Model Independent Search for Deviations from the Standard Model ...

Chapter 5
Monte Carlo Samples
5.1 MC Generator
The PYTHIA Program
This analysis uses only Monte Carlo samples with SM-processes. All except the t¯t-sample
are generated with PYTHIA (versions: 6.2 [41]), a leading-order parton shower generator.
What this means will be discussed in the following.
The PYTHIA program is frequently used for event generation in high-energy physics, simulating
the hard interactions in e + e , − pp and ep colliders. Especially at a hadron collider
at 2 TeV the emphasis is on multiple particle production. The best way to describe the
complexity of the interaction, its higher order corrections and the hadronisation, is to factorize
the problem in several components:
Initially two beam particles characterized by their parton density functions (pdf) approach
each other head on. Only two partons enter the hard process, which makes up the core of
the reaction. A typical example would be a 2 → 2 process with two initial partons and
two nal states. Leading order matrix element calculations are used to compute the exact
kinematics. A good example would be the W-production displayed in Figure 5.1. This is
the 2 → 1 production of a resonance followed by the leptonic decay of the gauge boson. As
hadrons have a substructure of quarks and gluons, one depends on the parameterizations
of pdf's which are based on experimental data (see Hera [4]). Uncertainty related to the
choice of a certain pdf will appear again in Section 8.2, as it results in a cross section error.
The second component simulated by PYTHIA is modeling perturbative corrections. The
problem now is that loop corrections and other higher order eects become increasingly
dicult to compute in higher orders. Often one is interested in the inclusive production
of particles, so the correct description of initial- and nal state radiation is crucial. As
the couplings of the strong interaction α s and of the electromagnetic interaction α em have
dierent magnitudes with α s ≫ α em , the gluon radiation dominates. Examples for these
radiativecorrectionscouldbetheinitialradiationofagluonbyoneoftheincomingquarks,