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

Chapter 4
Monte Carlo Simulation Samples
4.1 Introduction
Event generation consists of the production of a set of particles which is passed to detector simulation. Each
generated event contains the particles from a single interaction with a vertex located at the geometrical
origin. Several modifications, to account for the beam properties, are applied to the event before it is passed
to simulation. Particles with a life time cτ > 10 mm are considered stable by the generator, since they can
propagate far enough to interact with the detector material before decaying. Their decays are then han-
dled by the simulation. Generators produce complete events starting from proton-proton, proton-nucleus or
nucleus-nucleus initial states [75].
Several physics aspects must be considered by an event generator in the description of a typical high-
energy process. In colliders, like the LHC, two beams particles come in towards each other. Each particle
can be characterized by a set of parton distributions, defining the partonic substructure in terms of flavor
composition and energy sharing. The initial-state shower is built up by one parton from each beam, which
initiates one shower starting off a sequence of branchings (such as q → qg). Then, one incoming parton from
each of the two showers enters the hard process. Here, a number of outgoing partons are produced. The
nature of the hard process determines the main characteristics of the event. The hard process may produce
a set of short-lived resonances (like W/Z-bosons) whose decay into normal partons has to be considered
in close association with the hard process itself. The outgoing partons may radiate or decay building up
final-state showers. Further semi-hard interactions can occur between the other partons of the two incoming
hadrons. Many of the produced hadrons are unstable and decay. When a shower-initiator parton is taken
out of a beam, a beam remnant is left behind. This beam remnant may have an internal structure and a
net color charge that relates it to the rest of the final state, forming part of the same fragmentation system.
These additional hard or soft scattering processes comprise the underlying event. It must also be taken into
account that QCD confinement mechanism ensures that the outgoing quarks and gluons are not observable,
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