Measurement of the Z boson cross-section in - Harvard University ...

Chapter 5: Monte Carlo Simulation 153
involved. An event generator program can calculate cross-sections, i.e. , event rates,
for a process. It can determine differential cross-sections as functions of kinematic
variables, and then generates hypothetical events to populate the phase space of these
variables. It provides energy-momentum four-vectors of primary particles produced
in the hard interaction and of the decay products of all unstable particles. Differ-
ential cross-sections can be estimated using either exact matrix elements to a given
degree of accuracy, or an ‘all-order’ approach such as parton showering or QCD re-
summation. At the end of the decay process, partons are grouped into hadrons using
a hadronization model (Chapter 1). At a hadron-hadron collider such as the LHC,
the underlying event must also be modeled 1 .
A second class of MC generators are the so-called cross-section integrators. Unlike
event generators, these programs do not output events with full kinematic informa-
tion, but rather produce distributions of kinematic variables, e.g. , pT spectra, with
high precision. Cross-section integrators are often used for unambiguous interpreta-
tion of experimental results.
In the simulation step, final-state particles from the generation step are propa-
gated through a realistic description of the detector. Ideally, all parts of the detector,
including active detection volumes and inactive (‘dead’) material, would be included
in the detector description. The simulation program knows what volume a particle is
traversing at a given stage; interactions with the material in that volume are modeled
according to the particle type, properties of the material and known physics. Elec-
1 The underlying event arises from interactions between partons in the colliding hadrons that did
not participate in the hard process. These interactions can be significant and can lead to sufficient
activity in the final state so as to affect the measurement of the hard process. It is therefore essential
that the underlying event be accurately described in Monte Carlo.