mechanism?
\item Currently implemented MC models of MPI assume a factorisation between the 
$x$-dependence and impact-parameter profile of the incoming hadrons. Can this 
assumption be relaxed, and if so how large would the effect be?
\item Can the presence of rescattering events, i.e. where an incoming
parton scatters twice or more, be established experimentally, given that 
the natural signal of three outgoing jets competes with a large
QCD bremsstrahlung background? 
\item Can the initial-state branchings intertwine several 
$2 \to 2$ processes that are seemingly separate, and if so how?
\item A large amount of colour reconnection is favoured by the tunes of 
\textsc{Pythia} to $\langle p_{\perp} \rangle(n_{\mathrm{charged}})$ data; 
but is this
the correct interpretation and, if so, what is the physics and what are
the rules that govern colour reconnection?
\item To what extent can colour reconnection also affect the pattern of
perturbative QCD radiation? Can e.g. two dipoles each stretched between 
a final-state parton and (the hole left behind by) an initial-state one 
transform into a single dipole between the two final-state partons? 
\item Does the dense-packing of colour-field ``strings'' in central
collisions induce states that border on a quark-gluon plasma? 
\item Does the hadronization of these topologies give rise to a dense
hadron gas within which final-state rescatterings occur?
\item Given the above uncertainties, can we still assume that the 
composition of different particle species should be the same in 
hadronic collisions as in $e^+ e^-$ ones? 
\item How big a baryon-flow from the beam remnants to the central 
region should we expect?
\item How far can eikonal models be trusted to correctly relate 
different event topologies, including diffractive ones? Is maybe 
instead colour reconnection the proper way to think about the
emergence of diffractive topologies?
\item When tuning, how should the relative importance of various
data be judged? When are discrepancies due to poor physics or to poorly 
documented data? How can we avoid overtuning, {\it i.e.} avoid forcing 
the model to fit the data even if the data contain physics not included 
in the model? (Many experimental working groups and applications apply 
pressure to fit the data at any cost.)
\item Can meaningful uncertainties be attached to MC tunes, in particular 
for MPI? How far can particular physical effects be ruled out, or shown 
unambiguously to be present, based upon such tunes?
\end{itemize} 

In summary, the pride of recent successes should not blind us to the
challenges ahead. The LHC may well have surprises in store for us.

\end{document}
