10 - H1 - Desy

2.1 Monte Carlo simulation 31
by a struck quark and the proton remnant.
• Fragmentation
After the radiation step, as distances between partons increase, QCD becomes
strongly interacting and the perturbation theory breaks down. The coloured partons
are combined into colourless hadrons in a process called fragmentation. Since
the fragmentation region is non perturbative, some phenomenological models are
used.
In the Lund string model [57,58], two colour charged objects are bound together
forming a string. As two particles move apart, the potential energy of the string
increases until enough energy is accumulated to produce a quark-antiquark pair in
a break up of the string. Newly created quarks get connected with the loose ends
of the original string and the process continues until stable hadrons emerge.
An alternative to the Lund string model is provided by the cluster decay fragmentation
(CDF) model [59]. In this approach all gluons produced in parton showers
are decayed into quark-antiquark pairs. Colourless pairs of q¯q form clusters consequently
decaying into final state hadrons.
• Detector simulation
In the last step, the final state particles are passed through the detector simulation
producing a comparable output with information collected in real data experiment.
In the case of the H1 experiment (see chapter 3), the H1SIM [60] software package,
based on the GEANT [61] program provides a detailed description of the H1 detector
including all the instrumentation as well as passive material. Due to the high time
consumption of the detailed shower simulation in the LAr calorimeter [62,63] (see
section 3.2.2), it has been exchanged by the significantly faster H1FAST [64, 65]
shower parameterisation. It has been proven [66] that the used parameterisation
gives reliable shower description even in such a detailed shower shape analysis (see
chapter 6) as used for this thesis. In the same way as for real data, the output of the
H1SIM package is passed through the H1REC [67] reconstruction program, which
interprets the electronic signals and creates composed objects.
As pictured in figure 2.1, the event properties can be studied using partons, hadrons
or detector objects and are referenced later on as parton, hadron and detector levels.
The parton (generator) level takes into account all the particles after the radiation step.
The hadron level uses all the stable final state particles produced in the fragmentation
step as well as stable particles that do not hadronize. Finally, experimentally measured
detector level uses all the objects obtained after the reconstruction stage, such as tracks
and clusters. The direct comparison between data and theory is possible only on the same
level, usually being the hadron level. MC, having its representation on all three levels is
used for transition between different levels.