CERN-THESIS-2012-153 26/07/2012 - CERN Document Server

evolution equations for the q 2 dependance. The composite structure of hadrons allow for multiple parton-
parton scatterings to occur, in which case correlated parton distributions should be used to describe the
multi-parton structure of the incoming beam.
Initial- and Final-State Radiation
In every process that contains colored and/or charged objects in the initial or final state, gluon and/or photon
radiation may give large corrections to the overall topology of events. As the collision energy increases, hard
emission of this kind becomes more important, relative to fragmentation, in determining the event structure.
Two approaches are used to model the perturbative corrections. One is the matrix-element method, in which
Feynman diagrams are calculated, order by order. This is, in principle, the correct approach, since it takes
into account exact kinematics, and the full interference and helicity structure. However, these calculations
are increasingly difficult in higher orders, in particular for loop graphs. The second possible approach is
the parton-shower one. Here an arbitrary number of branchings of one parton into two, or more, may be
combined, to yield a description of multijet events, with no explicit upper limit on the number of partons
involved. Approximations derived by simplifying the kinematics, and the interference and helicity structure,
are used.
Hadronization
QCD perturbation theory, formulated in terms of quarks and gluons, is valid at short distance. At long
distances, QCD becomes increasingly interactive, breaking down perturbation theory. In this confinement
regime, the colored partons are transformed into colorless hadrons, a process called hadronization or frag-
mentation. The tight cone of particles created by the hadronization of a single quark is called a jet. The
hadronization process can not be derived from first principles, therefore different phenomenological models
are used to implement it in the MC. All current models are of a probabilistic and iterative nature. For
the description of soft scattering, parton shower modeling provides a relatively good description of collision
physics, but not as good for hard scattering events. In such case, higher-order perturbative calculations of
the hard scattering matrix element are needed. There are two main approaches for this: CKKM scheme and
the MLM scheme [89]. In both cases different jet multiplicities from matrix elements are combined, without
double counting with the parton shower emission.
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