10 - H1 - Desy

30 Theoretical predictions
partons
hadrons
detector objects
e ’
e
fragmentation
H1 - detector
p
hard scattering
parton shower
perturbative
fragmentation
not perturbative
simulation
reconstruction
Figure 2.1: MC generation process in the ep scattering case. The figure is taken
from [54].
• Hard scattering
The interaction between particles are described by matrix elements calculated according
to the Feymann rules. This perturbative step determines the main characteristics
of the event. The parton from the proton is chosen according to parton
density functions (PDFs) describing the probability of finding a parton in a proton
at a given Q 2 and x values. The photon PDF is used for choosing a parton from the
photon in case of resolved events. For more details concerning PDFs see section 1.2.
• Initial and final state radiation
The existence of charged or coloured objects before or after hard interaction may
lead to large corrections due to photon or gluon emissions. The electromagnetic
radiation is modelled according to QED.
QCD corrections may be modelled by the so called Parton Shower (PS) model [55]
describing the parton cascade by splitting of a parent parton into two daughters.
The possible transitions are q → qg, g → q¯q, g → gg and the evolution of the whole
shower is based on the DGLAP equations (see section 1.2) and splitting functions
P αβ (x/z). The PS model can be applied for both initial and final state radiation
and therefore the parton entering the hard interaction may already originate from
a parton splitting.
An alternative to the PS model is given by a colour dipole model (CDM) [56]
where each pair of coloured objects is treated as a colour dipole emitting gluons.
The radiation leads to additional dipoles emitting gluons and resulting in a parton
cascade. In case of ep scattering, the cascade is initiated by the dipole constructed